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Using old phones with Ooma Telo - why doesn't this work?
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Peabody
Feb 22, 2013, 12:29:14 PM2/22/13
to
I just wondered if there was anybody else in here as old as me that
knows something about POTS electroncs. This is just an interesting
curiosity, but I'd like to understand what's going on.
So I have a collection of older analog phones. They are new enough
to be touch-tone, but just barely. They have real ringers. When
you push a number button, mechanical switches connect selected
tuned circuits into the line to generate the appropriate tone pair.
The microphones are big round disks filled with carbon granules.
There are no integrated circuits.
These phones work quite well on my AT&T POTS land line. Clear as a
bell in both directions. But, I've decided to dump the land line
and switch over to a VOIP solution - the Ooma Telo in particular.
When I connect any of the older phones to the Telo, incoming audio
sounds fine, but I sound awful to people I call. I've left test
messages on my land line answering machine, and on my cellphone
voicemail, and I can confirm that I sound very rough and scratchy
and distorted. But remember that these phones do not produce that
result when used on the land line - I sound just fine.
In contrast, a more modern electronic phone works just fine with
the Telo in both directions. So there appears to be something
about the old phones that the Telo doesn't handle well. But of
course the Telo can't tell what kind of phone is being used - it
just sees what's on the handset line. And I can't figure out what
it is about the old phones that's different - in what way things
look different to the Telo.
The Telo is pretty powerful. It will drive an entire home POTS
network up to a total of 5 ringer equivalence. But maybe the
voltage on the line, or the current drawn by the old phones, is
messing things up. Perhaps the problem involves whatever "hybrid"
circuit the Telo uses to extract my outgoing audio from the
combined incoming and outgoing audio that appears on the handset
line.
Anyway, if anyone has any ideas about this, I would be interested.
It won't change the solution, which is to get some new phones, but
I'd still like to understand.
By the way, I'm pretty impressed with Ooma so far.
knows something about POTS electroncs. This is just an interesting
curiosity, but I'd like to understand what's going on.
So I have a collection of older analog phones. They are new enough
to be touch-tone, but just barely. They have real ringers. When
you push a number button, mechanical switches connect selected
tuned circuits into the line to generate the appropriate tone pair.
The microphones are big round disks filled with carbon granules.
There are no integrated circuits.
These phones work quite well on my AT&T POTS land line. Clear as a
bell in both directions. But, I've decided to dump the land line
and switch over to a VOIP solution - the Ooma Telo in particular.
When I connect any of the older phones to the Telo, incoming audio
sounds fine, but I sound awful to people I call. I've left test
messages on my land line answering machine, and on my cellphone
voicemail, and I can confirm that I sound very rough and scratchy
and distorted. But remember that these phones do not produce that
result when used on the land line - I sound just fine.
In contrast, a more modern electronic phone works just fine with
the Telo in both directions. So there appears to be something
about the old phones that the Telo doesn't handle well. But of
course the Telo can't tell what kind of phone is being used - it
just sees what's on the handset line. And I can't figure out what
it is about the old phones that's different - in what way things
look different to the Telo.
The Telo is pretty powerful. It will drive an entire home POTS
network up to a total of 5 ringer equivalence. But maybe the
voltage on the line, or the current drawn by the old phones, is
messing things up. Perhaps the problem involves whatever "hybrid"
circuit the Telo uses to extract my outgoing audio from the
combined incoming and outgoing audio that appears on the handset
line.
Anyway, if anyone has any ideas about this, I would be interested.
It won't change the solution, which is to get some new phones, but
I'd still like to understand.
By the way, I'm pretty impressed with Ooma so far.
Tim Wescott
Feb 22, 2013, 12:48:08 PM2/22/13
to
voltage, and then check the on-hook current through the phone.
The Ooma hardware is probably either supplying insufficient voltage, or
has too high a DC resistance, or both.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
mrob...@att.net
Feb 22, 2013, 12:50:51 PM2/22/13
to
Peabody <waybackNO...@yahoo.com> wrote:
> When I connect any of the older phones to the Telo, incoming audio
> sounds fine, but I sound awful to people I call.
Some things to try...
> When I connect any of the older phones to the Telo, incoming audio
> sounds fine, but I sound awful to people I call.
Reverse the polarity to your phone - take the cover off and swap the
red and green wires in the line cord. The Touch-Tone pad may stop
working when you do this. If that happens, use a modular "splitter"
to plug in a modified and unmodified phone. Pick up the unmodified
phone, dial, then pick up the modified phone and hang up the unmodified
one. Leave a message to see if the audio is any better.
Also, if the yellow and black wires from the line cord are connected
to anything inside the phone, note where they are connected, and
disconnect them, to see if that makes a difference. If that helps, tape
off the yellow and black wires from the line cord and run the phone that
way.
You can measure the DC voltage across the red and green wires in the
line cord. With the phone hung up, it should be 48 V DC, but this
isn't super critical. When you pick up the phone, the voltage should
drop. If it goes all the way down to 1 or 2 volts you will have
trouble; if it stays near 48 volts you may also have trouble.
This is less likely, since it works OK on a real land line, but:
unscrew the transmitter cover, take out the carbon microphone capsule,
hold it up to your ear, and shake it. You should hear a "sshh-sshh"
swishing noise from the carbon granules shaking around. If not, bang
the capsule on the table a couple of times. Also make sure the
contacts on the bottom of the capsule, and the springs in the handset
that touch them, are not covered with gunk.
> The Telo is pretty powerful. It will drive an entire home POTS
> network up to a total of 5 ringer equivalence.
supplies "talk battery" (the DC voltage to run the phone) will be
different and is maybe not up to the job - measuring the DC voltage
at the phone will tell you that.
Matt Roberds
tm
Feb 22, 2013, 1:54:49 PM2/22/13
to
"Peabody" <waybackNO...@yahoo.com> wrote in message
news:20130222-1...@news.astraweb.com...
likely the carbon granules are clumped together in the mic element.
Mechanical shock will loosen them up and make the mic work better.
Jim Thompson
Feb 22, 2013, 3:53:50 PM2/22/13
to
properly.
...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Tim Wescott
Feb 22, 2013, 6:17:22 PM2/22/13
to
phone works OK with a 24V battery, but I can't remember what we used for
a loading resistor. It's less than 25 years ago -- I think I'm going
senile!
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?
Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com
Jim Thompson
Feb 22, 2013, 6:25:40 PM2/22/13
to
On Fri, 22 Feb 2013 17:17:22 -0600, Tim Wescott <t...@seemywebsite.com>
I found this tidbit...
"The normal voltage of a CO line is about 48-50 VDC, and the Loop
Current of the line should be between 23ma and 27ma range, but in some
instances that may not be true. Most phone systems work fine with Loop
Current in the 23ma to 35ma range, to avoid any problems it is
recommended to be 23ma to 27ma range."
"The normal voltage of a CO line is about 48-50 VDC, and the Loop
Current of the line should be between 23ma and 27ma range, but in some
instances that may not be true. Most phone systems work fine with Loop
Current in the 23ma to 35ma range, to avoid any problems it is
recommended to be 23ma to 27ma range."
Joerg
Feb 22, 2013, 6:31:29 PM2/22/13
to
Sound too much like Obama to me :-)
>>>> ... hardware is probably either supplying insufficient voltage, or
>>>> has too high a DC resistance, or both.
>>> Yep. My guess as well. Not enough loop current to run a carbon mike
>>> properly.
>> My first job was at a company that did phone stuff. IIRC, an old-style
>> phone works OK with a 24V battery, but I can't remember what we used for
>> a loading resistor. It's less than 25 years ago -- I think I'm going
>> senile!
>
> I found this tidbit...
>
> "The normal voltage of a CO line is about 48-50 VDC, and the Loop
> Current of the line should be between 23ma and 27ma range, but in some
> instances that may not be true. Most phone systems work fine with Loop
> Current in the 23ma to 35ma range, to avoid any problems it is
> recommended to be 23ma to 27ma range."
>
When we moved here I found that ours is only around 20mA and some
>>> Yep. My guess as well. Not enough loop current to run a carbon mike
>>> properly.
>> My first job was at a company that did phone stuff. IIRC, an old-style
>> phone works OK with a 24V battery, but I can't remember what we used for
>> a loading resistor. It's less than 25 years ago -- I think I'm going
>> senile!
>
> I found this tidbit...
>
> "The normal voltage of a CO line is about 48-50 VDC, and the Loop
> Current of the line should be between 23ma and 27ma range, but in some
> instances that may not be true. Most phone systems work fine with Loop
> Current in the 23ma to 35ma range, to avoid any problems it is
> recommended to be 23ma to 27ma range."
>
speakerphone don't work reliably with that. Missy Bell said that's ok,
or at least that they ain't gonna do nothing about it. So I had to get a
speakerphone that works with this. Panasonic Easa-Phone did the trick,
in case any readers run into a similar situation.
--
Regards, Joerg
http://www.analogconsultants.com/
Michael A. Terrell
Feb 22, 2013, 7:08:28 PM2/22/13
to
Tim Wescott wrote:
>
> My first job was at a company that did phone stuff. IIRC, an old-style
> phone works OK with a 24V battery, but I can't remember what we used for
> a loading resistor. It's less than 25 years ago -- I think I'm going
> senile!
Peabody
Feb 23, 2013, 1:52:38 PM2/23/13
to
Peabody says...
I appreciate all the great suggestions. I've done some
testing:
The on-hook voltage of the Telo is 46.2V.
Telo off-hook voltage using the old analog phone
continuously fluctuates between 6 and 8V, and that appears
to vary with the audio input. However, if I remove the
carbon micropohone disk, it's 9.42V, and rock solid.
Telo off-hook voltage using the newer electronic phone is
rock solid at 5.71V no matter what the audio input is.
So then I decided to try all that on my land line. On-hook
voltage is 49.9V. Off-hook, the old phone fluctuates from 7
to 9V, and is solid at 10.42V with the microphone removed.
The electronic phone is 6.88V off-hook, solid.
I'm not clear why the Ma Bell off-hook voltage would be
higher than the Telo in all cases. But of course we don't
know what the Telo is putting out. And it seems the
resistance of the mile of 22-gauge twisted pair copper on
the Ma Bell side should have something to do with the
difference, but every time I try to figure that out, I
conclude I'm thinking about it backwards.
I made sure polarity was the same in all cases. I didn't do
any current measurements.
Doing some further reading, it seems the carbon mic is
actually a variable current device. Of course the current
changes get converted to voltage changes somewhere in the
process, but I think this technology is just incompatible
with the Telo. The fact that the voltage on the line is so
eratic probably doesn't help the Telo extract the actual
audio signal from it, and it ends up doing a poor job. Ma
Bell retains the ability to handle carbon mic input because
of people like me, but I think the Telo people probably just
concluded it wasn't worth the trouble. Actually, I pretty
much agree with that.
In any case, and whatever the explanation, it seems the
answer is to get a couple newer electronic phones, and I've
ordered one from Amazon to try out.
testing:
The on-hook voltage of the Telo is 46.2V.
Telo off-hook voltage using the old analog phone
continuously fluctuates between 6 and 8V, and that appears
to vary with the audio input. However, if I remove the
carbon micropohone disk, it's 9.42V, and rock solid.
Telo off-hook voltage using the newer electronic phone is
rock solid at 5.71V no matter what the audio input is.
So then I decided to try all that on my land line. On-hook
voltage is 49.9V. Off-hook, the old phone fluctuates from 7
to 9V, and is solid at 10.42V with the microphone removed.
The electronic phone is 6.88V off-hook, solid.
I'm not clear why the Ma Bell off-hook voltage would be
higher than the Telo in all cases. But of course we don't
know what the Telo is putting out. And it seems the
resistance of the mile of 22-gauge twisted pair copper on
the Ma Bell side should have something to do with the
difference, but every time I try to figure that out, I
conclude I'm thinking about it backwards.
I made sure polarity was the same in all cases. I didn't do
any current measurements.
Doing some further reading, it seems the carbon mic is
actually a variable current device. Of course the current
changes get converted to voltage changes somewhere in the
process, but I think this technology is just incompatible
with the Telo. The fact that the voltage on the line is so
eratic probably doesn't help the Telo extract the actual
audio signal from it, and it ends up doing a poor job. Ma
Bell retains the ability to handle carbon mic input because
of people like me, but I think the Telo people probably just
concluded it wasn't worth the trouble. Actually, I pretty
much agree with that.
In any case, and whatever the explanation, it seems the
answer is to get a couple newer electronic phones, and I've
ordered one from Amazon to try out.
Tim Wescott
Feb 23, 2013, 4:01:34 PM2/23/13
to
compresses and rarefies the carbon granules in response to sound
pressure, which makes the resistance go down or up. That changes the
current through the phone into a voltage.
Apparently the very earliest telco amplifiers were a speaker acoustically
coupled to a carbon mic, and the assembly actually achieved power
amplification (with enough drawbacks that Ma Bell was one of the earliest
adopters of vacuum tube technology).
Jeff Liebermann
Feb 23, 2013, 4:58:26 PM2/23/13
to
On Sat, 23 Feb 2013 12:52:38 -0600, Peabody
<waybackNO...@yahoo.com> wrote:
>The on-hook voltage of the Telo is 46.2V.
>
>Telo off-hook voltage using the old analog phone
>continuously fluctuates between 6 and 8V, and that appears
>to vary with the audio input. However, if I remove the
>carbon micropohone disk, it's 9.42V, and rock solid.
>
>Telo off-hook voltage using the newer electronic phone is
>rock solid at 5.71V no matter what the audio input is.
>
>So then I decided to try all that on my land line. On-hook
>voltage is 49.9V. Off-hook, the old phone fluctuates from 7
>to 9V, and is solid at 10.42V with the microphone removed.
>The electronic phone is 6.88V off-hook, solid.
This brings back fond nightmares. I ran a similar measurement on my
>The on-hook voltage of the Telo is 46.2V.
>
>Telo off-hook voltage using the old analog phone
>continuously fluctuates between 6 and 8V, and that appears
>to vary with the audio input. However, if I remove the
>carbon micropohone disk, it's 9.42V, and rock solid.
>
>Telo off-hook voltage using the newer electronic phone is
>rock solid at 5.71V no matter what the audio input is.
>
>So then I decided to try all that on my land line. On-hook
>voltage is 49.9V. Off-hook, the old phone fluctuates from 7
>to 9V, and is solid at 10.42V with the microphone removed.
>The electronic phone is 6.88V off-hook, solid.
POTS line. Incidentally, I'm 9500ft from the CO.
On hook is about 50.0 volts[1]. I tried various modern instruments,
all of which dropped the line voltage to about 9.2 to 9.5 volts. My
antique Northern Electric rotary dial butt-in should be a fair
simulation of an antique phone. It dropped the line voltage to about
9.3 volts. I confirmed the measurements with a Triplett Model 5 Loop
Tester and an Ameritec Model AM-44 VF tester (just to see if they
still were working). Loop current with a 600 load was 31.3ma.
5.71v seem far too low, even for your new electronic phone. It should
be more like 9 - 10VDC. Just to be sure, I tried about 8 assorted
instruments, all of which showed 9 to 10V off hook. Low DC and
probably low loop current, is a sufficient excuse to justify a
(hopefully free) service call from your local monopoly.
I don't have an Ooma box handy, but I have a pile of Linksys PAP2T-NA
awaiting installation. 49V on hook, 9.5V off hook, 19.5ma and 20.3ma
loop current (L1/L2), using the Triplett tester at 600 ohms.
I'm not sure what's wrong at your end, but these number may help. As
was suggested, try measuring the loop current. Anything less than
20ma will indicate a problem.
[1] Measured with my Fluke 77 DVM, after replacing the leaking 9V
battery. My Triplett had two leaky 9V batteries and my Ameritec had 4
dead AA batteries that were trying to leak. Time for a general
battery inspection and purge before electrolyte rot kills all my test
equipment.
--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Jan Panteltje
Feb 23, 2013, 5:31:56 PM2/23/13
to
On a sunny day (Sat, 23 Feb 2013 13:58:26 -0800) it happened Jeff Liebermann
<je...@cruzio.com> wrote in <ffcii85eshp11mppp...@4ax.com>:
>[1] Measured with my Fluke 77 DVM, after replacing the leaking 9V
>battery. My Triplett had two leaky 9V batteries and my Ameritec had 4
>dead AA batteries that were trying to leak. Time for a general
>battery inspection and purge before electrolyte rot kills all my test
>equipment.
Boeing may have some cheap Li-ion for you...
:-)
<je...@cruzio.com> wrote in <ffcii85eshp11mppp...@4ax.com>:
>[1] Measured with my Fluke 77 DVM, after replacing the leaking 9V
>battery. My Triplett had two leaky 9V batteries and my Ameritec had 4
>dead AA batteries that were trying to leak. Time for a general
>battery inspection and purge before electrolyte rot kills all my test
>equipment.
:-)
mrob...@att.net
Feb 24, 2013, 2:32:17 AM2/24/13
to
This is maybe a little low, but it doesn't actually matter that much.
The telephone pretty much doesn't use DC voltage in this state; it is
allowed to leak a tiny amount of current, but that mount is not enough
to do anything useful.
> Telo off-hook voltage using the old analog phone continuously
> fluctuates between 6 and 8V, and that appears to vary with the audio
> input.
If the room is quiet, the voltage should be fairly steady, but there
will be small fluctuations - the microphone is doing its job of sending
audio down the line.
On the other hand, if the Telo really is being loaded down by the
carbon microphone, its voltage output may be changing at the same time
the microphone is also trying to change the voltage, which may be the
cause of the bad sound.
> I'm not clear why the Ma Bell off-hook voltage would be higher than
> the Telo in all cases.
The Telo box is probably powered by an AC adapter that fits in your
hand and weighs a couple of ounces.
At your friendly local Ma Bell exchange (the brick building that looks
like it can survive a tornado, earthquake, and Godzilla simultaneously
- because it can), in the basement, there are 24 two-volt cells. Each
cell is about half the size of the dishwasher in your kitchen, and they
are bolted together with copper bars that are thicker than your thumb.
Ma replaces (or used to replace) the cells every 20 years or so whether
they need it or not.
That's why. :)
(Okay, in these latter days, your battery and dialtone is likely to be
coming from those cabinets that are popping up through neighborhoods
everywhere. These have four 12 V absorbed glass mat or gel-cell
batteries wired in series; each battery is about the size of one or two
car batteries.)
> And it seems the resistance of the mile of 22-gauge twisted pair
> copper on the Ma Bell side should have something to do with the
> difference, but every time I try to figure that out, I conclude I'm
> thinking about it backwards.
The resistance of the mile of wire does have an effect. If you carried
your old phone to the exchange and plugged it in right there, you would
see (say) 48 V on-hook and 47 V off hook. Put a mile of wire in
between you and and the exchange, and you get the drop to about 10 V
off hook that you see.
The important thing is that the batteries at Ma can sustain that 10 V
voltage at your phone very well. When you talk, the resistance of the
microphone decreases that voltage a little bit and then lets it come
back up, but it always comes back up to a steady 10 V.
The Telo box is dropping to 8 V, and I suspect it's hanging on "by the
skin of its teeth" to do that. When you start talking, the voltage
put out by the Telo box is probably sagging down, *in addition* to the
voltage decrease caused by the microphone.
The Telo box doesn't have the mile of wire to work against; I suspect it
is either very current-limited, which would make the voltage drop when
the phone is picked up happen "automatically", or it is programmed to
limit the current or drop the voltage when you go off hook, to simulate
a real Ma Bell line.
> In any case, and whatever the explanation, it seems the answer is to
> get a couple newer electronic phones, and I've ordered one from Amazon
> to try out.
I agree with that for a short-term answer. The long-term answer is to
complain to Ooma that their box can't operate a standard telephone. :)
(There is a small but nonzero possiblity that this can be fixed with
some kind of settings change or software upgrade. However, it's more
likely that the Telo box needs a better design for its internal power
supply.)
I had an interesting chat with the guy that was installing the new whiz-
bang VoIP PBX at a previous workplace. I asked if it could support
analog phones - yes. How about rotary dial? He laughed and said,
"Well, *now* it can." Apparently the first units could only do analog
Touch-Tone. This worked great until they got installed in churches and
elementary schools. It seems that those places tended to still have the
Western Electric rotary-dial phone from 195x or 196x still on the
wall... in the kitchen.
The first attempted fix was usually to replace that phone with a whiz-
bang electronic phone. A short time later, there would be a warranty
call on the whiz-bang phone, because it didn't handle humidity / grease
/ being dropped in a pot of chili nearly as well as the old phone. Once
the PBX got analog rotary dial capability, the old phone was restored to
the wall and everyone was happy.
Matt Roberds
The telephone pretty much doesn't use DC voltage in this state; it is
allowed to leak a tiny amount of current, but that mount is not enough
to do anything useful.
> Telo off-hook voltage using the old analog phone continuously
> fluctuates between 6 and 8V, and that appears to vary with the audio
> input.
will be small fluctuations - the microphone is doing its job of sending
audio down the line.
On the other hand, if the Telo really is being loaded down by the
carbon microphone, its voltage output may be changing at the same time
the microphone is also trying to change the voltage, which may be the
cause of the bad sound.
> I'm not clear why the Ma Bell off-hook voltage would be higher than
> the Telo in all cases.
hand and weighs a couple of ounces.
At your friendly local Ma Bell exchange (the brick building that looks
like it can survive a tornado, earthquake, and Godzilla simultaneously
- because it can), in the basement, there are 24 two-volt cells. Each
cell is about half the size of the dishwasher in your kitchen, and they
are bolted together with copper bars that are thicker than your thumb.
Ma replaces (or used to replace) the cells every 20 years or so whether
they need it or not.
That's why. :)
(Okay, in these latter days, your battery and dialtone is likely to be
coming from those cabinets that are popping up through neighborhoods
everywhere. These have four 12 V absorbed glass mat or gel-cell
batteries wired in series; each battery is about the size of one or two
car batteries.)
> And it seems the resistance of the mile of 22-gauge twisted pair
> copper on the Ma Bell side should have something to do with the
> difference, but every time I try to figure that out, I conclude I'm
> thinking about it backwards.
your old phone to the exchange and plugged it in right there, you would
see (say) 48 V on-hook and 47 V off hook. Put a mile of wire in
between you and and the exchange, and you get the drop to about 10 V
off hook that you see.
The important thing is that the batteries at Ma can sustain that 10 V
voltage at your phone very well. When you talk, the resistance of the
microphone decreases that voltage a little bit and then lets it come
back up, but it always comes back up to a steady 10 V.
The Telo box is dropping to 8 V, and I suspect it's hanging on "by the
skin of its teeth" to do that. When you start talking, the voltage
put out by the Telo box is probably sagging down, *in addition* to the
voltage decrease caused by the microphone.
The Telo box doesn't have the mile of wire to work against; I suspect it
is either very current-limited, which would make the voltage drop when
the phone is picked up happen "automatically", or it is programmed to
limit the current or drop the voltage when you go off hook, to simulate
a real Ma Bell line.
> In any case, and whatever the explanation, it seems the answer is to
> get a couple newer electronic phones, and I've ordered one from Amazon
> to try out.
complain to Ooma that their box can't operate a standard telephone. :)
(There is a small but nonzero possiblity that this can be fixed with
some kind of settings change or software upgrade. However, it's more
likely that the Telo box needs a better design for its internal power
supply.)
I had an interesting chat with the guy that was installing the new whiz-
bang VoIP PBX at a previous workplace. I asked if it could support
analog phones - yes. How about rotary dial? He laughed and said,
"Well, *now* it can." Apparently the first units could only do analog
Touch-Tone. This worked great until they got installed in churches and
elementary schools. It seems that those places tended to still have the
Western Electric rotary-dial phone from 195x or 196x still on the
wall... in the kitchen.
The first attempted fix was usually to replace that phone with a whiz-
bang electronic phone. A short time later, there would be a warranty
call on the whiz-bang phone, because it didn't handle humidity / grease
/ being dropped in a pot of chili nearly as well as the old phone. Once
the PBX got analog rotary dial capability, the old phone was restored to
the wall and everyone was happy.
Matt Roberds
Michael A. Terrell
Feb 24, 2013, 5:56:19 PM2/24/13
to
Michael A. Terrell
Feb 24, 2013, 6:19:52 PM2/24/13
to
mrob...@att.net wrote:
>
> On the other hand, if the Telo really is being loaded down by the
> carbon microphone, its voltage output may be changing at the same time
> the microphone is also trying to change the voltage, which may be the
> cause of the bad sound.
That causes the 'C.O.' line voltage to drop whenever the diaphram is
compressed by air pressure.
'Electronic' phones impose the AC voltae on the line without
modulating the loop current directly.
Tim Wescott
Feb 25, 2013, 1:26:36 PM2/25/13
to
you're really, really attached to those old phones you could regenerate
the power with a box that looks like a phone in one direction and a phone
line (suitable for your phones) in the other.
But it'd be way less effort, and probably less cost, to just buy new
phones.
mrob...@att.net
Feb 25, 2013, 2:07:13 PM2/25/13
to
Michael A. Terrell <mike.t...@earthlink.net> wrote:
> mrob...@att.net wrote:
>>
>> On the other hand, if the Telo really is being loaded down by the
>> carbon microphone, its voltage output may be changing at the same
>> time the microphone is also trying to change the voltage, which may
>> be the cause of the bad sound.
>
> A carbon mic modulates the loop current. Always has, always will.
> That causes the 'C.O.' line voltage to drop whenever the diaphram is
> compressed by air pressure.
What I was trying to say, and possibly not succeeding in saying, is that
> mrob...@att.net wrote:
>>
>> On the other hand, if the Telo really is being loaded down by the
>> carbon microphone, its voltage output may be changing at the same
>> time the microphone is also trying to change the voltage, which may
>> be the cause of the bad sound.
>
> A carbon mic modulates the loop current. Always has, always will.
> That causes the 'C.O.' line voltage to drop whenever the diaphram is
> compressed by air pressure.
if the power supply at the "CO" (has a high impedance / has a high
Thevenin equivalent resistance / isn't very "stiff") [pick your favorite
term], there won't be as much loop current available, which will have an
effect on the sound.
Here are some numbers to hopefully show what I am talking about. These
are not meant to be too close to "real world" values - they are just to
show the problem.
So, imagine as givens:
R1 Mic
+---/\/\/\---o---/\/\/\---+
+| |
V |
-| R2 |
+---/\/\/\---o------------+
"CO" | Telephone set
The carbon microphone resistance Mic varies between 400 and 800 ohms as
you talk. It does not vary based on any other factor.
There isn't any other resistance in the telephone set other than the
carbon microphone.
The "good" CO is a 48 V DC voltage source with R1 = R2 = 400 ohm fixed
resistor.
The "bad" CO is a 48 V DC voltage source with R1 = R2 = variable
resistors that can be between 1600 and 3200 ohms.
The idea with the variable resistance is that it models a power supply
that is not quite up to the job; something like a weak battery, or a
filter capacitor that's way too small.
Calculate for the "good" CO:
With the carbon mike at 400 ohms, the loop current is
48 / (400 + 400 + 400) = 40 mA.
The voltage at the telephone set will be (0.040 * 400) = 16 V.
With the carbon mike at 800 ohms, the loop current is
48 / (400 + 800 + 400) = 30 mA.
The voltage at the telephone set will be (0.030 * 800) = 24 V.
So, for the "good" CO, the voltage at the telephone set changes from 16
to 24 V, with a loop current between 40 and 30 mA. The changes in
voltage at the telephone set and loop current only depend on the change
of resistance of the carbon microphone.
Calculate for the "bad" CO:
With the carbon mike at 400 ohms, the "bad" CO has a loop current of
48 / (1600 + 400 + 1600) = 13 mA to 48 / (3200 + 400 + 3200) = 7.1 mA.
The voltage at the telephone set will be between (0.013 * 400) = 5.3 V
and (0.0071 * 400) = 2.8 V.
With the carbon mike at 800 ohms, the "bad" CO has a loop current of
48 / (1600 + 800 + 1600) or 12 mA to 48 / (3200 + 800 + 3200) or 6.7 mA.
The voltage at the telephone set will be between (0.012 * 800) or 9.6 V
and (0.0067 * 800) or 5.3 V.
So, for the "bad" CO, the voltage at the telephone set changes from 2.8
to 9.6 V, with a loop current between 7.1 and 13 mA. The changes in
voltage at the telephone set and loop current depend on the change of
resistance of the carbon microphone *AND* the change in resistance at
the CO.
The _higher_ resistance of the "bad" CO causes a lower amplitude signal
to be produced by the microphone. If this was the only thing happening,
the signal might sound OK, just not as loud as with the "good" CO.
The _changing_ resistance of the "bad" CO is adding more modulation to
the loop current that you would expect from the microphone alone. This
causes the resulting signal to be distorted and sound bad.
The higher and changing resistance is what I figured the Telo box is
doing; that's why I said it might be "loaded down". As you said, the
carbon microphone always loads down the CO. What I was trying to say is
that if your CO is not very good, there may come a point where the
normal load of the microphone, caused by talking, may be too much for
the power supply in the CO; the power supply in the CO will start to
sag even further than you would expect. That's all.
Matt Roberds
Michael A. Terrell
Feb 25, 2013, 3:08:59 PM2/25/13
to
hrough. As far as high impedance, they have 48 volt battery banks that
deliver thousands of amps. All of this is prety much meaningless in
,ost ares, since they have small switching center scattered all over
town, and no real C.O. Around here, it is fiber to the last mile, then
a SLIC in a large outdoor cabinet to connect you over underground copper
lines. How much drop do you see for two miles of AWG 24 or 22? Even
though it's the last mile, you still have to meet the Bell standards if
you want the line to work.
Peabody
Feb 25, 2013, 7:29:06 PM2/25/13
to
mrob...@att.net says...
> What I was trying to say, and possibly not succeeding in
> saying, is that if the power supply at the "CO" (has a
> high impedance / has a high Thevenin equivalent
> resistance / isn't very "stiff") [pick your favorite
> term], there won't be as much loop current available,
> which will have an effect on the sound.
-snip-
But it seems clear that a modern electronic phone doesn't
use a microphone that captures sound by varying resistance.
Instead, the phone draws enough current for its own
operation, and possibly some additional current to make sure
the CO senses it as being off-hook. But then the audio
signal is just injected onto what would otherwise be a
steady DC voltage on the line. And what's needed to extract
the audio signal at the CO or the Telo seems to be quite
different for the two phone types.
But it's still curious to me how much the voltage varied
with the old phone, while not at all with the new one, and
that's on both Ma Bell and the Telo. The variation is far
larger than would make any sense for an audio signal -
volts instead of millivolts. Of course this is as measured
with a digital meter - I wonder if an analog meter would
show the same variation. By contrast, the digital meter
shows a steady voltage even though the audio signal is
actually there.
In any case, I'm going to be saving a bundle of money on the
Ooma service over time, and I'm certainly not going to let
the cost of three or four new phones (at maybe $15 each, and
that's with Caller ID, redial, etc,) stand in the way - if
that's what's needed, and it looks like it is.
> What I was trying to say, and possibly not succeeding in
> saying, is that if the power supply at the "CO" (has a
> high impedance / has a high Thevenin equivalent
> resistance / isn't very "stiff") [pick your favorite
> term], there won't be as much loop current available,
> which will have an effect on the sound.
But it seems clear that a modern electronic phone doesn't
use a microphone that captures sound by varying resistance.
Instead, the phone draws enough current for its own
operation, and possibly some additional current to make sure
the CO senses it as being off-hook. But then the audio
signal is just injected onto what would otherwise be a
steady DC voltage on the line. And what's needed to extract
the audio signal at the CO or the Telo seems to be quite
different for the two phone types.
But it's still curious to me how much the voltage varied
with the old phone, while not at all with the new one, and
that's on both Ma Bell and the Telo. The variation is far
larger than would make any sense for an audio signal -
volts instead of millivolts. Of course this is as measured
with a digital meter - I wonder if an analog meter would
show the same variation. By contrast, the digital meter
shows a steady voltage even though the audio signal is
actually there.
In any case, I'm going to be saving a bundle of money on the
Ooma service over time, and I'm certainly not going to let
the cost of three or four new phones (at maybe $15 each, and
that's with Caller ID, redial, etc,) stand in the way - if
that's what's needed, and it looks like it is.
rickman
Feb 25, 2013, 9:48:47 PM2/25/13
to
On 2/24/2013 2:32 AM, mrob...@att.net wrote:
> Peabody<waybackNO...@yahoo.com> wrote:
>> The on-hook voltage of the Telo is 46.2V.
>
> Peabody<waybackNO...@yahoo.com> wrote:
>> The on-hook voltage of the Telo is 46.2V.
>
>> I'm not clear why the Ma Bell off-hook voltage would be higher than
>> the Telo in all cases.
>
> The Telo box is probably powered by an AC adapter that fits in your
> hand and weighs a couple of ounces.
>
> At your friendly local Ma Bell exchange (the brick building that looks
> like it can survive a tornado, earthquake, and Godzilla simultaneously
> - because it can), in the basement, there are 24 two-volt cells. Each
> cell is about half the size of the dishwasher in your kitchen, and they
> are bolted together with copper bars that are thicker than your thumb.
> Ma replaces (or used to replace) the cells every 20 years or so whether
> they need it or not.
>
> That's why. :)
Not terribly relevant. They intentionally add series resistance so the
>> the Telo in all cases.
>
> The Telo box is probably powered by an AC adapter that fits in your
> hand and weighs a couple of ounces.
>
> At your friendly local Ma Bell exchange (the brick building that looks
> like it can survive a tornado, earthquake, and Godzilla simultaneously
> - because it can), in the basement, there are 24 two-volt cells. Each
> cell is about half the size of the dishwasher in your kitchen, and they
> are bolted together with copper bars that are thicker than your thumb.
> Ma replaces (or used to replace) the cells every 20 years or so whether
> they need it or not.
>
> That's why. :)
line can be modulated by the carbon microphone as you describe in your
next post. So why does it matter how good the batteries are if they add
nearly a kohm to the circuit to your phone?
Yes, the telo is not putting out as much current or voltage, so it could
either have a higher in line resistance or it could simply be running
out of current. But since this current is spec'd and has been for a
long, long time, there is no reason why it wouldn't meet that spec, even
if only marginally. Otherwise it would be a reason for a lot of returns.
>> And it seems the resistance of the mile of 22-gauge twisted pair
>> copper on the Ma Bell side should have something to do with the
>> difference, but every time I try to figure that out, I conclude I'm
>> thinking about it backwards.
>
> The resistance of the mile of wire does have an effect. If you carried
> your old phone to the exchange and plugged it in right there, you would
> see (say) 48 V on-hook and 47 V off hook. Put a mile of wire in
> between you and and the exchange, and you get the drop to about 10 V
> off hook that you see.
phone. Do the math. I don't know what gauge they use or if it is even
copper, but the resistance would have to be pretty high to drop 48 volts
to 10 volts in just a mile. Your other post even talks about the
resistance they add to the line in the CO. Another reason they add that
is to protect the supplies from a short on the line. Instead of dozens
or hundreds of amps flowing out of the CO it is a few 10's of mA.
> The important thing is that the batteries at Ma can sustain that 10 V
> voltage at your phone very well. When you talk, the resistance of the
> microphone decreases that voltage a little bit and then lets it come
> back up, but it always comes back up to a steady 10 V.
>
> The Telo box is dropping to 8 V, and I suspect it's hanging on "by the
> skin of its teeth" to do that. When you start talking, the voltage
> put out by the Telo box is probably sagging down, *in addition* to the
> voltage decrease caused by the microphone.
>
> The Telo box doesn't have the mile of wire to work against; I suspect it
> is either very current-limited, which would make the voltage drop when
> the phone is picked up happen "automatically", or it is programmed to
> limit the current or drop the voltage when you go off hook, to simulate
> a real Ma Bell line.
voltage varies as a signal to the CO. The phone doesn't care about the
DC voltage on the line when it is off hook. The drop in voltage signals
the CO that the phone is off hook.
>> In any case, and whatever the explanation, it seems the answer is to
>> get a couple newer electronic phones, and I've ordered one from Amazon
>> to try out.
>
> I agree with that for a short-term answer. The long-term answer is to
> complain to Ooma that their box can't operate a standard telephone. :)
> (There is a small but nonzero possiblity that this can be fixed with
> some kind of settings change or software upgrade. However, it's more
> likely that the Telo box needs a better design for its internal power
> supply.)
It is rather inefficient to supply 48 volts to a 400 ohm load through
800 ohms. Perhaps you are right in that they have two supplies. One
provides 48 volts at *very* light currents and when the phone is on
hook, a second supply provides 10-12 volts to power the phone when off
hook. I guess there are phones that need the 48 volts to power the
electronics when off hook. There's not much to do, but I guess since
modern phones are electronic they need power even to make the annoying
bleep sounds which replace the ring of a phone (is it too much to expect
them to rectify the ring voltage?). So if they expect 48 volts, I guess
you need to give them 48 volts.
So one supply to power the very light load at 48 volts and a second
supply to power the phones when off hook and a third to supply the ring
voltage. The ringer equivalence only specs the power available when
ringing, not the DC voltages.
Another point is that if the off hook supply was not designed to work
with carbon mics, it might be *too* stiff, not letting the mic modulate
the line voltage. If they aren't monitoring the current, they wouldn't
get much signal from the "real" phone.
But this is all speculation until someone opens the Telo case and starts
tracing out some of the circuit. Like Matt says, contact Telo and see
what they say.
--
Rick
rickman
Feb 26, 2013, 6:40:18 PM2/26/13
to
On 2/25/2013 7:29 PM, Peabody wrote:
> mrob...@att.net says...
>
> > What I was trying to say, and possibly not succeeding in
> > saying, is that if the power supply at the "CO" (has a
> > high impedance / has a high Thevenin equivalent
> > resistance / isn't very "stiff") [pick your favorite
> > term], there won't be as much loop current available,
> > which will have an effect on the sound.
>
> -snip-
>
> But it seems clear that a modern electronic phone doesn't
> use a microphone that captures sound by varying resistance.
> Instead, the phone draws enough current for its own
> operation, and possibly some additional current to make sure
> the CO senses it as being off-hook. But then the audio
> signal is just injected onto what would otherwise be a
> steady DC voltage on the line.
Ah, that's what you aren't getting right. In the case of the old analog
> mrob...@att.net says...
>
> > What I was trying to say, and possibly not succeeding in
> > saying, is that if the power supply at the "CO" (has a
> > high impedance / has a high Thevenin equivalent
> > resistance / isn't very "stiff") [pick your favorite
> > term], there won't be as much loop current available,
> > which will have an effect on the sound.
>
> -snip-
>
> But it seems clear that a modern electronic phone doesn't
> use a microphone that captures sound by varying resistance.
> Instead, the phone draws enough current for its own
> operation, and possibly some additional current to make sure
> the CO senses it as being off-hook. But then the audio
> signal is just injected onto what would otherwise be a
> steady DC voltage on the line.
phone with the carbon mic, the voltage will vary along with the current.
> And what's needed to extract
> the audio signal at the CO or the Telo seems to be quite
> different for the two phone types.
resistance, even if it is synthetic (meaning produced by an op amp using
positive feedback through a sense resistance). Change the current or
the voltage on the line and the other changes. In the old fashion CO I
believe they used a transformer winding to pick up the signal, but it
can be done just as well by an op amp circuit.
> But it's still curious to me how much the voltage varied
> with the old phone, while not at all with the new one, and
> that's on both Ma Bell and the Telo. The variation is far
> larger than would make any sense for an audio signal -
> volts instead of millivolts. Of course this is as measured
> with a digital meter - I wonder if an analog meter would
> show the same variation. By contrast, the digital meter
> shows a steady voltage even though the audio signal is
> actually there.
voltage variation. Possibly it was the carbon mic and low frequency
vibrations.
> In any case, I'm going to be saving a bundle of money on the
> Ooma service over time, and I'm certainly not going to let
> the cost of three or four new phones (at maybe $15 each, and
> that's with Caller ID, redial, etc,) stand in the way - if
> that's what's needed, and it looks like it is.
--
Rick
josephkk
Feb 27, 2013, 9:57:24 PM2/27/13
to
On Mon, 25 Feb 2013 21:48:47 -0500, rickman <gnu...@gmail.com> wrote:
>On 2/24/2013 2:32 AM, mrob...@att.net wrote:
>> Peabody<waybackNO...@yahoo.com> wrote:
>>> The on-hook voltage of the Telo is 46.2V.
>>
>>> I'm not clear why the Ma Bell off-hook voltage would be higher than
>>> the Telo in all cases.
>>
>> The Telo box is probably powered by an AC adapter that fits in your
>> hand and weighs a couple of ounces.
>>
>> At your friendly local Ma Bell exchange (the brick building that looks
>> like it can survive a tornado, earthquake, and Godzilla simultaneously
>> - because it can), in the basement, there are 24 two-volt cells. Each
>> cell is about half the size of the dishwasher in your kitchen, and they
>> are bolted together with copper bars that are thicker than your thumb.
>> Ma replaces (or used to replace) the cells every 20 years or so whether
>> they need it or not.
>>
>> That's why. :)
>
>Not terribly relevant. They intentionally add series resistance so the
>line can be modulated by the carbon microphone as you describe in your
>next post. So why does it matter how good the batteries are if they add
>nearly a kohm to the circuit to your phone?
Well they did 50 years ago or more, now the current is limited by an IC.
>On 2/24/2013 2:32 AM, mrob...@att.net wrote:
>> Peabody<waybackNO...@yahoo.com> wrote:
>>> The on-hook voltage of the Telo is 46.2V.
>>
>>> I'm not clear why the Ma Bell off-hook voltage would be higher than
>>> the Telo in all cases.
>>
>> The Telo box is probably powered by an AC adapter that fits in your
>> hand and weighs a couple of ounces.
>>
>> At your friendly local Ma Bell exchange (the brick building that looks
>> like it can survive a tornado, earthquake, and Godzilla simultaneously
>> - because it can), in the basement, there are 24 two-volt cells. Each
>> cell is about half the size of the dishwasher in your kitchen, and they
>> are bolted together with copper bars that are thicker than your thumb.
>> Ma replaces (or used to replace) the cells every 20 years or so whether
>> they need it or not.
>>
>> That's why. :)
>
>Not terribly relevant. They intentionally add series resistance so the
>line can be modulated by the carbon microphone as you describe in your
>next post. So why does it matter how good the batteries are if they add
>nearly a kohm to the circuit to your phone?
Each IC supervises a dozen or more lines and does a lot of other things as
well.
>
>Yes, the telo is not putting out as much current or voltage, so it could
>either have a higher in line resistance or it could simply be running
>out of current. But since this current is spec'd and has been for a
>long, long time, there is no reason why it wouldn't meet that spec, even
>if only marginally. Otherwise it would be a reason for a lot of returns.
>
>
>>> And it seems the resistance of the mile of 22-gauge twisted pair
>>> copper on the Ma Bell side should have something to do with the
>>> difference, but every time I try to figure that out, I conclude I'm
>>> thinking about it backwards.
>>
>> The resistance of the mile of wire does have an effect. If you carried
>> your old phone to the exchange and plugged it in right there, you would
>> see (say) 48 V on-hook and 47 V off hook. Put a mile of wire in
>> between you and and the exchange, and you get the drop to about 10 V
>> off hook that you see.
>
>A mile of wire has very little effect on the resulting voltage at your
>phone. Do the math. I don't know what gauge they use or if it is even
>copper, but the resistance would have to be pretty high to drop 48 volts
>to 10 volts in just a mile. Your other post even talks about the
>resistance they add to the line in the CO. Another reason they add that
>is to protect the supplies from a short on the line. Instead of dozens
>or hundreds of amps flowing out of the CO it is a few 10's of mA.
Not that you could get that kind of current through 22 Ga twisted pair for
>Yes, the telo is not putting out as much current or voltage, so it could
>either have a higher in line resistance or it could simply be running
>out of current. But since this current is spec'd and has been for a
>long, long time, there is no reason why it wouldn't meet that spec, even
>if only marginally. Otherwise it would be a reason for a lot of returns.
>
>
>>> And it seems the resistance of the mile of 22-gauge twisted pair
>>> copper on the Ma Bell side should have something to do with the
>>> difference, but every time I try to figure that out, I conclude I'm
>>> thinking about it backwards.
>>
>> The resistance of the mile of wire does have an effect. If you carried
>> your old phone to the exchange and plugged it in right there, you would
>> see (say) 48 V on-hook and 47 V off hook. Put a mile of wire in
>> between you and and the exchange, and you get the drop to about 10 V
>> off hook that you see.
>
>A mile of wire has very little effect on the resulting voltage at your
>phone. Do the math. I don't know what gauge they use or if it is even
>copper, but the resistance would have to be pretty high to drop 48 volts
>to 10 volts in just a mile. Your other post even talks about the
>resistance they add to the line in the CO. Another reason they add that
>is to protect the supplies from a short on the line. Instead of dozens
>or hundreds of amps flowing out of the CO it is a few 10's of mA.
very long.
>
>
>> The important thing is that the batteries at Ma can sustain that 10 V
>> voltage at your phone very well. When you talk, the resistance of the
>> microphone decreases that voltage a little bit and then lets it come
>> back up, but it always comes back up to a steady 10 V.
>>
>> The Telo box is dropping to 8 V, and I suspect it's hanging on "by the
>> skin of its teeth" to do that. When you start talking, the voltage
>> put out by the Telo box is probably sagging down, *in addition* to the
>> voltage decrease caused by the microphone.
>>
>> The Telo box doesn't have the mile of wire to work against; I suspect it
>> is either very current-limited, which would make the voltage drop when
>> the phone is picked up happen "automatically", or it is programmed to
>> limit the current or drop the voltage when you go off hook, to simulate
>> a real Ma Bell line.
>
>There is no point in "simulating" a Ma Bell phone line because the line
>voltage varies as a signal to the CO. The phone doesn't care about the
>DC voltage on the line when it is off hook. The drop in voltage signals
>the CO that the phone is off hook.
>
Not really, it is loop current. It is called loop-start, DC current flow
>
>> The important thing is that the batteries at Ma can sustain that 10 V
>> voltage at your phone very well. When you talk, the resistance of the
>> microphone decreases that voltage a little bit and then lets it come
>> back up, but it always comes back up to a steady 10 V.
>>
>> The Telo box is dropping to 8 V, and I suspect it's hanging on "by the
>> skin of its teeth" to do that. When you start talking, the voltage
>> put out by the Telo box is probably sagging down, *in addition* to the
>> voltage decrease caused by the microphone.
>>
>> The Telo box doesn't have the mile of wire to work against; I suspect it
>> is either very current-limited, which would make the voltage drop when
>> the phone is picked up happen "automatically", or it is programmed to
>> limit the current or drop the voltage when you go off hook, to simulate
>> a real Ma Bell line.
>
>There is no point in "simulating" a Ma Bell phone line because the line
>voltage varies as a signal to the CO. The phone doesn't care about the
>DC voltage on the line when it is off hook. The drop in voltage signals
>the CO that the phone is off hook.
>
= off-hook, also handled by that supervisory IC. "Subscriber Line
Interface Circuit" (SLIC) ICs did the following functions "Battery,
Overvoltage protection, Ringing, Signaling, Hybrid (separation of
talk/listen or send/receive) and Test" (BORSHT). some 20 or so years ago
it was one line per IC, now it is many and ADC and DAC for each line are
included.
>
>>> In any case, and whatever the explanation, it seems the answer is to
>>> get a couple newer electronic phones, and I've ordered one from Amazon
>>> to try out.
>>
>> I agree with that for a short-term answer. The long-term answer is to
>> complain to Ooma that their box can't operate a standard telephone. :)
>> (There is a small but nonzero possiblity that this can be fixed with
>> some kind of settings change or software upgrade. However, it's more
>> likely that the Telo box needs a better design for its internal power
>> supply.)
>
>I think it is likely that the supply in the Telo is not doing the job.
>It is rather inefficient to supply 48 volts to a 400 ohm load through
>800 ohms. Perhaps you are right in that they have two supplies. One
>provides 48 volts at *very* light currents and when the phone is on
>hook, a second supply provides 10-12 volts to power the phone when off
>hook. I guess there are phones that need the 48 volts to power the
>electronics when off hook. There's not much to do, but I guess since
>modern phones are electronic they need power even to make the annoying
>bleep sounds which replace the ring of a phone (is it too much to expect
>them to rectify the ring voltage?). So if they expect 48 volts, I guess
>you need to give them 48 volts.
Nothing of the kind, just 48 V with a controlled source impedance. That
>>> In any case, and whatever the explanation, it seems the answer is to
>>> get a couple newer electronic phones, and I've ordered one from Amazon
>>> to try out.
>>
>> I agree with that for a short-term answer. The long-term answer is to
>> complain to Ooma that their box can't operate a standard telephone. :)
>> (There is a small but nonzero possiblity that this can be fixed with
>> some kind of settings change or software upgrade. However, it's more
>> likely that the Telo box needs a better design for its internal power
>> supply.)
>
>I think it is likely that the supply in the Telo is not doing the job.
>It is rather inefficient to supply 48 volts to a 400 ohm load through
>800 ohms. Perhaps you are right in that they have two supplies. One
>provides 48 volts at *very* light currents and when the phone is on
>hook, a second supply provides 10-12 volts to power the phone when off
>hook. I guess there are phones that need the 48 volts to power the
>electronics when off hook. There's not much to do, but I guess since
>modern phones are electronic they need power even to make the annoying
>bleep sounds which replace the ring of a phone (is it too much to expect
>them to rectify the ring voltage?). So if they expect 48 volts, I guess
>you need to give them 48 volts.
is what causes the difference between off-hook and on-hook voltages.
>
>So one supply to power the very light load at 48 volts and a second
>supply to power the phones when off hook and a third to supply the ring
>voltage. The ringer equivalence only specs the power available when
>ringing, not the DC voltages.
>
>Another point is that if the off hook supply was not designed to work
>with carbon mics, it might be *too* stiff, not letting the mic modulate
>the line voltage. If they aren't monitoring the current, they wouldn't
>get much signal from the "real" phone.
>
>But this is all speculation until someone opens the Telo case and starts
>tracing out some of the circuit. Like Matt says, contact Telo and see
>what they say.
Or you could find and purchase the relevant standards. Standards
>So one supply to power the very light load at 48 volts and a second
>supply to power the phones when off hook and a third to supply the ring
>voltage. The ringer equivalence only specs the power available when
>ringing, not the DC voltages.
>
>Another point is that if the off hook supply was not designed to work
>with carbon mics, it might be *too* stiff, not letting the mic modulate
>the line voltage. If they aren't monitoring the current, they wouldn't
>get much signal from the "real" phone.
>
>But this is all speculation until someone opens the Telo case and starts
>tracing out some of the circuit. Like Matt says, contact Telo and see
>what they say.
compliance is necessary to get FCC part 68 compliance certification in
order to sell the equipment.
See:
Administrative Council for Terminal Attachments (ACTA)
?-)
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