BF-F8HP/ARPS-K2 Vox

[Patrick Whitney]

I'm wondering if anyone has any advice.  I'm using aprs.fi iOS app with a Baofeng BF-F8HP.  When I start the audio TNC, if I have VOX enabled on the radio, set to any value other than "OFF", the radio keys up, but nothing is transmitted (and I don't have any settings in the app to transmit anything -- Beacon is off).  

When VOX is off, I receive just fine, and, with the aid of "voice recorder" on the iPad, I was able to confirm that the cable is working as expected, that is, audio is successfully received from the radio (also confirmed by successful packet flow in aprs.fi, when vox is off) and that VOX is triggered successfully when something goes out over the speaker/APRS-K2 cable.  

Any thoughts?  I've tried adjusting the volume on the ipad, as I thought that might be doing it, but no luck. I'd suspect the radio or the cable if it wasn't for the fact I can get VOX to work as expected using voice recorder...

Any advice would be greatly appreciated. 

Environment: 
iOS: 14.7.1
Device: iPad 6th Gen
APRS.FI Version: 2.3.1.4

Radio: Baofeng BF-F8HP

Audio Cable: BTECH APRS-K1 

Best,

-Pat ~ K1PJW

[Geoffrey Dann]

I experienced something similar with a iPhone 5s and a iPhone SE (2016). The HT would sometimes key up with no audio input. I tried adding some ferrite beads to the cable, and that seemed to help, but not entirely. I do not know if the ferrites were the right type (mix of ingredients). I eventually gave up on the cable and bought a Mobilinkd TNC3. 

It would be nice if a similar cable existed for Icom HTs, of which I have a couple. 

geoff, N3CFX

[Heikki Hannikainen]

On Thu, 2 Sep 2021, Patrick Whitney wrote:

> I'm wondering if anyone has any advice.  I'm using aprs.fi iOS app with a Baofeng BF-F8HP.  When I start the audio TNC, if I have VOX
> enabled on the radio, set to any value other than "OFF", the radio keys up, but nothing is transmitted (and I don't have any settings
> in the app to transmit anything -- Beacon is off). 

Pat,

Can you clarify this with a few more details:

* Does the radio key up *continuously* when audio TNC is started?
* Or, does it get keyed up repeatedly with short bursts?

Do you have an oscilloscope available for some tests? Or, do you have
another radio to listen the transmitted audio with?

It would seem like some audio is leaking to the radio, and triggering VOX,
and it would be interesting to know what it is exactly. With the cable
connected to the iPad, it should be possible to feed in some audio through
the connector that goes in the earphone output of the radio, and check
with an oscilloscope or a mic-level audio input device check if there is
something coming out from the connector which goes to the microphone jack
of the radio.

There has been at least one case of a faulty cable, in which the received
audio from the radio's headphone output leaked back to the radio's mic
input. That audio then triggered VOX, at which point the received audio
stopped since the transmitter was keyed, and the VOX then released the
PTT. After which the received noise triggered the VOX again...

But if it works with the voice recorder, that would indeed to point
something being wrong with the app itself.

- Hessu

[Chris Clement]

I have a related issue and did some probing of the interface I bought. It is summarized in a graphic to preserve the table alignment.

[Heikki Hannikainen]

Chris,

The btech cables indeed do not provide actual PTT signaling, since the
audio connector on the iPhones and iPads do not have such signal
available. VOX keying does work if it can be adjusted to react fast
enough and if a long-enough txdelay setting is used on the iPhone app
side. I have used it myself, and quite a few others have used it as well.
I haven't used the Baofengs though (I don't have any), I've only tried it
with Kenwood D72 & D74 and chinese Puxing radios.

When setting it up, remember that the audio output level from the iPhone
to the radio is controlled by the iPhone volume control buttons. If it
doesn't key, or transmit anything, you may need to increase the volume.
The iPhone will remember a separate volume setting for the earphone
connector and switch to using and adjusting that whenever something is
plugged in there. The aprs.fi app does not have a separate output level
setting, since it could not bypass the iPhone volume setting - there would
then be two gain controls which would both have to be adjusted to get the
desired sum gain instead of just one control.

The main drawback of VOX is that the transmitter stays on and sends a
carrier for quite some time after the packet data ends. It ties up the
channel and causes interference - many other stations have their squelch
open all the time, and they use a DCD circuit instead to figure when to
transmit. They will happily transmit on top of a carrier, as long as no
data is being sent. The local digipeater may well do this, and digipeat
your freshly-transmitted packet before your transmitter has stopped
keying.

Since the audio output on the connector is stereophonic, one method that
has been suggested before would be to provide the transmitted audio on one
channel, and a tone to trigger PTT on the other channel. A little
electronics circuit (analog VOX circuit without any delays built in) could
then ground the PTT line when a tone is received. A rectifier of the audio
signal, and a transistor, basically.

This does not require any modification to the aprs.fi software, as it
already transmits the modem audio on both left and right channels. The
simple electronics probably would not care if the PTT signal would be a
single tone of X hertz, or an alternating 1200 bit/s data of two tones.

I think someone had drawn a circuit for this already, but I fail to find
the link to it now.

It'd be neat if someone could produce cables with such a circuit
commercially.

On Fri, 24 Sep 2021, Chris Clement wrote:

> I have a related issue and did some probing of the interface I bought. It is summarized in a graphic

> to preserve the table alignment.ptt3.png

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- Hessu

[Chris Clement]

Hessu - I am an old school ham and did packet in the 90's. PTT is an essential part of the technology. I tried to find a way, photosensing off the screen (in the mp4) but even that was frustrated by the long "red" tail. The Btech cable seems to be equally at fault and their documentation is vague. I just assumed that the little module would do the job. My idea (commercial) was just telemetry but on appropriate bands. I was to test viability with my own equipment. Sorry it didn't work out.

Chris - ka4upc

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