FM de-emphasis switch, what does it do?
David R. Kuipers
perfect. Not surprisingly, no manual though. The back panel has an "FM
de-emphasis" switch with two settings, 25uS and 75uS. Can anyone tell me
what this does and the rationale for it? I don't see such a switch on
modern tuners.
Similarly, front panel has a switch for "MPX filter", on or off. Any ideas?
TIA................David
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dkui...@door.net (remove _nospam from email addy to send a reply!)
Fatman
USA is not the only one out there! Europe and other countries use other
standards.
Been a long time since I have thought about this, so I don't remember which
is the USA setting.
As far a MPX filter goes, there is a 19khz pilot signal in the fm
transmission, to indicate stereo signal I believe.
This could leak through the circuitry and into the audio.
This was a problem because this 19khz signal would interact with tape decks
(their bias signal) and cause various noises on the recorded tapes. The MPX
filter
filtered out the 19khz signal from the audio in a more "intense" manner.
"David R. Kuipers" <dkui...@door.net_nospam> wrote in message
news:dkuipers-010...@125door197.door.net...
John Asbury
In FM transmission, the process of restoring (after detection) the
amplitude-vs.-frequency characteristics of the signal.
Mark D. Zacharias
mentioned 25uSec was also used in some other countries. I believe that was
50uSec, not 25. The uSec rating describes the rolloff characteristics of the
de-emphasis filter.
Mark Z.
"David R. Kuipers" <dkui...@door.net_nospam> wrote in message
news:dkuipers-010...@125door197.door.net...
Richard D Pierce
Fatman <fat...@my-deja.com> wrote:
>Been a long time since I have thought about this, so I don't remember which
>is the USA setting.
75 us
>As far a MPX filter goes, there is a 19khz pilot signal in the fm
>transmission, to indicate stereo signal I believe.
>This could leak through the circuitry and into the audio.
>This was a problem because this 19khz signal would interact with tape decks
>(their bias signal) and cause various noises on the recorded tapes. The MPX
>filter
>filtered out the 19khz signal from the audio in a more "intense" manner.
Not exactly correct: the MPX filter does what you state, but the
reason for it's includion was for those stations at the time
that were broadcasting their signals with Dolby-B encoding. The
presence of excess high frequency spurious signal would cause
the decoder at the receive end to mistrack.
The bias oscillators of most tape decks is far enough away so
that sort of interference, in and of itself, is a non-issue.
--
| Dick Pierce |
| Professional Audio Development |
| 1-781/826-4953 Voice and FAX |
| DPi...@world.std.com |
Richard D Pierce
David R. Kuipers <dkui...@door.net_nospam> wrote:
>I bought a vintage Kenwood tuner (KT-7300) which sounds great, works
>perfect. Not surprisingly, no manual though. The back panel has an "FM
>de-emphasis" switch with two settings, 25uS and 75uS. Can anyone tell me
>what this does and the rationale for it? I don't see such a switch on
>modern tuners.
In order to reduce the apparent audible noise, the high
frequencies are boosted, or "emphasized" before broadcasting.
The frequency above which the highs are boosted is related to
the time constant settings. At the receiver end, the highs must
be attenuated in a complementary fashion, or "deemphasized." In
doing so, the original frequency response is restored, but any
noise that is picked up along the way is attenuated.
The 25 vs 75 uS swicth determines the corner frequency where the
emphasis/deemphasis takes place, and must be complementary. @5
uS was for the old, now largely abandoned, Dolby-B FM
broadcasting. 75 us is the stdnard to us in the United States.
Engineer
>
> The correct setting is 75uSec. 25uSec was used for Dolby FM. Someone else
> mentioned 25uSec was also used in some other countries. I believe that was
> 50uSec, not 25. The uSec rating describes the rolloff characteristics of the
> de-emphasis filter.
It was 50 uS in the UK, likely still is.
> Mark Z.
>
(snip)
--
Roger Jones, P.Eng.
Thornhill, Ontario,
Canada
E-mail: LandR...@sprint.ca
also: Roger...@ieee.org
David R. Kuipers
.......David
--
Fatman
remember that.
"Richard D Pierce" <DPi...@world.std.com> wrote in message
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Richard D Pierce
>The FM MPX option on receivers was there LONG BEFORE Dolby was out. I do
>remember that.
The MPX option or the MPX filter option? Two different things.
And Dolby was used in FM quite some time ago. The old Luxman
T310 tuner had built in Dolby in the mid-1970's, and people were
doing Dolby FM broadcasts around 1972-73 or so.
Robert Orban
As far as I know, Dolby FM has been adandoned since the late 70s. It was
an interesting idea at the time, but improved transmission limiters (which
allowed more apparent HF response to be preserved through the headroom
limitations of the pre-emphasis/de-emphasis process) doomed it, because
its compatibity with existing 75us tuners was poorer than the results
obtained from the newer technology.
Just to verify earlier posts--in general terms, 75us is used in the
Western Hemisphere and 50us elsewhere in the world (with a few
exceptions). 75 and 50us are the only two FM pre-emphasis standards in
current use.
As an aside, pre-emphasis/de-emphasis is a powerful noise reduction
technique for FM because its noise spectrum is "triangular" ("blue
noise"). The explanation is this: For a given frequency deviation of the
FM carrier, the "modulation index" (which determines the noise advantage
of FM modulation over AM modulation) decreases proportionally to
modulating frequency. (This is also why there is such a severe noise
penalty in FM stereo--the modulation index of the entire stereo
subchannel, representing L-R modulation and located between 23 and 53kHz
in the baseband, is much smaller than the modulation index of frequencies
in the stereo main channel from 0 to 15,000Hz, representing the L+R
signal.)
Since the noise spectrum is so dominated by high frequencies, a simple
6dB/octave rolloff is very effective in removing it.