It is not a sub carrier in the rf spectrum, meaning it can be fully
seen with or without any other carrier (video), but when detected in
the TV receiver it becomes a sub carrier of 4.5 mhz in the TV's IF.
The audio carrier is mixed with the video carrier to produce the 4.5
mhz.
Without the video carrier the TV receiver will not see the 4.5 mhz.

As others have said the sound carrier is usually generated by a
separate transmitter at the sound carrier frequency. But it could also
be generated as a sub carrier of the main video carrier with the same
results. It would still be visible in the RF spectrum just the same as
it is when generated by a separate transmitter. The main carrier would
also have to be present at the same time of course.

However if the TV video signal was an FM modulated signal rather than
AM then the sound sub carrier would not be visible without the main
video carrier if it was generated as a sub carrier.

The same happens with an FM broadcast SCA sub carrier or the stereo
sub carrier on the main signal. The main signal must first be
demodulated in order to demodulate any sub carrier.

73
Gary K4FMX


On Tue, 27 Dec 2005 23:28:16 GMT, John Ferrell
wrote:

I like your answer best, probably because I understand how it works!

On Tue, 27 Dec 2005 13:46:45 -0500, "Fred W4JLE"
wrote:

TV audio is not a sub-carrier. The audio is an independent transmitter from
the aural transmitter.

They may have come up with some other way since I worked at WXEL in the
fifties, but as a fifty's TV set still works today, I suspect the result is
the same.

"John Ferrell" wrote in message
. ..
You guys are testing my gray matter today...
The question: Is it possible to detect an FM subcarrier without
receiving the base carrier?

Perhaps another cup of coffee will help...



John Ferrell W8CCW

"Roy Lewallen" wrote:
With no information other than looking at the composite signal including
video and audio, would you be able to tell if it was generated by two
separate transmitters or by modulation of a single transmitter by the
video and an audio subcarrier? That is, if you could do the modulation
without generation of intermodulation products.
______________

Given all that, then no. But that isn't the reality.

RF

"Gary Schafer" wrote
However if the TV video signal was an FM modulated signal rather than
AM then the sound sub carrier would not be visible without the main
video carrier if it was generated as a sub carrier.
The same happens with an FM broadcast SCA sub carrier or the stereo
sub carrier on the main signal. The main signal must first be
demodulated in order to demodulate any sub carrier.
_____________

The r-f spectra of TV aural signals and FM SCAs are standalone entities.
Each may be fully demodulated by a suitable FM receiver capable of tuning to
that r-f spectrum. No "main" carrier is needed. Given, this isn't the
conventional practice -- however it would be possible to do.

Likewise, the L-R audio information of an FM stereo transmission is a
standalone entity, and given a suitable r-f receiver and detector, it can be
fully demodulated back to L-R without needing the main carrier. Not
conventional or cost-effective, but possible. Of course, the L+R waveform
conveyed by the main carrier is necessary to matrix with L-R to produce the
L&R channels needed for stereo reproduction by the receiver system.

RF

On Tue, 27 Dec 2005 20:53:54 -0600, "Richard Fry"
wrote:

"Gary Schafer" wrote
However if the TV video signal was an FM modulated signal rather than
AM then the sound sub carrier would not be visible without the main
video carrier if it was generated as a sub carrier.
The same happens with an FM broadcast SCA sub carrier or the stereo
sub carrier on the main signal. The main signal must first be
demodulated in order to demodulate any sub carrier.
_____________

The r-f spectra of TV aural signals and FM SCAs are standalone entities.
Each may be fully demodulated by a suitable FM receiver capable of tuning to
that r-f spectrum. No "main" carrier is needed. Given, this isn't the
conventional practice -- however it would be possible to do.



It may be "visible" but it is not a standalone signal like a TV aural
signal is. With an SCA signal there is more than one side band on the
main carrier involved. With TV aural there is not.

73
Gary K4FMX