Radium,

Some television is actually transmitted using FM modulation schemes, but not
for commercial broadcasting. Examples of FM TV are ham/amateur ATV/SSTV,
unlicensed 2.4 GHz surveillance links, and a number of point to point and
studio to transmitter systems. FM signaling provides benefits in noise
immunity and also permits better channel re-use, since it exploits one of
the best features of FM called "the FM capture effect", wherein nearly equal
strength signals which would otherwise interfere in AM systems will cause an
FM receiver to "capture" only the stronger signal and ignore the weaker,
even if differences of 1 dB of signal strength exists.

There is no specific answer to what you would see as far as video patterns,
and there is no reason whatsoever to expect to see sawtooth waveforms in
particular. The demodulated signal from FM will conform to the spectral
changes just as the demodulated signal from an AM detector would conform to
amplitude changes, and random noise would be considered "snow" in either
case. Unless a transmitted signal with a frequency ramp (sometimes called "a
chirp") is present, the video would have no sawtooth. A Doppler radar, for
example, could generate such a waveform, since some radars create chirped /
swept signals. The video scan rate(s) would additionally need to be in the
range of the chirp rate to create the appearance of a sawtooth.

FM disturbances in the SHF band are likely to be man-made and not
atmospheric, and thus only "viewable" if the "FM Video Receiver" you
envision had a demodulator / discriminator whose bandwidth was tailored to a
specific transmitted waveform, and even then only if sweep rates were
suitable. Absent a man-made transmitter, the SHF environment is mostly
thermal noise (both circuit and atmospheric) and only a radio telescope or
other enormous aperture / antenna will see beyond the atmosphere.

The choice of using AM versus FM is really way more complicated than "AM for
audio" or "FM for digital video". When designing communication systems of
any type, the engineer is faced with balancing many issues, and the channel,
media, noise environment, interference sources, power budget, multipath,
complexity, and cost are only a few of the considerations involved. A highly
reliable cable modem to transmit fast digital content may indeed by phase
modulated with an amplitude trellis; a secure and interference resistant
link may use spread-spectrum frequency hopping AM for digital signaling; and
FM winds up being used heavily in many voice communication systems mostly
because the capture effect reduces co-channel interference.

The closest I can suggest to what you might enjoy exploring would be a
satellite dish and receiver designed for L band which will see and decode
some broadcasting which is unprotected / unencrypted. It gets you into the
range of SHF, has true TV signaling for public viewing, and is a hobbyist
activity with others involved.

Smarty






"Radium" wrote in message
ups.com...
Hi:

Video signals for NTSC, PAL, and SECAM television are transmitted on
AM carriers.

My question is, let's say I have a television set that is capable of
receiving and demodulating FM video carrier waves. What would I see on
the TV? I am aware that no company uses FM video. Would I see sawtooth-
like patterns on the screen due to frequency-modulated electric fields
present in the environment?

I'd really like buy a TV with a FM-video receiver; I want to find out
what FM-video disturbances in the SHF [Super High Frequency ]
frequency-range look like. I am sick n' tired of AM video.

AM should be used for analog audio. FM should be used for digital
video.


Regards,

Radium