"Radium" wrote in message
oups.com...

Why not carry the luminance-signal on FM and the audio-signal on AM?

See the previous post; it's basically a matter of the enormous bandwidth
requirements of FM (note that the standard "deviation" in FM *audio*
broadcast - a measure of how "wide" the overall signal will be - is +/-
75 kHz from the nominal carrier frequency, for a 15 kHz audio bandwidth.
The relationship between the transmitted signal bandwidth and the original
signal bandwidth in FM is not a simple one, but let's just leave it at the
point of noting that video signals are bandwidth hogs, and TV doesn't
even use regular-old-AM as a result of that. (The luminance signal is
actually sent via "vestigal sideband AM," one step removed from full
suppressed-carrier SSB.)

The audio is FM both to avoid the problems of interference bothering the
sound (just as in FM radio), AND to minimize the effects of the
video portions of the signal possibly interfering with the audio. A TV
channel, though, has relatively lots of room for audio.

Nonsense. The choices of AM and FM within the original analog
standard definitions were made for some very, very good reasons.
Digital television is a completely different beast, and is presently
broadcast using two very different modulation schemes - the
U.S. standard (ATSC) using 8-VSB, while the rest of the world
(mostly) will be using COFDM under the DVB-T standard.

Couldn't FSK [the digital equivalent of FM] be used for luminance [Y]
signal of the digital video?

FSK isn't exactly "the digital equivalent of FM" in the first place, and
the short form answer is no. Digital video is carried in a completely
difference manner, and there isn't exactly a readily-separable
luminance "signal" as such in the transmitted signal, at least not as
something you could identify on a scope as in analog TV - it's all just
bits, and it's all packetized.

Bob M.