[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Geosynchronous Satellites



Jon, I started to rebut your message point by point, but because of
its high error density I thought it would be much clearer to just give
the analysis from scratch.

A NTSC video signal is 6 MHz wide, and it needs a SNR of at least 50
dB in this bandwidth to provide studio quality. Transmitting this
signal with SSB-AM implies a RF SNR of 50 dB in 6 MHz, or a C/No
(carrier to noise spectral density ratio) of 50 + 10log10(6MHz) =
117.78 dB/Hz.

Now let's look at FM. Standard analog satellite TV practice is
wideband FM in a 36 MHz transponder bandwidth. The exact RF C/N
required to provide good picture quality varies with the design of the
qdemodulator, but it's around 12 dB. In a 36 MHz bandwidth, the C/No
works out to 12 + 10log10(36MHz) = 87.56 dB/Hz. That's 30 dB less than
SSB-AM!

In other words, if a 10-watt transponder is enough for FM video, then
it would need to run 10 *kilowatts* to provide the same picture
quality with SSB-AM. Not bad for a modulation method that you're
completely ruled out for satellite use, eh?

So where did the truism come from, that FM is bad on amateur
satellites? Simple, actually. It comes from the assumption that hams
will tolerate poor audio SNRs. If you're willing to tolerate a average
SNR of, say 10 dB in a 3 KHz bandwidth, then the average RF power is
indeed less than the 10 dB or so C/N needed to quiet a 15 KHz NBFM
detector. But as soon as you up the required audio SNR, even by a few
dB, FM once again starts to look better. And digital voice (with good
compression, FEC and modulation) is better still. That's why the
cellular networks started with FM and are rapidly going digital.

You might say that hams should just stick with SSB and accept a crappy
10 dB SNR. But that's much easier said than done. Human nature being
what it is, every SSB operator is continually tempted to turn up the
wick to improve the SNR and reduce operator fatigue. Because SSB is
linear, every additional dB of audio SNR requires another dB of RF
power. So on HF you end up with a lot of people running linears to get
"armchair copy" in their ragchews. And the satellites have their power
hogs trying to grab all of the transponder power for themselves.

So it really seems to me that SSB's claimed power efficiency is
largely illusory in practice. We might as well switch to nonlinear
methods such as FM (or better yet, digital voice) that can give us the
high audio SNRs we really want while using much less power than SSB.

Phil
----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home