Re: DAB sounds like decent AM

On Thu, 04 May 2006 19:14:28 GMT, "DAB sounds worse than FM"
<dab.is@xxxxxxxxxxxxxx> wrote:

Don Pearce wrote:
On Thu, 04 May 2006 18:48:23 GMT, "DAB sounds worse than FM"
<dab.is@xxxxxxxxxxxxxx> wrote:

Don Pearce wrote:
On Thu, 04 May 2006 18:08:45 GMT, "DAB sounds worse than FM"
<dab.is@xxxxxxxxxxxxxx> wrote:

Hang on - half of FM is actually AM (The L-R bit, that is). Now
I'm confused. Which does it sound like again?


The L-R is not AM at all. FM has a massive SNR advantage over AM,
so irrespective of the fact that the L-R bit is upconverted using a
kind of AM modulation it still cannot be described as being an AM
transmission.

No, not a "kind of" AM, actual AM.


I meant "a kind of" as in "a type of", i.e. envelope AM and
double-sideband suppressed carrier AM.

OK - agreed. It is, I'm sure you will agree, a trickier kind of AM to
deal with than the usual envelope kind.


It may be trickier, but double-sideband suppressed carrier also has an SNR
advantage over envelope AM. But anyway, I fundamentally disagree with
referring to it as an AM signal, because it's part of an FM multiplex.

Whether you like it or not, it is AM.


In fact it is even harder than the
usual kind, in that the carrier is suppressed, and has to be
re-created from the 19kHz pilot tone before you can even think about
demodulating it.


But the L-R channel is still FM modulated, so it still has the 20 dB
or so SNR advantage over normal AM transmissions. So I think it's
totally incorrect to refer to it as being AM, because it is the
outer modulation that matters, and the AM is only used for
up/downconversion purposes.

The outer modulation is just a kind of wrapper.


As an example, say you had nothing on the R channel, so L-R = L, and you
upconverted this L channel to 38 kHz as usual, FM modulated it and
transmitted it. Then you transmitted this L channel using AM with equal
power transmitters, the FM transmission would sound good when the AM
transmission would probably sound crap because of FM's massive SNR
advantage.

So?


Once the signal has
been through the FM demodulator, what you are left with,


I know.


besides
baseband L+R is an absolutely bog standard suppressed carrier AM
signal, which must be recovered by an AM demodulator just like in any
AM radio.


No, it depends on the receiver architecture. It doesn't have to use an
envelope detector.

Did I say it was an envelope detector? When you are recreating a pure
carrier, you can do much better with a proper synchronous detector.

d

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Pearce Consulting
http://www.pearce.uk.com
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