A Basic Question or two on CD Emphasis/Deemphasis

On Mon, 20 Sep 2004 19:06:08 GMT, eric.jacobsen@ieee.org (Eric
Jacobsen) wrote:

>The idea is that if FM modulation is used and one
>wishes to keep the transmitted spectrum relatively flat (which is a
>good thing to do to maximize SNR in the receiver), then some sort of
>pre-emphasis is needed if the input spectrum isn't flat.   I think
>that'll be true for any transmissions system with linear modulation.

Don't forget the "triangular noise" spectrum for FM.  If we assume
that the IF noise is flat (a reasonable assumption in the power
limited case) then the demodulated noise will increase 20dB/decade
with the baseband frequency.

This makes preemphasis much more important for FM that most other
communications channels.  Not only does the programme material have
less power at higher frequencies, the noise floor increases too.

Here's my derivation:
http://groups.google.com/groups?selm=37733912.21210660%40newshost

Regards,
Allan

Allan Herriman wrote:

> On Mon, 20 Sep 2004 19:06:08 GMT, eric.jacobsen@ieee.org (Eric
> Jacobsen) wrote:
> 
> 
>>The idea is that if FM modulation is used and one
>>wishes to keep the transmitted spectrum relatively flat (which is a
>>good thing to do to maximize SNR in the receiver), then some sort of
>>pre-emphasis is needed if the input spectrum isn't flat.   I think
>>that'll be true for any transmissions system with linear modulation.
> 
> 
> Don't forget the "triangular noise" spectrum for FM.  If we assume
> that the IF noise is flat (a reasonable assumption in the power
> limited case) then the demodulated noise will increase 20dB/decade
> with the baseband frequency.
> 
> This makes preemphasis much more important for FM that most other
> communications channels.  Not only does the programme material have
> less power at higher frequencies, the noise floor increases too.
> 
> Here's my derivation:
> http://groups.google.com/groups?selm=37733912.21210660%40newshost
> 
> Regards,
> Allan

There's yet another way to look at FM preemphasis. PM has a flat noise
spectrum but the detector is very insensitive to low frequencies. FM
with high-frequency preemphasis is indistinguishable from PM with
low-frequency pre-emphasis. The signal is FM below the preemphasis
turnover and PM above.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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