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A Basic Question or two on CD Emphasis/Deemphasis

Started by Unknown September 16, 2004
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. �����������������������������������������������������������������������