FM Broadcast Pre-emphasis ?

[Gregory]

Hello guys,

I'm designing a little Stereo FM baseband processor and I have some doubts about it.

In FM broadcast a pre-emphasis filter is used before the frequency modulator, to improve the SNR. From a mathematical perspective, with the pre-emphasis filter, the modulator becomes a hybrid FM/PM modulator. (FM for lower frequencies and PM for higher frequencies).

This makes sense and seems right for mono signals.

For stereo signals, we need to DSB modulate the stereo image L - R with a 19khz/38khz sub-carrier.

I'm seen a lot of block diagrams on the internet where the pre-emphasis is applied in the L and R signals at the input of this scheme, before the stereo processing.

But this does not make sense for me.

It sounds much more right to apply the pre-emphasis after all the processing, just before the FM modulator.

This would guarantee the hybrid FM/PM for all the 53khz baseband stereo signal and would not need two pre-emphasis filters.
Also, for the mono part of the signal (0 - 15khz) it would work exactly equal a mono only modulator.

[szoftveres]

Successful stereo demultiplexing and good separation is only achievable if the group delay is constant, which is the case if there's no pre- and de-emphasis applied on the MPX signal, or the pre-and de-emphasis cause the exact same phase shift. Any slight difference between the phase shift of the pre-and de-emphasis would ruin the separation, had the phase shift was introduced before the stereo decoding in the receiver. FM stereo was conceived in the early 1960s in the days of vacuum tubes, cheap carbon-composition resistors and paper capacitors.
Had the pre-emphasis been applied on the FM MPX signal, the de-emphasis RC network would need to be made of ultra precision components and the radios would have been much more expensive to manufacture. Hence it was decided that pre- and de-emphasis would only be applied on the separated audio channels, where the poor quality and cheap RC filters didn't cause any harm.