FM in Mono

[TimmyWA]

Probably a simple question... If you are running an FM station in Mono (no pilot), do you still have to follow the 15khz cut-off? My limited understanding would think that if that 15khz cut-off is to protect the pilot (Stereo Indicator), and you're running in mono, couldn't you conceivably push up the high end to accomodate for the dull-ish sound of being in mono? My guess as to why this would not fly is maybe there are some interferance issues that also come into play at high frequencies. Educate me please?

Thanks!

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[tpt]

You could, but the receivers wouldn't see it.

Since most stations do use stereo, they have a 19 khz. pilot. So the receiver would be rolled off not to let this pilot appear on the main channel audio. Also, in older radios, audio material in the 19 khz region would trigger the stereo detector. Not a problem in most all modern radios, which dig-out the remnants of the 38 khz suppressed subcarrier, and lock to it for stereo.

As a practical matter, most receilvers begin to roll-off much beyond 12 khz anyway.

 

[westlife]

> You could, but the receivers wouldn't see it.
>
> Since most stations do use stereo, they have a 19 khz.
> pilot. So the receiver would be rolled off not to let this
> pilot appear on the main channel audio.

With regard to how they treat the 19 kHz FM Stereo pilot tone, there are basically three different types of receivers:

* Cheap receivers, which don't perform any filtering on the audio. They can get away with this because the 19 kHz pilot is transmitted at a low modulation level (around 8%, I believe) and is easily "drowned out" by today's heavily-processed FM audio. With this kind of receiver, people with exceptionally good hearing who are listening to lightly processed Classical music stations at high volume levels might be able to hear a faint 19 kHz whistle/squeal in the audio. But most listeners in most circumstances won't hear a thing.

* Mid-level receivers, which roll off the audio above 15 kHz in order to filter out the 19 kHz pilot tone. If done poorly, this can make the FM audio sound muddy because of the gradual slope of the filter.

* High-level receivers, which have a sharp notch filter at 19 kHz to cut out the pilot tone, while leaving the rest of the audio spectrum unaffected. Ideally, this kind of receiver would be able to reproduce FM audio -- from Stereo or Mono stations equally -- up to almost 18 kHz. And with this in mind, I have heard of at least one FM broadcast audio processor which cuts off the audio at 17.5 kHz, instead of 15 kHz, in order to open up this extra bit of high end treble on these receivers (as well as on the "cheap" receivers which don't have any pilot filtering). I don't think this is the DSP-X, but it is one of the new DSP-based FM audio processors which was featured in Radio World recently.
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