Stereo Tool

This may end up being the weirdest and/or dumbest question to hit the forums. I'm at a blockade when it comes to the distortion I'm getting out of the final limiter and clipper stage for FM Output when pre-emphasis is enabled. It's what gives the Optimod in particular its characteristic bright "classic rock" sound where the cymbals and hi-hats can become a sloshy distorted mess if overdone. A set of stations in my area used CRL's digital Millennium processor years ago and never had this issue at competitive loudness levels and did not seem to have very much if any less treble than the competitors.

Doing a bit of research with a very limited amount of info from archived trade magazines, brochures and user manuals... it would appear that Circuit Research Labs had some sort of proprietary distortion-cancelling clippers they called "transparent transient fidelity" ... from their catalogue: "The special design concepts of the pre-emphasis gain control circuits and the low pass clipping filter are selected to minimize the FM limiting degradation of transient fidelity which exists in other pre-emphasis limiting techniques." I'm assuming this is the part of the circuitry that does it. With that said...

Would "Highs" sub-section of Advanced Clipper be what I'm looking for to potentially cut back on that treble distortion? It's especially bad at the 75us pre-emphasis setting. De-essing and then changing to a 50us pre-emphasis with treble boost in the EQ section to regain 75us treble levels works somewhat without making the HD stream unlistenable, but the de-esser dulls some of the transients. Does ST even DO any HF limiting when the pre-emphasis setting is enabled, or is this just the nature of pre-emphasis? I've had mild success turning off pre-emphasis and simply applying a radical EQ curve under the Equalizer section, but this of course interferes with the HD/Web output stream. Is what I'm asking about even possible with ST at all?

Hi,

Is your issue that you don't get enough highs, or that they distort?

If they distort, that's not normal and it sounds like something must be wrong.

For not enough, I recently discovered a simple trick. The issue is that the loudest highs won't get through the clipper due to pre-emphasis, which makes the FM out much duller than the HD out. The trick is to raise the compressor levels for the high frequencies, and reduce the limiters by about the same amount (in fact slightly more). If you do this, the loudest highs won't get louder, but there's much more highs overall, and that gives a very similar effect. What's going into the clipper is actually slightly lower in peak levels, so this doesn't increase the amount of clipping overall.

Will that also in same time make HD brighter too? Ratio is always same.

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Thank you both for your replies. After experimenting around some with your suggestions, I was unfortunately still not able to get 75µs pre-emphasis to stop dulling the highs to a satisfactory level. "Dulling" is a far more fitting term for what I was trying to describe than distortion. The sharpness of fast transients like hi-hat and the snap of a snare drum get entirely lost with 75µs pre-emphasis on.

Whenever I attempted no pre-emphasis and relying entirely on the EQ module to boost treble, I noticed that the amount of dulling effect and transient blurring is marginally reduced whenever I only boost a narrower range of frequencies above 12kHz, rather than applying the standard 75µs EQ curve. So perhaps that's how CRL accomplished it, by using a non-standard pre-emphasis curve that had a steeper filter with frequencies below 10 or 12kHz not boosted at all.

A couple of other settings factored in as well: the speed of the limiter on Hard Limit Output, disabling Simple Clipper, as well as changing from SSBSC to normal DSBSC mode.

So I think my existing workaround involving use of the 50µs pre-emphasis setting along with some extra EQ boost before the clipper, for use with 75µs receivers, is likely what I'll stick with.

Thank you both for your input!

And of course much thanks to Hans for creating an amazingly versatile, indispensable tool for everything related to audio processing.