Peak Limiting

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K9ACT:
I am beginning to think that I have a fundamental misunderstanding about peak limiting, especially negative peak limiting.

I set up my Volumax so that whenever the needle enters the red area, the bottom of a sine wave or a recording of my voice starts to clip.

If I feed this into the speech amp and put out RF, I can see this limiting at the base line.  It appears as a bulge, more precisely, like a breast with the nipple facing the previous wave.  As I increase the gain toward 100% the nipple end separates from the previous wave and looks just like overmodulation just as the top reaches the 100% line.

The net result is no different from bypassing the Volumax.

I am usuing the 811 rig for testing and there is plenty of audio for over 100% modulation.

It seems to me that, as long as negative peak limiting is done in front of the modulator, no matter what the audio input looks like going into the speech amp, one can still overmodulate with the audio gain control on the speech amp.

Is this correct?  If so, what good is peak limiting if one still has to ride the gain on the speech amp?
Perhaps not ride it but set it to a point where it can not overmodulate but I don't need the Volumax to do that.

js

WBear2GCR:
Jack,

Yes, you can clip the negative peaks and then send a level that is in excess of 100% negative (baseline the transmitter) anyhow.

However, if you put in the 3 diode limiter circuit into the modulator it self, that will never happen... it will still clip the negative peak, but just before you can baseline, and splatter. That's why it works - no splatter.

In theory you should never have to ride the ,odulator gain control - it should be set, fixed and locked down! Then the Volumax should ride the gain preventing overmodulation, in whatever fashion you set it up. There shoudl be a gain control in front of the Volumax, that will set up the "density" of your sound. So, below a certain mic level in, the Volumax is not doing much and your sound will be fairly unprocessed (depending of course on how you set up the Volumax...) then as you crank up the mic gain (before the Volumax) it will begin to compress/limit/ride whatever it does the audio signal. In theory, if you really blast the input to it, you'll get a highly limited/compressed but very "dense" audio out of it...

The other thing, a flattened baseline look that is NOT actually at the baseline due to the negative clipping/limiting in a processor might look a lot like "baselining" the rig! You may have to look closely at the flat part to see if it really reaches the baseline or stops just short! Or put another way, as you hit it harder, the flattened area on the negative side will get wider due to the clipper/limiter working longer on that wave because it is flattening a "higher mountain" making that width greater...

Shooting from the hip...

        _-_-Want Both Two Gooey Chicken Ranch

Ian VK3KRI:
Quote from: K9ACT on March 23, 2008, 07:55:43 PM


Is this correct?  If so, what good is peak limiting if one still has to ride the gain on the speech amp?
Perhaps not ride it but set it to a point where it can not overmodulate but I don't need the Volumax to do that.


As Mr Bear said, once you have set the correct level at modulator it should be left alone.  You should be setting it up so that the carrier doesn't quite dissappear on negative modulation peaks. Being a peak limiter with a relatively fast release time (140ms) , you wont be able to see the effects directly  on a sine wave signal , and probably not on voice either.

The Volumax isnt an automatic level control, it alters the dynamics of the signal, effectively reducing the level of the peaks so that the average level can be higher before you hit 100% modulation.
                                                                                   Ian VK3KRI

                                                                       

W2INR:
Here is an article written by W2ZE on Processing in the Audio section of our handbook. This may help.

Quote

http://amfone.net/ECSound/processing101-1.htm

G

steve_qix:
Negative peak limiting (clipping) ahead of the modulator only "really" works if the modulator is DC coupled, and doesn't have any overshoot or bumps in the frequency response.  Otherwise, the modulator will modify the clipped waveform and you will not achieve the desired result.

Someone in a previous response pointed to the 3 diode negative peak limiter (clipper).  For a transformer-coupled modulator, this is the only way to achieve true negative peak limiting.  The circuit is simple and very effective !

With DC coupled modulators such as PWM, negative peak limiting is performed ahead of the modulator and audio filtering, and the modulator does not alter the waveform. (if everything is designed properly).

Here is the 3-diode circuit:


Regards,

Steve

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