janedoahlovesbrentlee said:
We have an optimod 8100A ( I think? Its the last big blue one before you get into the series with latency issues). We have a CHR with a direct format competitor in market. So, the processing has to be aggressive. The problem is that if the jock isn't watching the levels on the console, you'll hear distortion on the air. Other wise, if they keep it below 0, it sounds very clean, and loud. Our engineer is a RF guy, and not a sound guy, so he's out of ideas. Is there something in the optimod I can change to fix that, or is there a unit I can install inline to correct the problem? A compressor or something to stop the problem before it gets to the optimod?
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The first step in eliminating distortion is isolating it, that is, find out where it is occurring.
Unless you have a broken piece of gear, chances are that the overload is just mis-aligned levels, where one piece of equipment's output is hotter than needed by the following piece, whose input overloads.
If this is the case it's really easy to fix, especially if you have a 'scope (any production or audio guy worth his soap should know how to do this anyway, as a matter of good engineering practice)
The idea is to align the clipping points of all equipment in the complete chain, from console out to processor in, using a test tone. This way they all overload at the same point, and no single piece is the weak link in the chain. You can then be assured that the signal-to-noise has been set to be as good as that particular equipment combination will allow.
You'll have to take the station off the air , or at least subject your listeners to test tones, which my not be a good idea during afternoon drive. ;-) I myself prefer late nights with lots of coffee.
You will need a tone generator, and a means to monitor: either an oscilloscope, a set of powered speakers with bridging inputs or, better yet, both. A calibrated audio meter will come in handy, but you may not need it.
Connect your monitor (speakers, scope, etc.) directly paralleled to the console output, leaving the console's output still connected to the following equipment.
Send a sine wave (1 kHz will do) from the tone generator into a mono console fader at normal operating level. Set the channel balance (if available) so both left and right are matched in level. Monitoring the audio at the console's output, turn up the fader level until you can just barely hear (or see, with a scope) the tone clipping [at this point the console meters may be pegging. Ignore them
]. If it does not clip, send more level from the generator. Just be careful that the generator itself is not clipping.
If you can measure the console output level with a meter, the difference in dB between "operating level" and "clipping level" is your "headroom." It should be, at a bare minimum, 15 dB; 20 or more if you can get it. Headroom is your 'insurance' against casual overload by sloppy board operators.
If you have a calibrated audio meter, you can measure the actual peak level where clipping occurs. In a 'standard' professional analog audio environment, if the overload point is not well over +20 dBm, you may have found your problem.
The exact level where the audio clips will probably be fairly audible, and visible on the waveform of the 'scope if you are using one. Back off the fader level very slightly until it sounds clean again, and leave it there. If the console has clipping indicator LEDs, now would be a good time to calibrate them. If there are master output level controls on the console, set them to the identical output level.
You should observe whether one channel clips at a noticeably different level than the other. This would indicate a problem in the lower-clipping channel, and should be addressed.
Now, hang your monitor/speaker/scope on the NEXT piece's OUTPUT, and (leaving the console levels exactly where they were), reduce or increase that next piece's own INPUT controls until its output is also just below clipping (with some pieces of equipment it may be necessary to do a similar stage-to-stage level adjustment to prevent clipping internally). Once again, be careful to keep both channels at the same level. The idea is to prevent clipping inside that piece of gear, distortion that would be passed onward, while keeping levels as high as possible.
You may note that the clipping levels for the input and outputs of any particular piece of equipment should be matched as closely as you can get them. Do this by initially reducing output level well below its amplifier's clipping point so you can calibrate the input level; once the input levels are calibrated, do the same with the output controls by raising them to just below clipping.
Note that some equipment has amplification -- which may overload-- BEFORE its input level controls. If reducing the input control does not reduce the clipping, you may need to turn down the OUTPUT of the PREVIOUS piece driving that unit.
If your setup sends console output directly to the processor, your job is done. Otherwise, repeat the procedure with every piece of gear all the way through the entire system (leaving the generator sending level to the console input where you originally set it, just below overload), and you will have optimized inter-equipment levels and, hence, signal-to-noise, for the entire chain.
BTW, when you send audio to a dynamic system such as a processor, you may find setting input clipping level to be somewhat tricky, as the processor will counter your increases. Your processor's manual may give you specific advice concerning this. But there should still be an audibly clear point where the audio hits clipping, and you just back down from there, using its input level controls, or the previous equipment's output controls, whichever is appropriate.
I know this procedure sounds lengthy. Hopefully I haven't forgotten anything vital. ;-) But it's pretty simple once you've done it a time or two, and it will give you a peace of mind to know your setup is working at its best.
Kind Regards
David Reaves
