Saturday, September 1, 2012

Dear SNR: Compression and Feedback

It looks like the feedback article from yesterday has sparked some thought in one of our young Jedi-In-Training. Nate DeMare who has contributed here before dropped us these questions via the Facebook page.

So I just read the feedback article and the title reminded me of a conversation I had with the Lighting Professor does sound here at school. During the conversation I discovered we actually have a nice Soundcraft analog board with outboard gear consisting of two channels of compression. When asked about why we're still using the 01v's instead of investing in an EQ and using the Soundcraft, The professor informed me that in the past they've tried it and they need to compress every channel individually to get max volume out. So two questions are  involved, 
1.) Does compressing a mic actually help prevent feedback? 
2.) Can the feedback issues be from the system setup and have nothing to do with the room?
The PA consists of a cluster of 4 speakers hung above the proscenium with monitors put on stage when needed.
Well Nate,  while dynamic processing on each channel allows you to tailor things a bit better, if those channels are going to show back up in a monitor mix then you're more likely to have feedback issues. Compressing adds volume but also raises the noise floor. There's more signal there and also more non-program signal like shoes and such which all come back raised up from the compression.  That's why I hardly ever compress on the channels if I'm also doing the monitor mixing. For rock, a desk with four or eight subgroups, each with a compressor inserted can work wonders. We've discussed it on the blog before so go back and look that up.

If it's a case of getting a bunch of lav mics to be heard, the same bus compression technique can be used, you just have to EQ carefully. Again, there's a theatre post on the topic so go check it out. But in short, inserting a graph across the lav group or groups will let you ruthlessly take out any frequencies that are going to feed back and leave you the channel EQs to shape each mic so the actors sound as natural as possible. Having good lavs and good radios is important too. A cheap element on a radio that compresses the signal a ton on transmit and then expands it on receive (compansion) can do more harm than you can fix sometimes.  But it's important to do that processing just on the lav groups. If you do it on the stereo bus then you're also processing any front mics, instrument inputs and playback with the lav mic settings.

If it came down to it and there was a little money in the budget I'd invest a few hundred in compressors and graphs for each group on the Soundcraft and put those Yamahas to better use as training pieces or door stops. The lack of headroom and sonic degradation you get from those low end digital desks just isn't worth it in my humble opinion. Not that I'm against digital mind you, but like anything you need to listen and decide how your signal is being affected.

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