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Author Topic: At concert, substituting SB for M mic in M-S setup  (Read 797 times)

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Offline thatjackelliott

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At concert, substituting SB for M mic in M-S setup
« on: July 03, 2017, 10:59:24 AM »
Hi, we are a community radio station and we broadcast four summer music festivals each year. I'm going to be using an M-S setup at our next festival (we need full mono-compatibility due to a transmitter issue that forces the FM broadcast to be mono).

With M-S micing, the normal configuration is to put that M mic as close as possible to the figure-8 S mic. But it occurs to me that the sound board feed from the PA, being mono, could be substituted for the M and might offer some increased clarity, if it could be time-aligned to match with what the M mic would "hear." I already use a delay inline with the board feed so that from our listening and broadcasting location, the feed from the board hits our earphones at roughly the same time that the acoustic version arrives at our booth. This dramatically increases listening clarity at our location, and eliminates the 50 to 70 ms "echo" that listeners hear when we open our announcer mics 50 or 70 feet away from the stage.

So. The delay is adjustable in 1ms step, which is not nearly enough resolution to dial in the delay. Being off by 1ms would be the equivalent of having the M mic as much as 1.1 foot forward or behind the S mic. So I reckon I'll have to fine-tune things by moving the mic stand a little bit at a time to align the M mic's signal with the SB feed. I read about a cute trick where you invert the polarity of one of the two feeds (SB or M mic) and sum them and adjust the mic distance to obtain as much of a null as possible.

Does this sound crazy? I can just see the mic stand waving about if someone touches it and we get some kind of weird phase stuff going on when the S mics move but the M feed from the SB doesn't. The festival is this coming Friday, so talk me down if what I'm thinking is nuts.

Offline thatjackelliott

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Re: At concert, substituting SB for M mic in M-S setup
« Reply #1 on: July 08, 2017, 01:23:44 PM »
Update: it took several days to get this post approved, me being new here and all that. The festival started last night (Friday) and goes until Sunday. I had a chance to listen to the recordings I made last night. Viewing the waveforms (I captured the M and S mics, and the sound board feed separately) and can see that I need to remove about 15ms of delay from the SB's feed to time-align it with what the M mic is hearing. This is likely due to built-in latency in the digital SB.

I used an omni for the M mic, and that was a mistake: it picked up the bass rumble of nearby generators, trucks, other stages the next block over, and general low-end hubub of a city. I didn't use much of that mic in the mix.

I'm setting up a card for today's broadcast and recording.

Because the station is limited to broadcasting in mono only, whatever micing I use has to be compatible with a mono mixdown. The sound board feed is pure mono (the sound board guy cares about the PA, not the recording) so adding in some S mic (figure-8, matrixed) gives a nice spaciousness to the sound. If I turn off the S mic, the soundfield collapses into plain mono. The M-S micing also picks up crowd noise, applause, cheering, which the onstage mics don't. So I think this is going to work out real good.
« Last Edit: July 10, 2017, 01:24:15 PM by thatjackelliott »

Offline Gutbucket

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Re: At concert, substituting SB for M mic in M-S setup
« Reply #2 on: July 10, 2017, 10:45:00 AM »
Just found your thread.  Good to hear it's working well for you.

As you've found, it's probably best to use a cardioid or supercardioid Mid microphone from the soundboard location rather than an omni, unless you are only using it to wet the soundboard PA feed with some audience and room sound.  Otherwise an omni Mid by itself is likely to sound overly distant and reverberant as the mono FM broadcast.   

If using the mono soundboard PA feed as your Mid, you may want to mix a touch of audience microphone in with that so the mono FM broadcast isn't completely dry.  You won't want much audience mixed into the mono broadcast, but just a touch will make it sound much more natural, and also help cover the rough edges of a soundboard mix intended for PA rather broadcast.  This is to say, mix a touch of Mid mic from the soundboard location in with the mono PA feed, as long as the timing is close and you hear no comb-filtering problems.

That's the critical mono part, now on to stereo-  If you get the timing close enough, your fig-8 Side mic at the soundboard should work well to stereo-ize things.  You have a figure-8 available to do that so you're good, but actually any mic can work as a Side channel to provide a sense of stereo breadth and openness if not actual left/right imaging.  The main thing is to listen and see if it sounds acceptable in both mono and stereo.  I've used a single omni or rear-facing cardioid as a Side channel when I had problems with my primary Left/Right mic pair before and that worked out quite well.  Likewise a pair of mics can serve as Side.  Flip polarity on one and sum them and you basically get the bi-directional component of the pair as Side channel.
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