Soundboard Pre Mix

[Benjamin Maas]

Originally Posted by Greg Miller

Ben, everything you say there is entirely true. Mixing a close mono mic and a distant mono mic could give you some rather strange results. That is probably less of an issue if the "reverb" mic is mixed in at a relatively low level. For example if the level from the "reverb" mic is 1/10 the level (-20dB) of the "close" mic, then the comb filtering would not go to zero, it would go only to 90% (-.915dB). That doesn't make it a good idea, but if used very judiciously it would be less of a problem.

However, I notice that the scenario suggested above by Eric Olsen is a special case. He suggests mixing the "reverb" mic as if it were the "side" mic in an M/S setup. Let's look at the math. We'll call the mic close to the stage the M mic, and the distant (reverb) mic the S mic.

So if he mixes the two together in a typical M/S matrix,

L = M + S
R = M - S

Now if you further mix those together equally, to get a mono signal, you get:

Mono = (L) + (R) = (M + S) + (M - S) = 2M. In other words, in the mono mix the "reverb" mic, which we're calling the "side" mic or "S" in this equation, disappears completely. In this special case only you end up with complete mono compatibility, although with no reverb mixed in... just a close-up mono recording from the mic closest to the stage.

Be that as it may, I would not choose to do this, for three reasons. First, you're taking pressure-related signal from the "reverb" mic and putting it into the two stereo channels completely out of phase. If the level is at all significant, it could result in that "hollow" or "sound inside one's head" effect. Second, if the "reverb" level is at all significant, you will have some comb filtering. And finally, as mentioned above, the reverb will completely drop out of the mono mix.

IMHO if I wanted a close mono mic, with additional ambience mixed in, I'd use a stereo ambience mic, mixed L/R like a normal stereo source, and mix the close mic signal to the center, after delaying it so it is coincident with the ambience mic. Not ideal, but perhaps workable. Just my opinion... deposit 2¢ please.

Oh boy... where to even begin. I guess I'll leave it at this because there is so much wrong with this I don't know where to start. As an audio engineer, this really makes me cringe.

The problem with this whole line of thought is that this is not mid-side. Mid Side is a very useful stereo microphone technique, especially when it comes to mono compatability. A ambient mic does not make a side mic. the whole point of that mic technique is sum and difference. The middle mic has everything that a figure eight mic (as the side) does not have. To take a sum and a difference of those two mics makes a stereo image. Two individual microphones in different locations will never make mid and side.

I don't mean to be blunt or rude here, but I do need to try to clarify a correct usage of a technique. Two mics as originally described is mono only. It cannot be stereo.

--Ben

[Greg Miller]

Ben:

I think you and I are actually in agreement. In my next to last paragraph (as quoted above) you'll note that I say I would not do what the OP suggests (namely using a mono mic -- presumably a pressure mic -- deep in the house, then mixing it in as if it were a "side" mic). And I give my reasons for disagreeing with that proposed technique.

Indeed, I think the only time the M/S matrix really produces true stereo results is with a coincident pair of proper M/S mics (i.e. the Side mic is figure-8 facing sideways). That is not what was proposed by the OP so the results will be entirely artificial and technically prone to numerous problems.

--

But I did find it an interesting sidelight that, if one followed the OPs proposed setup, the "reverb" mic would be added to the left channel, and subtracted from the right channel; therefore it would entirely drop out of the mono mix. (That's what I was trying to illustrate with the math.) So this particular proposed means of getting a bogus stereo mix, would in fact produce a clean mono mix, consisting of only the front mic.

--

Be that as it may, I agree with you, the proposed mix is not the best way to go about things.

[Eric Olson]

Originally Posted by Benjamin Maas

I don't mean to be blunt or rude here, but I do need to try to clarify a correct usage of a technique. Two mics as originally described is mono only. It cannot be stereo.

No offense, however, I was not suggesting that mixing the ambient mic and monaural stage mike as M/S was correct usage, only that this might be better than mixing it as an X/Y pair. One way to create pseudo stereo is by adding a delayed signal to the left channel and subtracting it from the right. This is called the complimentary comb filter technique and similar in spirit to mixing the ambient mic as if it were M/S.

[Greg Miller]

Originally Posted by Eric Olson

One way to create pseudo stereo is by adding a delayed signal to the left channel and subtracting it from the right. This is called the complimentary comb filter technique and similar in spirit to mixing the ambient mic as if it were M/S.

Yes, exactly. That is a known technique. If you cannot record more than two channels that are perfectly locked in time, and if you keep the level of the "reverb" mic relatively low, compared to the "stage" mic, you will get similar results: it will have some apparent stereo spread, although it won't be true stereo.

Of course it would be much better if you had a recorder with at least three tracks, so you could put a stereo mic out in the house. Better yet if you could then delay the stage mic to be coincident in time with the house mic.

[Paul R Johnson]

Going back a tad to this..

Your biggest headache is going to be the drums. In many venues drums won't be mic'ed at all and their only presence in the mix for your recording would be through bleed into other performers mics. You're going to need a full set of mic's for the kit, 5 to 7 or even more mics just for the drum kit, each going to their own input on your mixer.

Electric guitars and basses might not be mic'ed either, depending on their cabinets to provide enough volume for the audience. DI boxes to split their signals to your recording mix might be necessary for them.

If the live sound people have not miked up the kit with individual mics, it's because there was no need - if this is the case, then for a recording there is no need to individually mic the kit - unless maybe it's a heavy rock sound, and if that was the case, then the kit would be miked for the audience too!

Boundary mics above work rather like an omni, or very, very wide cardioid - they cannot work as a boundary mic unless they're on a large surface - other than that, they're not boundaries. Sure - you can try clever placement on the drummer, but why not, if you really must mic the drums, just use an overhead, or an overhead with snare, or the verity common kick, snare overhead system. To complicated for a casual video without real sound people to work it!

[Steve House]

Originally Posted by Paul R Johnson

Going back a tad to this..

If the live sound people have not miked up the kit with individual mics, it's because there was no need - if this is the case, then for a recording there is no need to individually mic the kit - unless maybe it's a heavy rock sound, and if that was the case, then the kit would be miked for the audience too!

...!

There's no need because the audience can hear the drums directly, true enough. But the RECORDING has no direct sound - if the kit isn't picked up by a mic somewhere it doesn't exist in the recording. Close mic'ing some instruments while relying on bleed or a house mic to capture others is asking for trouble. If you're recording the entire ensemble with a well-placed stereo mic then you are correct ... you're picking up whast the audience is hearing. But if you want high-quality concert footage you're going to have to mic up the group as if you were in the studio laying down a track destined for CD

[Paul R Johnson]

That is NOT what I'm saying. I'm merely pointing out there is no absolute requirement to stick a mic on every drum. The style of music dictates the drum mic technique you use. Individually eq'd and processed rock drums with perhaps sampled sound slavered over need isolation, whereas many other drums styles just need a more modest amount of mics to sound good. I'm quite aware that there won't be drums or other instruments in the audience mix, so you need to replace them - but a 3 mic approach works really well for most styles, and for big band, just one or two is quite sufficient. The reality is that most video people do not have sufficient facilities to hand to manage multi-mic audio mixing themselves. As it happens, I do - and can record to hard drive 24 channels, or 16-32 on a computer based system. There is no 'standard' solution, it's worked on fluidly to get the best results for each project. My point is simply that you don't need to mic up every single thing just because you can.