Soundboard Pre Mix

[Richard Crowley]

Then they must have some special definition for multi-frequency "phase shift" for audio. It is still a mathematical impossibility no matter how you slice it.

The delay required to produce a "90° phase-shift" at 1 KHz will produce a 180° phase-shift at 2 KHz. and a 360° phase-shift at 4 KHz. But if you could somehow separate the various frequencies and delay each of them for long enough to produce exactly 90° phase-shift (or whatever), you will end up smearing the sound all over the map. We typically try to AVOID doing things like that.

[Benjamin Maas]

Originally Posted by Pete Cofrancesco

It gives a fuller sound with depth that is more pleasing to my ear than one mono mic. Whether it meets the definition of stereo is not of great concern to me.

Actually, it should be of tremendous concern to you. A true stereo recording will mix down to mono much better than this will. One of the things that so many people fail to take into account is the speed of sound. Sound travels slowly and if you're sitting there pulling microphones from two completely different locations, you are going to have a really bad lag between the two channels. When that sums to mono, you are going to end up with phasing and comb filtering- both qualities of sound that any good sound engineer tries very hard to avoid.

Never mind the issues of sync to your video. Sound travels approximately 1ms per foot. That means that if you are 40 feet away and you're shooting 24 frames, you're about a frame out on that microphone. If you are shooting a faster frame rate, you'll be further out of sync in relation to your frame rate.

If you want that to come close to working as a mono recording, you will need to compensate for the time lag in post.

--Ben

[Greg Miller]

Originally Posted by Richard Crowley

Then they must have some special definition for multi-frequency "phase shift" for audio. It is still a mathematical impossibility no matter how you slice it.

I am quite impressed that you know more about mathematics than Ben Bauer and all the other engineers at CBS Labs who were involved in development of the SQ-quadraphonic system.

Originally Posted by Richard Crowley

The delay required to produce a "90° phase-shift" at 1 KHz will produce a 180° phase-shift at 2 KHz. and a 360° phase-shift at 4 KHz.

No argument there. The point is that an "all pass filter" does not use a fixed time delay.

Originally Posted by Richard Crowley

if you could somehow separate the various frequencies and delay each of them for long enough to produce exactly 90° phase-shift (or whatever), you will end up smearing the sound all over the map.

I won't debate that point, either. It certainly does seem that the resulting waveform would be changed significantly, for anything except single-frequency tones. As we both agree, the SQ system did not gain consumer acceptance. Engineering tests showed that it had very limited channel separation, and many reviewers mentioned "artifacts" which might have been a result of such time smearing.

If you read the lengthy reference I posted, you'll have noted that the encoder, and the prototype decoders, had very accurate 10-pole filters (to produce the phase shift). Those apparently sounded acceptable. But many consumer decoders had only 2- or 3-pole filters. Those would produce a very inaccurate phase shift, and undoubtedly made the whole system sound much worse.

Be that as it may, I have never made any claims about SQ's audio quality. I have only stated, and here reaffirm, that it is possible to make a filter which produces a reasonably accurate 90º phase shift across the audio spectrum which by CBS's definition was 20Hz - 20kHz. You denied that it is possible to make such a filter. I stand by my original statement: it is possible. CBS Labs did it, and I built a prototype (based on a schematic from CBS) which I did observe to work as stated. So such a filter is, indeed, possible... whether you or I can explain the math, or not.

[Richard Crowley]

Then we will have to agree to disagree. I am unwilling to accept "I can't explain the math" or "it's patented" or "it's magic". No, I have no respect for the technical competence of the USPTO. You would be amazed at what they have granted patents for.

[Greg Miller]

Originally Posted by Benjamin Maas

Actually, it should be of tremendous concern to you. A true stereo recording will mix down to mono much better than this will. One of the things that so many people fail to take into account is the speed of sound. Sound travels slowly and if you're sitting there pulling microphones from two completely different locations, you are going to have a really bad lag between the two channels. When that sums to mono, you are going to end up with phasing and comb filtering- both qualities of sound that any good sound engineer tries very hard to avoid.

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.

[Greg Miller]

Originally Posted by Richard Crowley

Then we will have to agree to disagree. I am unwilling to accept "I can't explain the math" or "it's patented" or "it's magic". No, I have no respect for the technical competence of the USPTO. You would be amazed at what they have granted patents for.

Richard, we can agree to disagree. I know the filter I described is workable, and have found a few references to it in places like The Audio Cyclopedia, but I don't understand the supporting math (I checked out of math after first year calculus). But I built one, and tested it, and saw it work as predicted. No offense intended, but I give more weight to my own measurements and observations than I give to anyone's blanket statement that something is "impossible." After all, manned flight was once considered impossible. Just because something seems impossible to you does not make it so.

Having said that, I don't know why you keep trying to obfuscate the issue. First you wanted to use "SQ was never successful" to prove that a 90º phase shift filter is not possible. There is no logical connection between the economic success of the SQ system, and the feasibility of this filter (which was only one very small part of the SQ scheme).

Now you are invoking the USPTO to prove that a 90º phase shift filter is impossible. In the first place, I haven't made any claims, one way or the other, about the filter's being patented. Actually, I doubt that the filter, itself, can be patented, any more than a two-way 12dB/octave crossover can be patented. Ohm's Law isn't patented, but we accept it as workable. Thevenin's Theorem isn't patented but we accept it as workable. OTOH, at one point phonographs, motion pictures, and FM radio were patented and we know that they work, too. A given device can work, whether or not it is patented... there is no logical relationship between the two. So whether this particular filter is patented or not, there is no logical relationship between the patent status and the question of whether such a filter can exist and function as predicted. Your continued obfuscation simply makes me feel that you are arguing from a position that can not be logically supported. IMHO that doesn't strengthen your argument; if anything it weakens your credibility. Although you might make a good politician.

At the present time, this seems to be all we really have to go on: I have built one and have seen it work and believe it is possible; you have not built one and have not seen one work and believe it is not possible.

[Brian P. Reynolds]

Wow this subject has gone off in a different direction than the OP must have wanted...

Phase from what I grasp is frequency and there fore time dependant, many years ago in the early days of stereo TV broadcasting we ran a "Howe phase chaser" in the control rooms when we were producing music clip shows, they corrected any head alignment errors that occurred in the VTR machines.
They were slow acting but worked VERY well for fixing small alignment errors but caused problems when used when with MS mics as there often sensed the side components as an error and tried to correct it, with some disastrous on-air audio results. (And hence my personal dislike of MS mics for broadcasting)
Here is a link to the patent for the Phase Chaser. www.google.com/patents/US4890065.pdf

The role of a patent office is NOT to determine the validity or practicality of an idea but purely a place to lodge an idea and to have it documented.

But I have to agree it would be difficult to produce a 90 deg phase shift on ALL frequencies of 20Hz -20k Hz

[Greg Miller]

Brian,

Yes, we have gone off on a tangent. When I first briefly mentioned a 90º constant phase shift, I never could have imagined that someone would argue so vehemently and persistently that such a thing was "an impossibility," and would continue to do so when there is so much literature available online supporting their existence.

You're certainly correct in saying that phase is frequency- and time-dependant... no question about that. That much is intuitively obvious to me.

Yet there are a lot of complex engineering principles that are not intuitively obvious, which certainly are true. For example, I don't understand anything about thermodynamics. I can't prove mathematically that an internal combustion engine works; yet I've driven my car, so I know that it does work. I am certainly not a nuclear scientist and couldn't design an atomic bomb or atomic power plant; yet there is enough evidence to convince me that both of these things exist. Likewise, I can't begin to explain how a constant phase-shift filter works; yet I've built and tested one and I know that such a thing is possible. My point is simply that, although a given person doesn't understand the theory behind some device, it doesn't mean that device can't possibly exist. (Heck, I can't explain mathematically how we are here on the earth, orbiting the sun, which is in the milky way... etc. Yet obviously we are here, despite my limited mathematical comprehension.)

Since this discussion began, I've been reading many articles about constant phase-shift circuits. They are not intuitively obvious to me. I suppose if I had stuck with calculus for a few more years, and had taken some filter design courses, then I might begin to understand how the things actually work. I will readily admit that I don't have that deep of an engineering background. Nevertheless, there are plenty of relevant articles online. These filters seem to find a lot of uses; the most recent one I've read about was demodulating SSB (single sideband) radio transmissions. They are out there.

(I decided, at age 18, that I wanted to have a "hands-on" career. I did not want to spend my life playing with calculus and mathematical theory. But my hat's off to the people who do that for a living, and who are smart enough to design filters with this degree of complexity... filters that I really can't comprehend.)

Yes, Brian, you are clearly correct in saying that "it is difficult to produce a 90 deg phase shift on ALL frequencies of 20Hz - 20kHz." It is difficult, but not impossible. The detailed article I read about the SQ system (of which the phase shift is only one part) says that the encoders, and prototype decoders, needed 10-pole filters, with unusually precise components, to produce a phase shift which was accurate to 1º over that frequency range. A 10-pole filter is a very complex circuit, compared to most typical audio filters, crossover networks, and filters that I've worked with. Indeed, it is difficult.

I'll freely admit that I wouldn't have even thought of such a thing, if I hadn't read the article about SQ, and I certainly couldn't have built one if someone hadn't given me a detailed schematic. Yet here we are on the earth, orbiting the sun, flying through space... with cars, nuclear power, and constant phase-shift filters.

Thanks for your interest and input.

[Steve House]

Originally Posted by Tom Morrow

Desperation audio is a pretty strong term for something that's more reliable than the alternatives, but I would agree this is more for documentary type work than music quality.

Nothing can kill a documentary, or any other type of film, faster than crappy, amateurish sound. Just because the equipment availability has been democratized doesn't mean that the standards the finished product should adhere to have come down - if anything, the fact that everyone today has grown up listening to "that Hollywood Sound" means the standards are higher today that they ever were in previous generations. I don't care if you're recorder costs $500, $5,000, or $25,000, your finished sound track must adhere to the highest level of quality that is possible to deliver or you're not doing your job as a filmmaker properly. The only time it is acceptable to compromise on quality is when it's a situation so desperate that it becomes a choice of settling for second best or coming home with nothing at all.

Rather than micing a speaker, you need to bite the bullet and do whatever it takes to mic the stage properly for your recording mix. Don't have a second mixer? Buy one. Don't have someone to operate it? Hire one. Can't afford to do it right:? Don't do it at all and spare the world from the presence of yet another POC production.

[Eric Olson]

Originally Posted by Steve House

Don't have a second mixer? Buy one. Don't have someone to operate it? Hire one. Can't afford to do it right:? Don't do it at all and spare the world from the presence of yet another POC production.

There is nothing done so well that it couldn't be done better. Not having an additional sound person for recording the band does not prevent most videographers from covering a wedding reception. The goal is to get the best sound possible given the constrains imposed on the shoot.

The original post considers what a single shooter using a single video camera can do to improve sound without going dual system or bringing so much equipment that it takes a crew to move it. Having said this, an inexpensive improvement could be made using reasonable recording techniques along with something like a $500 Zoom R24 which can record 8 simultaneous 48khz wav files through XLR using 6 AA batteries.

Zoom — R24

[Mark Ahrens]

Thanks Eric,
That could be the best response to my original query.
So, $500 for the unit, another $180 for splitters and patch cords.
And probably an additional 3-4 hours of post for mixing and it should be the best possible solution.

A Couple questions though . . .
Say the performance is:
2 Vocalists (using hand mics & headsets for different songs)
Keyboard (piano + synth)
Drums
Bass
Brass (3pcs)

-do i request a submix from the board combining the different mics for each vocalist?
-same for drums
-same for keyboards

I guess all XLR cables aren't made the same . . . what should i look out for?
I don't specialize in this niche . . . but this is getting close to a solution. I'll have to raise my rates for this setup.

Thanks again.

[Eric Olson]

Originally Posted by Mark Ahrens

So, $500 for the unit, another $180 for splitters and patch cords.

Consider also the trick of using channel inserts as recording outputs:

http://www.hosatech.com/product/0/DOC-106/_/

Amazon.com: Hosa DOC106 1/4 TRS (M) to 1/4 TS (F) Direct Out Insert Cable 6 Inch: Musical Instruments

 

Amazon.com: Hosa DOC106 1/4 TRS (M) to 1/4 TS (F) Direct Out Insert Cable 6 Inch: Musical Instruments

This will look less suspicious than a bunch of Y-cables and then you don't need to worry about isolating yourself from the phantom power. I would also suggest using one well placed stereo microphone to make sure the brass players get recorded and as a backup. What's possible in terms of sub-mixes depends on the exact mixing desk and level of cooperation.

If you have a notebook computer and don't mind bringing it along, you may prefer using a USB interface such as the Tascam US-1800 for multi-track sound recording.

Product: US-1800 | TASCAM

[Steve House]

Originally Posted by Mark Ahrens

Thanks Eric,
That could be the best response to my original query.
So, $500 for the unit, another $180 for splitters and patch cords.
And probably an additional 3-4 hours of post for mixing and it should be the best possible solution.

A Couple questions though . . .
Say the performance is:
2 Vocalists (using hand mics & headsets for different songs)
Keyboard (piano + synth)
Drums
Bass
Brass (3pcs)

-do i request a submix from the board combining the different mics for each vocalist?
-same for drums
-same for keyboards

I guess all XLR cables aren't made the same . . . what should i look out for?
I don't specialize in this niche . . . but this is getting close to a solution. I'll have to raise my rates for this setup.

Thanks again.

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.

As been said before, mic'ing a group for recording and mic'ing them for a stage performance are two very different things

[Mark Ahrens]

Dead End, for most budgets i'm associated with.

[Eric Olson]

Originally Posted by Steve House

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.

I've used an overhead boundary microphone with success. Rumor has it that some drummers will allow you to gaffer-tape a boundary microphone to their chest, but I don't know about that. Anyway, without a crew it needs to be simple and not require too much time to set up.

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