Large Ensemble Recording Problem

Discussion in 'Ensembles' started by Henrique Fantato, Jan 14, 2018.

  1. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    Yeah I remember we talked about this and I made many tests. A condenser and a ribbon doesn't work in MS with a condenser...
     
  2. paulears

    paulears Well-Known Member

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    Hang on - we're a bit mangled now - velocity sensitive? That's a new one on me. Maybe just British, but we have pressure gradient and pressure operated as the two types of mic, with the mics with rear vents operating in the PG mode, and single entry mics being pressure operated. isn't the velocity of sound fixed, as in 300m/s? Is this a US used term that doesn't;t travel well?

    I've had no troubles with mixing a ribbon with a dynamic or condenser. My M/S preference is actually two condensers, but if I do use a ribbon (as I only have one!!) I'd match it with a condenser normally. I've tried hypers too with some success in sport, and even a short shotgun - for music they're not too nice - the side movement isn't linear. You get all dead centre, but then noises left and right leap out - great for football and sport.
     
  3. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    Velocity of the wave is the speed of sound. The velocity of the particles that move as the wave passes is variable.
     
  4. paulears

    paulears Well-Known Member

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    Sorry - surely this phenomenon applies to every mic? I'm not sure I'm understanding what we're talking about now? Whats the condenser and ribbon problem?
     
  5. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    I think the idea is that their phase responses are fundamentally and irreparably incompatible due to the different ways they respond to sound waves.
     
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  6. Boswell

    Boswell Moderator Distinguished Member

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    Yes, there is a confusing mix of terminologies. Basically, fig-8 ribbons generate an output according to how fast the ribbon is moving through the magnetic field (velocity-sensitive). In the case of an acoustic sinewave, maximum velocity occurs at zero displacement. Most other types of microphone transducer give maximum (peak) output at maximum displacement or pressure. This is in quadrature with the velocity, hence the two types will not decode to L-R correctly when used as an M-S pair.
     
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  7. paulears

    paulears Well-Known Member

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    This is the first time I've heard of this, but your explanation does explain it very well, and I'd just never thought of this at all. Is the phase lag recoverable if the mid channel was delayed through 90 degrees?
     
  8. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    I can't answer that, but it does make me ask: delayed through 90° at what frequency?
     
  9. paulears

    paulears Well-Known Member

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    My thought are that this lag is the same as when two microphones are separated in space - but that's obviously in cm sizes. In the case we're talking about here, the phase error is less than a half cycle, so probably any electronic shifting in the time domain couldn't do this level of micro shifting in time. The difference between a ribbon side and a condenser side appears to be so small that I'm surprised it's critical - especially as physically the diaphragms of the two mics are not time aligned anyway. If the problem is the shift in the time alignment as frequency rises, so it isn't a constant - doesn't this also happen in non-ribbon diaphragms.

    To be honest, my thinking is getting very circular on this - every attempt to understand why the condenser vs ribbon doesn't work also suggests that it shouldn't work in the other examples of M/S. As Yoda said - Confused now, I am.
     
  10. Boswell

    Boswell Moderator Distinguished Member

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    It has nothing to do with time or spatial alignment. This is a phase issue, and it's a constant frequency-independent 90 degree difference. It's easy to see and understand with sinewave sources, less easy with complex waveforms. Whether you see it as a lead or a lag depends only on which face of the ribbon you designate as the front.

    The only type of device that can align the two signals is an all-pass phase shifter. It's one of the reasons I have an Audient Mico pre-amp with a +/-180 degree continuous phase adjustment on the second channel.
     
  11. paulears

    paulears Well-Known Member

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    Does that completely solve the problem and restore the stereo field to what it should be?

    Perhaps a draft question - but is this adjustment only visible in the scope, or can you tune the phase shift by ear? I guess you'd need to have a central tone source, and then look at the X/Y display on the display, or is this too simplistic and easily wrecked by room reflections?

    Learned quite a bit here today, to tuck away for future use. Cheers.

    EDIT
    Just been thinking more - (dangerous). I have a stereo mic - two omni/cardioid/fig-8 capsules one above the other. Made for me as a special from a Chinese supplier I've dealt with for years and I've used this for Blumlein (in the rotten room) and M/S recordings, although most of it's life is spent in X/Y mode. As it derives it's fig-8 from coupling two back to back cardioids together, there must be similar phase errors created by the coupling of the two? Obviously the ribbon is the only practical 'real' fig 8 capable design - has anyone discovered that the large diaphragm condenser different modes do similar things? Clearly, all the omni have little dips in the polar pattern where the patterns interact - is this as destructive to coherence as the ribbon/condenser combination?
     
  12. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    In my experiment with M/S using a condenser and a ribbon, I lived exactly what Boswell is talking about. There was always one side with less energy.
    I was able to tricked it by delaying the mid channel but never were able to make it sound as good as a proper mic choice.
     
  13. Boswell

    Boswell Moderator Distinguished Member

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    It's the physical type of transducer that determines the phase characteristic.

    Multi-pattern condenser microphones are always pressure-sensitive, whichever pattern you select. A condenser set to fig-8 will work just fine in M-S with another condenser set to cardioid.
     
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  14. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

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    Now that I have the voltage peak vs. displacement image clear in my mind it all makes sense. And I was going to ask if an all pass filter was an appropriate treatment.
     
  15. Boswell

    Boswell Moderator Distinguished Member

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    Well, yes, a particular type of all-pass filter that is a phase-shifter. Any filter that does the phase-shifting cannot on its own be perfect, as all physical filters have group delay, which imparts a pure time delay as well as whatever the filter does. The way round this is to use a phase-shifting filter in both the M and the S channels, one set to +45 degrees and the other to -45 degrees.
     
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  16. Henrique Fantato

    Henrique Fantato Active Member

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    My bad, what I meant to say is that I used a piece of the audio that NOBODY was playing to use as a "white noise"
     
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  17. paulears

    paulears Well-Known Member

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    That's really room sound - it isn't white noise - which is that horrible hiss you hear between FM radio stations. I think your use of that word, as with pink noise, do mean very specific things - and the absence of sound for audio 'fingerprints' isn't white noise in any description.
     
  18. pcrecord

    pcrecord Quality recording seeker ! Distinguished Member

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    That's what I thought ! ;)
    There is alot of misuses of terminology already, I didn't want someone to go with that info after a google search pointing here...
     
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