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M/S and mixing ribbons and condensers - possible phase issues?

I posted recently about a job coming up where I was going to use a condenser mid mic, and a ribbon side mic, and useful piece of information emerged. There was some evidence that as ribbons use a different operational mode there would be a 45 degree phase shift, and this could wreck M/S recordings which use combining channels to produce the stereo channel that can then be adjusted in width after the recording - a useful technique.

The mid channel in the DAW is panned centre, but the side channel is duplicated to two channels, one is panned hard left, and the other panned hard right, and very crucially, has it's polarity inverted. If the two pans were centre, raising the two faders would do nothing - the two channels would cancel each other out, but by panning them, raising the faders introduces the stereo content, right channel components being nulled out of the left, and vice versa.

The idea that the ribbon channel being effectively late could, I thought, interfere with the stereo integration when mixed with the mono mid mic - but I wanted to be sure. I started to do research, and for me that means real research sources, that are validated and have published and verifiable facts, not opinions. Straight away I found mention of the 45 degree phase lag, but it was linked to a mode of velocity operation. More research showed that although many people are familiar with pressure operation and pressure gradient modes of operation, there was also velocity operation. yet more research produced the conclusion that this was a theoretical mode of operation and shared much theory with the difference in pressure mode - the pressure gradient. One paper indicated research by the RCA company in America centred on their pre-WW2 ribbon microphone. I was left with 45, 90 and even 180 degree phase lag being possible. A big problem was the mathematics - My A level Maths was sorely ill equipped for formulii of this complexity, so I fell back on my old tool - experimentation.

I could simply try it, and that is what this video is about. I have also made three clips available, MP3 versions of the mid, side condenser and side ribbon if anyone wishes to experiment. With hindsight, guitar was perhaps not the best source material as the mid channel is mostly sound hole, but the side channel is my left hand, so string noise and fingers, the other side was pretty much empty. You can see this in the screen display, where the stereo field is favouring the right.

I did get a proper result though - I played tones into the three mics from where the guitar was, and then magnified the waveform in the DAW to produce the result. I was going to let you hear this, but it was just ten seconds or so of a tone, so rather boring. 

Doing this video also made me realise that M/S is very useful for acoustic guitar recording. I've never been a lover of two mics on a guitar, but M/S lets you thicken the sound very simply. I'd love to try this on a recording at some point on a nice guitar and nice player.

  • Guitar Condenser (Mid)
  • Guitar Condenser (Side)
  • Guitar Ribbon (Side)
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