AV News 195 - February 2014
Let's start with a stereo signal like that in Figs 7 and 8. In fig 7, both channels
are 'doing the same thing' ie both rising at the same time and falling at the
same time - a condition known as 'in phase'. If we were to add these 2
channels together, we would get a similarly-shaped waveform, but twice as
big. In Fig 8, the two channels are doing opposite things - one is rising as the
other is falling - they are mirror images of each other. This is 'out of phase.' If
we were to add these two signals together, they would cancel each other out
and we would get silence.
This simple fact is the clue to Alice's problem. She was putting out-of-phase, Fig
8-like, signals onto the two channels of her soundcard. When she recorded in
mono (ie summed the two channels) they cancelled out and she heard nothing.
When she 'accidentally' recorded in stereo, she heard Fig 8. Both channels were
clearly audible. They would be out of phase, and the resulting sound would be a
bit odd and disorienting, but it would sound a lot better than complete silence!
Incidentally, there is a dramatic demonstration/test of phasing on
'Understanding Audio', which saved one of our members a lot of money - but
that's another story.
So, how was Alice managing to get
out-of-phase signals from her single
mic? The answer comes back to
connecting leads and the article in
Issue 194.
She was using a balanced mic
which is intended to be used in
conjunction with a balanced input as in
Fig 2. BUT....She had connected it
directly, via a simple adaptor, to the
stereo minijack of her computer
soundcard as in Fig 9.
As you can see, the positive-going
mic output was feeding the left
channel and the negative going output
was feeding the right. The channels
were out of phase.
The correct connection is as in Fig 5 the same signal is applied to both channel
inputs, and, as we have already seen, it doesn't matter whether you record in
mono or stereo, you get the same result.
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