Problem with phase

Nat · October 2007

Hello, we're evaluating the demo version of Electroacoustic Toolbox. We use it in a lab to teach acoustics to musicians.
One of our example involves showing how phase works. To do so, we generate 2 Sines with the Signal generator and pipe it to an XY Plotter. We found out that to have the sines in phase, we need to dial -7 degree in the Signal generator which seems a bit strange as it should be 0. So basically it seems to be offset by 7 degrees. For example, at -180 degrees, the XY Plotter should display a straight diagonal line, but it shows an ellipse instead.
Any ideas ? We're using the builtin audio of the Mac for our tests.
Thank you.

benfaber · October 2007

I suspect that the reason you need a phase correction is the phase error between the two channels of your built-in audio device. On my MacBook Pro, I had to use a phase correction of around 4 degrees (with 1 kHz sinusoids). By measuring the phase response at 1 kHz between the two built-in input channels, using the Dual FFT Analyzer, I found the phase error to be 3.7 degrees.

The best way to perform your demonstration would be to select the same output device in your XY Plotter that you are using in the Signal Generator (i.e. the built-in audio of your Mac). When you do this, the signals do not need to be routed through an actual audio device. Instead, they are routed directly from the Signal Generator to the XY Plotter (although, the signals will continue to be routed to the output device, so you can hear them, if you desire). Then, you will see a straight line in the XY Plotter when you use 0 or 180 degrees of phase in the Signal Generator.

Ben

Nat · October 2007

Thanks for your answer,

I'm not sure what you mean when you say select the same output device. What I did on the Macbook Pro we had was the following : In the signal generator, select Builtin output A for first sine and Builtin output B for second sine, hardpan both sines left and right respectively. In the XY Plotter, I chose as the input source, the Builtin Output, with channel A and channel B. Is that what you mean ? If so, we had a 7 degrees difference by doing it that way.
To add some confusion, I just tested the same thing on my Macbook (non pro) and all is fine.. No offset.

Nat

benfaber · October 2007

Well, it sounds like you are already setting it up the way I suggested. In that case, I don't have an explanation for the 7 degree phase error. Before, I thought that perhaps you were routing the audio via a patch cable from the audio output to the audio input on your Mac. I certainly haven't been able to duplicate the problem when routing signals directly from the Signal Generator to the XY Plotter.

Is there something different about the way things are set up or configured with your MacBook Pro? Are you using version 1.2.1 of the Toolbox?

If you could duplicate the problem on the MacBook, and then tell me how you did it, perhaps I could then duplicate the problem also (and zero in on a solution).

Ben

Nat · October 2007

I will try to replicate the problem and get back to you.

Thanks,

Nat

Problem with phase

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