Audacity Forum

Been using Audacity 1.26 on Win XP Pro SP2 for some time. I use it to capture signal sweeps from audio systems, with the signals later used by DRC for generating digital room correction files. Captures are done using an E-Mu 0404 USB device. One input port is used for the recording mic, the other for the capture of the right / left control sweeps. In order to use DRC the captures must be exported as 44.1KHz, 32 bit floating point (which is how Audacity is set).

I am attaching images showing the Audacity captured control sweep signal only (the speakers were not outputting the signal at this time). Watch what happens when the left channel is exported to WAV.

I know what that is--or what it looks like. That's what happens when you combine two sweeps with a delay or echo between them. The energy adds up and cancels out at a rate deterrmined by the absolute frequency. It's called combing for obvious reasons. I don't know how you got that, except that Windows machines are famous for having sound pathways that only the gods can figure out.

I need to go back and read that again. The first three times didn't form an image of what you're doing.

Koz

Nope. Can't do it. You didn't include enough of the connection information for me to reconstruct the setup.

<<<One input port is used for the recording mic, the other for the capture of the right / left control sweeps.>>>

No matter how many times I read that, I can't figure out where to plug what. Sweeps from where? Is it a three-input mixer?

Koz

Koz, I am using the E-Mu 0404 as a two input device. The control sweep is being fed from my digital source device in my stereo set-up, through my stereo preamp, and from there to the power amp and speakers. The mic (a calibrated Behringer ECM8000) is set up to capture the speaker output; this is fed into one input of the E-Mu. My digital source has two identical sets of line outputs, one goes to my stereo preamp and then to my amp and speakers, as I said a moment ago. I use either the right or left channel of the second set of outputs (one at a time) into the other input of the E-Mu. The reason it is done this way is because the left and right speakers are fed with Left Channel Only and Right Channel Only sweeps. In other words, the speakers are done one at a time with their corresponding control signal fed into the E-Mu via the second set of Line Outs from my digital source.

Here is the link to the E-Mu

http://www.emu.com/products/product.asp ... duct=18388

You can see the volume controls for the inputs as the two knobs below the square button over to the left side of the top panel (it's Item #9 on the Product Tour tab).

Oh, also keep in mind that for this experiment I was only capturing the control signal (input 2 on the E-Mu). I didn't even have the capture mic on input 1 turned on. That's why the top trace on each of the graphs shows nothing. The same thing sometimes happens when I do capture the mic input along with the control signal. Actually, it's worse.

What is the signal that you are recording? What frequency is it?

stevethefiddle wrote:What is the signal that you are recording? What frequency is it?

Steve, it's a sweep at 44.1kHz, at 50% full amplitude, from 5Hz to 22kHz. The sweep lasts 90 seconds. It's in two separate tracks. One is left channel only with right channel mute; the other is the opposite.

I think I see the problem.

In Audacity, in a stereo track, the upper channel is the Left channel and the lower channel is the Right channel.

Looking at your screen shots, both of your stereo tracks have the signal recorded on the right channel and the left channel is muted. When you Export the audio, Audacity is mixing the two tracks together. The two muted left channels produce (near) silence, and the two right channels show that interesting beating of the mixed Right channels. The shape is caused by phase differences between the two right channels which is probably because the two tracks do not start at exactly the same time.

Steve, this may be it, but it would be my own fault of course for not knowing the subtleties of Audacity. I did have the two captures open at the same time (I think), but I was under the impression that when I clicked in the one I wanted (and it highlights) that the Export would then just export that one capture. Not the case?

edit: If this is the case, is there a way for me to take the clean right channel WAV (which I exported originally before capturing the left channel), the combed left channel WAV, and "de-combify" that waveform using the right channel waveform somehow?

I had to lie down for a minute.

<<<I was under the impression that when I clicked in the one I wanted (and it highlights) that the Export would then just export that one capture.>>>

That's the part I can't follow. So far you have a complete textbook example of using a stereo recording system to record a test sweep sample on one side and the test results (the microphone) on the other. I don't get why we're muting certain tracks and exporting partial captures.

I, with these fingers, have done tests like that with similar equipment and simplicity is a really good thing.

Start a fresh Audacity, start the sweep, stop Audacity, Export a stereo WAV file. What is it about that process that would not fulfill the test requirements?

What is the specific signal that the DRC needs?

Koz

@Koz I think what Tony has done is to record one track with the "Right Channel control sweep from his audio system" + "microphone (no signal), and in the same Audacity project recorded a second track with "Left Channel control sweep from his audio system" + "microphone (no signal).

@Tony - is that correct?

When he then Exported from Audacity, the two tracks were mixed together (this is what is supposed to happen when you Export from Audacity) and created the comb effect seen in the third screen shot.

@Tony, what you should have done is to select one of the tracks and then use "Export Selected" rather than "Export".

tony22 wrote:edit: If this is the case, is there a way for me to take the clean right channel WAV (which I exported originally before capturing the left channel), the combed left channel WAV, and "de-combify" that waveform using the right channel waveform somehow?

Yes you could - but if you use "Export Selected" to export one track at a time, then you will not need to.