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Calibrating azimuth on VCR: Can any software report or visualize phase
SuspiciousLizard
post Jul 27 2012, 00:06
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I'm capturing VHS home movies on a VCR with stereo linear audio heads. The tapes I'm currently working with have only a mono track, but I get occasional distracting hitches in the audio that are only present in the right/left captured channel and not the other, so I need to collapse the audio into a single mono channel in software to fix them. (Well, I have to normalize the channels to equal volume first, since their relative volume depends on how the tape was recorded.) This means I have to worry about sound cancellation from phase differences. I'll probably end up correcting the audio phase in software anyway with Stereo Tool, but I'd like to start out with a properly aligned VCR at least.

As a result, I'm now trying to calibrate the azimuth for my VCR's linear audio heads, and it's a huge pain. I've done it before by comparing channel differences with a pretty high-latency method: Capture part of a tape with Virtualdub, export the audio to .wav, import to Audacity, normalize the channels separately, split the stereo channels, invert one, rejoin the channels into stereo, then collapse the channels into mono for the difference between the two channels after normalizing. Then turn the azimuth screw a bit, do the whole process over again, and compare the two differences to see whether I made it better or worse...lather, rinse, repeat. I've spent all day doing this before, and I eventually got my VCR aligned to my satisfaction (the difference track was very faint), but I've recently found that it's all out of alignment again. I'd hate to spend just as much time doing it this way all over again, so I'm looking for better tools.

Is there any Windows or Linux software that can visualize or otherwise report phase differences in real-time (or with some reasonable latency)? I would love to have something that worked on line-in audio, so I could turn the screw and get real-time feedback, but even having something that worked on a .wav file would save me a lot of time. ProPhase seems to do this, but it's MacOS only, and I'd like to find something I can actually use. Thanks smile.gif

This post has been edited by SuspiciousLizard: Jul 27 2012, 00:14
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SuspiciousLizard
post Aug 17 2012, 00:26
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After I read the first reply here, I realized that going by a Lissajous pattern wouldn't be as good as actually hearing the difference channel myself. I wish I had kept up with this thread though, because I never knew - as splice pointed out - that I could actually listen to the difference channel directly. I ended up using a VST server instead with an appropriate plugin that showed the difference channel and levels. It made it a cinch to adjust the levels and then the azimuth!

Unfortunately, the VCR's sound circuitry has gotten so wonky (likely bad caps or something) that aside from high noise levels, the audio levels are totally inconsistent for multiple captures of the same tape. Sometimes the levels are too weak, and sometimes they clip. It's the best VCR I have for video by far, but after all this, it looks like I'll be recapturing the audio with something else anyway and piecing them together...go figure.

Anyway, thanks everyone smile.gif

This post has been edited by SuspiciousLizard: Aug 17 2012, 00:27
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