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From: | John E. Perry |
Subject: | Re: [Discuss-gnuradio] synchronizing sound cards in a cluster (fwd) |
Date: | Fri, 14 Mar 2003 00:19:08 -0500 |
User-agent: | Mozilla/5.0 (Windows; U; Win98; en-US; rv:1.0.2) Gecko/20030208 Netscape/7.02 |
Chris Albertson wrote:
Actually, I'm glad I didn't. Reading the intervening stuff piqued my memory...This is to long, skip to last paragraph.
This is actually an excellent way to synchronize any number of channels at practically zero cost!One more idea: This is harder and it costs you bandwidth but not a whole channel like above. Add a reference signal to your adalog signal. You will later filter this out Like above the reference signal must be delivered in phase to each sound card. ... My opinion is the sooner a stable clock gets mixed with your data the better. More of their paths will be the same. but also the harder to do. The best an hardest is to mix a time code signal with the analogdata before it is sampled.
A simple resistive combiner at the input of each channel of the input cards will allow you to apply a low-level pseudorandom sequence, say, one or two bits amplitude, to every channel in your system. Straighforward postprocessing tells you the timing to within a sample interval or so of every channel at intervals of the sequence length. Removal of the probe sequence from your input data is also straighforward. Fancy processing of the probe signal should be able to give sub-sample timing, though I'm not sure what you'd do with it (I know what I did
with it in a very different application).Quick, easy, noninterfering, accurate. You have to make sure you don't saturate your input channel, or you lose the sequence information in that period (although you could infer it from whatever you did get by comparing it with timing from other channels).
John Perry
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