OT - VAC resampling, killing I/Q phase relationship?

N7CXI
Posts: 6
Joined: Sun Apr 09, 2017 5:20 pm

OT - VAC resampling, killing I/Q phase relationship?

Postby N7CXI » Wed Apr 25, 2018 5:42 pm

OT, but not sure where else to ask:

I have an audio-rate I/Q test "sink" that I use when working with "sound card sampling" I/Q stream sources.
I thought it would be useful to set up VAC, so that I could use multiple "sink" (input/receiving) programs at the same time for comparison.

I've tried both Eugene's VAC and VB cables with the same results. Everything is running at 96K both in and out, so no resampling is explicitly required, except perhaps for clock-matching.

The problem I'm having with this is that when I look at the spectra from a 96K I/Q stream that's come across the VAC cable, I see it's been band-limited to 48K and the I/Q relationship has been spoiled. Again, I have both source and sink set to sample at 96K, and am using 16-bit signed samples on both ends as well. I've looked at available options for the VB cables at least and haven't seen anything relevant beyond setting the "internal" rate to 96K, which I did.

Any ideas? I'm hoping it's something I'm doing, I'd like to have all my test apps running in parallel.

Thanks,
Jim N7CXI
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w-u-2-o
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Joined: Fri Mar 10, 2017 1:47 pm

Re: OT - VAC resampling, killing I/Q phase relationship?

Postby w-u-2-o » Wed Apr 25, 2018 6:25 pm

Jim,

It does sound like you may be doing something wrong. When the resampler was being tested it was tested for both normal audio and for IQ data. IQ data was tested using CW Skimmer. The resampler did not have any negative effect on CW Skimmer performance, and the sample rate could be run at 192KHz if desired, with a full 192KHz visible in CW Skimmer. The connection was made using Muzychenko VAC.

Rather than using your tools, perhaps you could try this with CW Skimmer and see if it works for you. That might help narrow down the problem a bit.

73,

Scott
N7CXI
Posts: 6
Joined: Sun Apr 09, 2017 5:20 pm

Re: OT - VAC resampling, killing I/Q phase relationship?

Postby N7CXI » Wed Apr 25, 2018 6:30 pm

Thanks, Scott. I'll do exactly that.
It helps to know that it *should* work, that gives me reason to pursue it further.

The other unknown variable in the system is PortAudio V19, but since that is working fine with physical devices I don't see why it in itself would be a problem when used with VAC.

73,
Jim N7CXI

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