VAC Sampling Rates

Joe
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Joined: Tue Aug 22, 2017 5:58 pm

VAC Sampling Rates

Postby Joe » Sat Jun 19, 2021 3:51 pm

“The ChannelMaster in Thetis always runs at 48 ksa/s.
What would be the motivation to run VAC1 (or VAC2) at 192k?

73, Bryan W4WMT”

Just trying to understand, I thought using the setting of 192 ksa/s was a faster transfer? If Channelmaster is set at 48 ksa/s wouldn’t it be better to match the setting? Why would Thetis allow a mis-match, is this a coding opportunity / change needed?

Thanks,
Joe
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w-u-2-o
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Re: VAC Sampling Rates

Postby w-u-2-o » Sat Jun 19, 2021 4:01 pm

I just answered this in the original thread here: viewtopic.php?f=9&t=3817&p=14642#p14642
W4WMT
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Re: VAC Sampling Rates

Postby W4WMT » Sat Jun 19, 2021 4:44 pm

Hi Joe,

When Doug & Warren developed Thetis their design goal was to normalize buffer latency to 1.333ms irrespective of the user selected sampling rates of the DDC and DUC. The ChannelMaster is completely flexible, but in Thetis it is hard coded to 48k sa/s @ 64 frames per buffer.

It's okay to run VAC1 at 192k of course, but you're just forcing VAC to do an unnecessary rate conversion which eats up cpu cycles that could be put to better use, like for instance, keeping PureSignal supplied with data :-)

73, Bryan W4WMT
Joe
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Re: VAC Sampling Rates

Postby Joe » Sat Jun 19, 2021 5:14 pm

Sorry I missed your response Scott.

Warren, thanks for your explanation, I am probably getting by with this setting because I am running a pc build with a 5950 cpu. I was just planning to make the setting changes you mentioned if it was a more feasible setup.

Thanks and 73’s
Joe
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Joe-W4WT
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Re: VAC Sampling Rates

Postby Joe-W4WT » Sat Jun 19, 2021 6:36 pm

Well, I guess the question now is Why was ChannelMaster fixed at 48khz with Thetis and what would happen if it was allowed to be set to what the user specifies for Sample Rate? Any benefit or would our radio melt down to slag.

Joe W4WT
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w-u-2-o
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Re: VAC Sampling Rates

Postby w-u-2-o » Sat Jun 19, 2021 9:18 pm

Apples and oranges here, gents.

The 1st IF sample rate is set in Setup > General > F/W Set. This sample rate sets your maximum panadapter width, and sets the range over which the 2nd IF can be tuned. It should be noted that because this data consists of IQ data pairs, the sample rate you set equals the bandwidth you see. The actual data rate is twice that defined in setup if you consider that each I and each Q sample count as separate data, and the Nyquist limit is therefore not exceeded.

The 2nd IF, which is also the baseband, sample rate in ChannelMaster is fixed at 48KHz. This is why your maximum receive and transmit bandwidths are limited to 20KHz.

The data buffer size defined in Setup > DSP > Options controls either the 1st IF data buffer or the 2nd IF data buffer, I'm not sure which one. Bryan: do you know?

The audio sample rates that VAC will entertain exist not only to move audio, but also to move raw IQ data from the 1st IF. This is an old fashioned way of moving IQ data that is a holdover from the very early days of openHPSDR and other amateur radio software. It would be much better to move it via an IP channel, TCP or UDP, and then software that can process IQ data could have access to as much as the 1st IF sample rate allows. Alas, this is not the case. However, for example, if you are using CW Skimmer you can use it at 192KHz and see 192KHz of bandwidth in CW Skimmer (again the data here is IQ data pairs using the left and right audio channels, so aggregate data rate is 2 x 192KHz and the Nyquist limit is not exceeded). The VAC buffers are separate from the 1st and 2nd IF data buffers. A VAC buffer at 96KHz will be half as long as the same VAC buffer at 48KHz.
Joe-W4WT
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Joined: Sun Apr 09, 2017 5:27 pm
Location: Cumming, GA

Re: VAC Sampling Rates

Postby Joe-W4WT » Sun Jun 20, 2021 2:19 am

Scott, thanks for the nice indepth reply! Very useful information that I hadn't run across before.

Joe W4WT
Joe
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Re: VAC Sampling Rates

Postby Joe » Sun Jun 20, 2021 2:15 pm

Thank you Scott,

Very in depth! A lot to take in.

Thanks
Joe
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