Thetis, buffers, latencies and VAC settings.

ea3aqr
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Thetis, buffers, latencies and VAC settings.

Postby ea3aqr » Fri Nov 22, 2019 6:01 pm

Hi,

I have a questions about what are the best settings in Thetis.

The current VAC tutorial seems to be a little bit outdated...

My present settings are:

DSP.jpg
DSP.jpg (103.61 KiB) Viewed 867 times


VAC.jpg
VAC.jpg (120.16 KiB) Viewed 867 times


My TO/FROM VAC ratio is quite stable... jumping up and down only the last 2 digits and sometimes just the last one.
From time to time I have a few over/underflows, but nothing serious to worry about.

Yesterday I was reading some old posts about setting up VAC and most user were using buffer sizes about 512 with the RingBuffers latency set to zero in "Manual" mode.

So my question is... what is better, a bigger DSP/VAC buffers with RingBuffers latency set to zero or the smallest DSP/VAC buffers I can achieve, but with some manual RingBuffer latency?

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

Re: Thetis, buffers, latencies and VAC settings.

Postby w-u-2-o » Fri Nov 22, 2019 9:12 pm

Where VAC is concerned, there are no "best settings". It is different for everyone. There are some rules of thumb, though.

If you are using ASIO PortAudio buffer sizes can be set to 0 using the manual checkboxes, they are meaningless for ASIO. And, even for other drivers you can usually, but not always, set these to 0.

Latency is a trade-off between VAC buffer size and the ringbuffer size (length, really). Consider that at 48KHz a buffer of 128 is 2.7ms long. Or, in your case at 96KHz, 1.35ms long. Total delay is complicated and I hope Bryan jumps in, but it is going to be at least the ringbuffer at 4ms plus the VAC buffer at 1.35ms, so 5.35ms.

If you could get the ringbuffer down to 2ms by going to a VAC buffer of 256 then the total delay would be 4.7, a tiny improvement. In other words, you have to play with it and figure out what combination gives you the lowest delay.

But...that configuration also needs to play nice with the resampler if you want to use the resampler. I have found that at very small delays and buffer sizes the resampler will sometimes run wild on my and I have to reset it by cycling one of the manual checkboxes. Some combinations won't run at all according to other folks reports (that's never happened to me).

Another thing to consider is that if you've shaved everything down as far as it will go and have hit the smallest VAC buffer size, i.e. 128, you can go to a higher audio sampling rate, i.e. 96KHz, to effectively half the delay in that buffer. I don't know if that was your intent, but it is a practical side effect.

At any rate, you've got some very nice, very FAST, settings there. 96KHz with buffer 128 and the only 4ms in the ringbuffer is excellent. I have the exact same setup except I've got 5ms "In", 3ms "Out". With the filters set to Low Latency that's just about as fast as you can get things to go. There are a few people who have gotten all the way to all zeros.

73,

Scott
ea3aqr
Posts: 52
Joined: Mon Mar 04, 2019 10:50 pm
Location: BCN

Re: Thetis, buffers, latencies and VAC settings.

Postby ea3aqr » Fri Nov 22, 2019 9:57 pm

Scott, thanks for the very interesting reply, it is very clarifying!

Just one more question, what is the weigh of DSP SSB/AM buffer size into the whole delay?
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w-u-2-o
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Joined: Fri Mar 10, 2017 1:47 pm

Re: Thetis, buffers, latencies and VAC settings.

Postby w-u-2-o » Fri Nov 22, 2019 11:56 pm

Unfortunately I don't know the answer to that question :( But I haven't noticed any problems in leaving those buffers at the minimum setting.

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