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Pro Tools buffer size and round trip latency

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Pro Tools buffer size and round trip latency

PostPosted: Sat Oct 31, 2020 6:26 am
by cashhewn

I'm a bit confused as to what's happening here, but perhaps I'm thinking about it incorrectly:

I'm using Pro Tools and have the hardware buffer set to 256 samples. I have an audio clip of a click on a track in Pro Tools that I'm sending out of my interface's converter to an analog console channel then assigning that channel to the main mix bus of the console and taking a mult/half-normal of that mix bus out to a headphone amp and then out to a pair of headphones, then I'm holding the headphone earpiece up to a mic that's going back to an external preamp and back to the analog console and finally back into the interface/converter and onto another adjacent track in Pro Tools, and I'm only seeing a timing difference in the two tracks of about 190 samples.

I would have expected to see a latency of at least 256 samples since that's is what my hardware buffer is set to in Pro Tools. The session is 24 bit 48 kHz.

Am I either misreporting something or thinking about it incorrectly?

Thanks for you help!

Re: Pro Tools buffer size and round trip latency

PostPosted: Sat Oct 31, 2020 10:24 am
by Sam Inglis
Like most DAWs, Pro Tools compensates for latency that it knows about when placing regions on the timeline. (I think this behaviour can be turned off in Preferences somewhere, or possibly in the Playback Engine dialogue.) So the delay of 190 samples you're seeing represents the additional delay that it doesn't know about. This is either incurred in the analogue path, or represents unreported latency in the audio interface.

The theoretical round-trip latency at a 256-sample buffer size should be at least 512 samples, since the audio is buffered twice, once on the way out and again on the way in. In practice it will be a fair bit higher because of 'safety buffering', A-D and D-A conversion and so on, all of which add additional delay. Not all interfaces report this additional delay to the host DAW correctly.

Re: Pro Tools buffer size and round trip latency

PostPosted: Sat Oct 31, 2020 4:28 pm
by cashhewn
Aha, this makes sense, thank you for the response and providing some clarity for me!

Re: Pro Tools buffer size and round trip latency

PostPosted: Sat Oct 31, 2020 9:50 pm
by DC-Choppah
I assure you that when I do the same test that you describe, the tracks are lined directly up no matter setting I use for the buffer size.

I just tried it again for 256, 512 or 1024 buffer size. The track recorded from the mic on the headphones is sample-aligned with the original click track audio. No displacement that I can discern. Same correct position for all buffer sizes.

The waveforms are not exactly the same, but you can see the 'click' and they start and end at the same place at the sample level.

There really is something wrong if these are not aligned. Protools should know everything it needs to line everything up.

I have seen this happen where they don't line up and it indicates a problem with the state that the audio interface is in. Restarting Protools can clear this error. Are you getting any other errors from ProTools complaining about problems with the interface?

You might try restarting ProTools and repeat your test for various buffer sizes. If it clears then there is a problem with how Protools started up with the interface.

If the offset is always there, you can tell ProTools about it and it will compensate for it. That would mean that the extra latency is really in your system.

Re: Pro Tools buffer size and round trip latency

PostPosted: Sun Nov 01, 2020 3:05 am
by cashhewn
@DC-Choppah hey thanks for this response.

The latency behavior I'm describing here has been going on since I began using my current studio setup and has persisted across many power cycles of the interface and MacOS. I wish I had the same results that you have with perfect alignment!

My workflow is that I've created a custom nudge-length exactly the input latency in samples, and then by hand move the audio back in time that amount after every single overdub/take, and not only is it tedious but it's easy to forget to do and can get confusing. 190 samples at 48kHz is only around 3.95 milliseconds, and I might be fooling myself, but I really seem to feel the difference if the latency is there between the tracks in the music.

I tried using the Pro Tools "Delay Compensation" feature and input my custom user offset in samples (I tried both positive and negative values) but frustratingly I couldn't seem to get that to work to move the clip any earlier in time, seemed like it only moved the clip later from what I could suss out. I have a message out to Avid support but haven't heard back from them yet. I need to troubleshoot it some more and don't want to spread misinformation about that feature, as it might work just fine and it's simply my user error.

I just did a quick search while writing this and found a plugin that looks like it might do the delay compensation for me, I might have to try it...

Thanks for your response!

Re: Pro Tools buffer size and round trip latency

PostPosted: Sun Nov 01, 2020 4:33 am
by DC-Choppah
This kind of thing is NOT what automatic delay compensation (ADC) is for. ADC just delays every track by the longest (reported) delay reported by any track. ADC is only relevant for mixing when you have high latency processing where the latency of that problem plugin is soo long that it goes outside of the available time window normally available (buffer size). It's not relevant for recording with analog monitoring like you are doing. ADC will only delay tracks.

Your problem is that the round trip delay of your interface, which should be published to PT is wrong?

To fix that, you need to look into the controls for your audio interface. Somehow it is not reporting its own round trip latency correctly? So you need to override it and put in a correct setting? But you should not have to do this of course.

This is a problem with your interface, not with Protools.

Does the delay of the recorded track increase when you increase the buffer size? Does the delay follow the buffer? Or is it always just wrong by 190 samples?

It sounds like Protools thinks you are using one buffer setting, but your interface is actually using another, so the round trip delay is simply wrong. You should check with your interface control software if it is actually getting the correct buffer setting that Protools is using.

There are some setting that don't allow the DAW to control the interface so they can get out of whack.

Re: Pro Tools buffer size and round trip latency

PostPosted: Sun Nov 01, 2020 6:08 pm
by Sam Inglis
Do you have any other digital gear in the signal path? For instance is your external preamp connected via ADAT or S/PDIF or something like that?

As DC-Choppah says, if the test path is purely analogue you shouldn't expect to see a delay.

What's the interface? I have come across interfaces that don't report their latency correctly but 190 samples is quite a large error.

Re: Pro Tools buffer size and round trip latency

PostPosted: Sun Nov 01, 2020 10:53 pm
by cashhewn
Hi DC-Choppah and hi Sam,

Thanks for sticking with me on this!

To DC's point: I'm away from the studio right now for a week so I can't test to see if changing the buffer size in Pro Tools changes the latency I'm seeing. I'll have to do that when I'm back at the studio.

To Sam's point, and he brings up an interesting one: for this test I thought it was simply a purely analog signal path, but now I'm not so sure when I think about it and here's why- I'm using a Focusrite Clarett USB 8pre and it's synced to a Focusrite Clarett Octopre to give me 16 channels, but I'm using the ADAT of the Octopre as the clock source within the Focusrite Control app...could that have something to do wit it?

So although my signal path is entirely analog from when it leaves the 8pre converter and goes out to the console, the headphone amp, the headphones, the mic, the mic pre, the console, and finally back to the 8pre converter again, even though I'm not using the Octopre's I/O I am however using it's ADAT as the sync source.


That said I have found a plugin that seems to be designed to fix exactly this problem, but it seems a shame to have to resort to that and have this plugin on every recording input channel in Pro Tools to fix this error.

Thanks for your help!

Re: Pro Tools buffer size and round trip latency

PostPosted: Mon Nov 02, 2020 6:22 am
by DC-Choppah
Everything in the system needs to be using the same clock. Protools (computer) and both digital interfaces need to be using the same master clock, and anything else that is digital that you connect to in the future.

Protools will default to the internal clock source of the first interface that you connected.

You can change the Protools clock source under the menu Setup/Hardware.

If your interface has more than one option for clock source it should show up there as an option. This a function of your interface, not really Protools. Protools is just showing you whatever your hardware offers.

Make sure that everything is using the same clock.

Re: Pro Tools buffer size and round trip latency

PostPosted: Tue Nov 03, 2020 2:48 am
by cashhewn
Excellent suggestion on checking what the clock source is set to inside the Pro Tools setup/hardware menu. I have to admit I don't think I've ever explicitly checked to see what it is set to inside Pro Tools, only at the interfaces' settings themselves.

I'll also test to see whether or not changing the hardware buffer setting in Pro Tools affects the latency I'm measuring.

I'll investigate all these things when I get back into the studio and will report back in case it's a help to others that might turn this thread up in a search some time.

One additional thing- I heard back from Avid support and the representative there said, besides nudging the clips back by hand, the only other thing to try is:

"Input track delay compensation
This will be applied to the entire track and not just one clip, assuming that delay/difference will be the same for all clips in the particular track.
On the Mix window (Command + =)
View (menu) > Mix Window Views > Delay Compensation.
Then input the amount of offset (in samples) negative value to play the clips in the track earlier, positive value to play the clips later. In effect, it's like moving the clips (earlier or later) in the track by the amount you set."

I'll give that a go as well, though I thought I had already tried this and couldn't seem to get it to work.

Thanks again for all your help!

Re: Pro Tools buffer size and round trip latency

PostPosted: Tue Nov 03, 2020 3:18 am
by DC-Choppah
No that won't work, as you already know.

Delay compensation just makes things equally late, as late the latest track. When mixing, this just means that you hit play, and the music is delayed a bit before it starts.

Re: Pro Tools buffer size and round trip latency

PostPosted: Tue Nov 03, 2020 10:27 am
by Sam Inglis
It would be worth running the same test in another program. You can download Reaper to try out for free, so give it a go there and see if you get the same error. If you do then it's definitely not the fault of the software!

I don't think that clocking the Clarett USB externally should make any difference.

Re: Pro Tools buffer size and round trip latency

PostPosted: Tue Nov 03, 2020 4:52 pm
by cashhewn
Thanks Sam, good point that I could try a different DAW to try and deduce whether or not it’s a software issue.

Thanks again for all your help. I’ll report back when possible.

Re: Pro Tools buffer size and round trip latency

PostPosted: Wed Nov 04, 2020 5:14 pm
by DC-Choppah
There is an option in Protools to force it to ignore play back and record errors. You should turn this option OFF normally so that any errors in the setup will be caught by Protools.

Go to Setup>Playback Engine and make sure that "Ignore Errors During Playback/Record" is not checked.

Re: Pro Tools buffer size and round trip latency

PostPosted: Thu Nov 05, 2020 11:45 pm
by cashhewn
@DC-Choppah Thanks for this suggestion about “Ignore Errors During Playback/Record”. I’ve read a lot around the web and it seems this fixes the problem for some folks using Pro Tools and UA interfaces, maybe it will fix the problem for me with the Focusrite interface, fingers crossed. I’ll be happily back in the studio this weekend and can troubleshoot!

Thanks again for your help!