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Old 11-29-2020, 11:53 AM
gandlz gandlz is offline
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Join Date: Mar 2011
Location: Austria
Posts: 68
Default Compensate Input/Output Delay with external converters – not H/W Insert delay!

Compensate Input/Output Delay with external converters – not H/W Insert delay!

I found this topic discussed in a few threads but mostly there are only a few people understanding what’s meant and often it’s confused with the ability of H/W Insert delays.

Although my personal tests where made with Pro Tools 10 HD with 192 I/O and Pro Tools 12 native with RME Fireface UC, there is absolutely no different behavior in most recent versions with new Avid I/Os, neither HDX or native.


The problem explained:
Enable Delay compensation and also check the according options in I/O Settings (input and output).
Playback an audio track (1kHz square) on 192 I/O analog output looped to 192 I/O analog input and recorded on new track. This track is perfectly in sync with the original track.
Do the same but record it through any external converter connected via AES or ADAT to your 192 I/O. The result will be too late.
If you go one step further and output the original track through an external converter connected to 192 I/O via ADAT or AES you will get additional delay.


Examples:
Focusrite Octopre MKII dynamic via ADAT: 16 samples late from 192 I/O, 34 samples late from Octopre
Universal Audio 4-710d via ADAT: 14 samples late from 192 I/O, 32 samples late from Octopre
Universal Audio 4-710d via AES: 12 samples late from 192 I/O, 30 samples late from Octopre
Octopre analog out to 192 I/O analog in: 18 samples late


How did I recognize this as an issue?
I wanted to use my Octopre analog outputs for the Cue-Mix outputs. I had a singer and the microphone preamp was connected to the additional line inputs on the 4-710d. This makes the vocal track 30 samples late as explained above. This is 0,68ms at 44.1kHz and in fact is not acceptable. The timing issue was recognized by everyone in the studio.


In general:
You can change the 192 I/O in this example with any other interface (also native) which has it’s own analog and digital inputs for use with external converters. In native situation the ASIO driver tells Pro Tools about the input and output delay created by DAC/ADC of your interface and this is compensated correctly. But if you use external converters connected to your interface, the driver has no idea about the time your external converter needs for conversion and therefore the tracks will not be in sync.
Pro Tools HD and HDX know about the delays of Avid interfaces and compensate correctly but have also no idea about external converters.

Delays change when disabling I/O compensation in I/O settings or disabling Delay compensation at all (or any possible combination). That shows that every settings adds a fixed amount of samples to compensate.


The solution would be...
...to make the amount of samples compensated for "after recording pass" (as said in the I/O Settings window) variable! In my situation I would have to give all Octopre outputs 18 samples of additional compensation. And so on for the different kinds of inputs I have. I think about one more column in the I/O Settings window, where we can just add this amount of samples.


This is really annoying…
…and I tested a lot, and also tried to develop some workflows. But you always have to nudge and if you combine a few converters the situation is really complicate and you miss to focus on the overall work you have to do. And also you cannot use spot function with original time stamp as far as this timestamp is what you really want.

So I really bought a fourth 192 I/O and changed my 4 stereo Cue-Mix to those analog outputs. So everything is heard at the same time.
I don’t use the 8 channels ADC on the 4-710d anymore and connected it to the 8 inputs of the fourth 192 I/O.
ADAT Octopre is used for gate triggers at the moment. I put these tracks 3000 samples ahead to work with my sidechain gates, so original timestamp is not important.
So all four 192 I/O do only 8 channels of I/O (except the last one with the Octopre) and I need a fifth if I buy one more preamp!


What I also noticed during testing:
when looping AES output to input the recorded signal is 15 samples too early.

Post-Fader sends are compensated same as explained above but Pre-Fader sends are compensation different and recorded signal is additional 6 samples late. Adding any Plug-In inserted on Master-Fader of that PRE-Fader send, makes the recorded signal 4 samples late. (Maybe that’s different on newer Pro Tools versions?!)

Realtime SRC on ADAT inputs when working with higher sample rates adds additional 28 samples delay.


What can we do - Metaplugin:
For making all outputs synced (Avid IOs and external Converters, and solving the PRE-Fader Bug described above) you can use something like Metaplugin by DDMF. Sadly I didn’t find anything else.
The idea is to add this insert on the Master-Fader and define a Plug-In-Latency that does not exist.
For example on my Cue-Mix outputs (ADAT-to-Octopre) I would define 18 samples of latency (as mentioned above) or 24 samples (additional 6 samples) if the Cue-Mix is PRE-Fader.
As the result, all other outputs are delayed by 18 (24) samples and therefore in sync with the Octopre-Output. As far as this playback is recorded to a new track using the 192 I/O, it is in sync.

The problem with Metaplugin:
Metaplugin itself adds heavy latency additional to the fake-latency you define and therefore it is not usable for realtime monitoring. Also it is only available as native and not as DSP Plug-In, which means that your Master-Fader needs 2 additional voices for mono (4 for stereo).
I did not find any other Plugin to define a fake-latency and metaplugin is pretty overloaded for just doing this. A simple DSP-Plugin just doing this would be the solution for making outputs in sync.


Conclusion:
As far as you count Metaplugin as a solution for outputs (I would not recommend this) there is still no solution for inputs.

I hope you got what I wanted to show up with this writing. Just think about your individual recording situation and the different analog and digital inputs you are using on your main interface (either its Avid or any third party native interface) and be aware of, that different inputs have different conversion latency which is not compensated in Pro Tools.

You may say these are only a few samples, but this is when doing loops. In real life you have physical distances adding latency. Distance from Headphone or Monitor to ear, from instrument or speaker to microphone. So you may end up nudging tracks more because of other reasons.
But I think we should start up from a perfect professional solution and be able to mess it up later.

I’m excited about any response (if any) to this topic either technical input, workarounds or solutions, or why this is not discussed anywhere and therefore not recognized as a problem.
Or why in 2020 we are not able to record through different converters and have it in sync in our timeline because we are not able to define the latencies of the converters used.

Kind regards,
Gandlz
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