• Registered: 2010-05-08
  • Posts: 32

Topic: FF Safety Buffer Operation with Pro Tools 9 ADC

Hello to all RME friends,

I have a FF800 and a Mac Pro and use Protools 9 as my DAW.
I previously posted a few questions about the general operation of the FF800 safety buffer and specifically how it works with Pro Tools 9..
Thank you Matthias for your answer and suggestion of the loop back test with Pro Tools 9.
I will now outline the test I did and results I got.
I am trying to find out if Pro tools 9 automatic delay compensation will recognize the FF800 safety buffer so that overdubs are correctly synced and so that the automatic delay compensation works properly?


I created a session in PT 9 with a single guitar audio track and a master fader.
I didnt use any plug ins on the audio track or the master fader.
Hardware buffer setting 1024 samples
Session sample rate 44.1 kHz
Automatic Delay Compensation "on" and set to long.
Using a short balanced jack cable I patched the output of FF800 Hardware output number one into the input of number two and recorded this onto another track that I will call audio two.
The session now has two audio tracks and a master fader and if everything is operating correctly then the two tracks should be in perfect sync.
But they are not.
Believe it or not the newly recorded track has been placed AHEAD in time of the source track.
I measured the difference and it is tiny ...only about 14 samples or 0.317ms..insignificant unless attempting perfect phase.
But its the concept I don't understand...how can the recorded audio be ahead of the source.
I am fascinated by this whole subject and I would appreciate any guidance I can get.

PS For those interested I measured the difference in time between the two tracks by inverting the phase of just one of them and then listening to the sum at the output.
If they are in perfect phase I should hear silence.
I didnt. I heard a sound that was heavily comb filtered but not silent.
I then nudged the recorded track earlier in time but immediately heard that the comb filtering was reducing.
I  was very surprised so I tried nudging the source track earlier in time and found a point at 14 samples track difference where the two tracks nearly nulled.
Strangely they didnt quite create silence at the 14 samples difference point.
... I dont know why?
So I looked at the waveform zoomed way in and sure enough ...the two tracks are out by about 14 to 15 samples with the newly recorded track EARLIER in time.
Mmmm.

Kind Regards
Jim

2 Reply by MC

  • MC
  • MC
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  • From: Germany/Thailand
  • Registered: 2007-04-06
  • Posts: 25,599

Re: FF Safety Buffer Operation with Pro Tools 9 ADC

The placement before is logical, and also mentioned in the manual of your preferred soundcard. At 44.1 sample rate you can not fully null the waves, as there is an additional, very small delay, that is smaller than the sample steps in which you shift. 14 - I don't know why, but I am sure our driver reports the correct values (and 14 is not matching the safety buffer size either).

BTW, 1024 is pretty high. You could do the same test at 256, maybe the error gets smaller then. Or maybe you have to switch the AutoDelayComp off?

Regards
Matthias Carstens
RME

MC's Website

  • Registered: 2010-05-08
  • Posts: 32

Re: FF Safety Buffer Operation with Pro Tools 9 ADC

Hello to RME folk,

Thank you Matthias for your help. You were right about the reason why I couldn't get perfect silence from the phase flip test.
I increased the sample rate from 44.1khz to 192kHz and was able to get a much deeper null but still not complete silence.

In attempting to understand the recorded track appearing ahead of the source by 14 samples I did some more tests.
I tried various combinations of sample rates and buffer sizes with ADC on and off.
I am perplexed to report that today I cannot reproduce any of the 14 sample differences that were apparent yesterday.
I believe I got my methods right but something must have changed.
I am sorry for any concerns my bad results may have caused.

However, what I believe I can assume from my tests is that the FF safety buffer must be recognized by core audio because even with PT 9 ADC off there was no significant delay between the source and looped back track.

It is also worthy to note that according to the PT reference manual, the ADC does not compensate for external A/D and D/A conversion delays. ( PT HD version does though)

Best Wishes
Jim