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Computers- Variable Work Rate vs Latency
Old 17th June 2009
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Computers- Variable Work Rate vs Latency

With particular reference to DAW (PT8 HD3 Win XP) parallel processing. As I mix hybrid using consoles, outboard and HW FX, I find that I have to calculate offsets b/n PT tracks and busses and sends making round trips to H/W. The real annoying thing is this offset can vary by a few samples from day to day. What causes this, and is there an easy way to measure it or better yet, to avoid needing to recheck?

For those interested, I'm using a setup where orig PT tracks are fed out to indiv anlg console tracks (up to 46), the simultaneously printing up to 46 stems from the separate channel outs from the console. I'm monitoring the input for these (soon to be) printed stems whilst splitting them to PT subgroups for further insert processing (eg Neve H/W comp across cln gtr grp). In addition, the monitored stems are sent to H/W FX, submixed in the anlg console and returned to another PT sub group. All sub groups are then further bussed to a 2 mix destination via another round trip to an outboard C2 compressor. The instrument sub groups need to be offset later by a few thousand samples at 48K in order to be in sync (within 1 sample) with the H/W FX rtn. The complicated way that all the sub bussing works causes the latency issues, if I de-activate the orig PT source tracks once the stems are printed, the latency changes considerably. I wonder if this should help overall, despite the recalculation necessary? I understand that when I add plugins or new buss paths that the latency changes elsewhere, but if I change nothing from one day to the next, should I be experiencing random latency shifts?

If anyone understands PT architecture to offer an insight I'd be very grateful. I hope no-one relies on PT to do parallel compression, a single sample drift is catastrophic!
Old 18th June 2009
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sooner or later, somebody's gonna chime in with something....
Old 20th June 2009
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yup, any day now....
Old 20th June 2009
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dubrichie's Avatar
i'm not quite sure exactly how you are going about this, but would like to help if i can.

can you explain your signal flow in a more explicit and methodical fashion?

for example, i understand that you are sending many audio tracks out to console channels and recording them back in on new tracks, but have you set this up as hardware inserts in PT? as in, each console channel is a hardware insert on a PT channel?

are you feeding your hardware effects from sends on the console while those channels are being used as hardware inserts in PT?

you should not be experiencing "random latency shifts" with PT if you are using it correctly.

what interfaces (and other converters) are you using? how are they clocked?

do you have delay compensation enabled? are you staying within its limit?

do you have the boxes for "compensate for input delays" and "compensate for output delays" checked in the i/o dialogue?
Old 21st June 2009
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cheers for the reply. I'm gonna check my settings and get back to you about that. I'll also try to be more specific, but I know that there are others out there that complain of PT's variable latency dependent upon CPU stress. You've never heard of this issue?
Old 21st June 2009
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dubrichie's Avatar
no, i have not heard of this issue with PT HD.

i can understand the possibility of it occurring in a native system like PT LE, but find it hard to comprehend with PT HD, if the system is configured and operated correctly.

you see, PT HD is essentially a digital mixer, a hardware digital mixer that operates on the DSP cards which are connected directly to the i/o including the AD/DA.

the mixer itself, the amount of tracks, voices, i/o, etc. including all related latencies are fixed and guaranteed to be repeatable.

this digital mixer is housed inside a general purpose computer in order to integrate it with a recorder / editor. the computer is used to proved the record / edit functions (storage - HDDs), for human interface and host-based "native" processing tasks, ie RTAS plugins, which can essentially be viewed as a "digital rack" which is patched in and out of the mixer on demand.

the mixer itself is hardwired and hard-coded, which is the beauty of the HD system and what makes it so much more predictable and reliable than any native system.

all of the above is my attempt at explaining my understanding of the system. some of it may be wrong, slightly or seriously, and i am open to being corrected by anyone who knows better.

i'm looking forward to your reply as this is just the kind of tech-out system conundrum busting that i get my kicks out of!

let's get our **** sorted badass and PWN that damned computater!

it ain't the boss of me, or YOU!
Old 22nd June 2009
Lives for gear

just give me a little more time so I can really put the finger on where the culprit might be, but I will say this, I like to use a couple of VST wrapped plugs, sometimes on a submaster. Another thing which is a definite, If I wish to re eq a printed stem of say a snare (focusrite TDM), then it goes out of phase sync with the rest of the kit. I have to put the same eq in bypass on the rest of the drum stems to get sync back. All this with heaps of headroom as regards ADC. Go figure...
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