ADC and Master Fader Issue

[wyman]

hi folks,

came across something interesting tonight. i searched a little on it, but didn't come up with anything that was quite like this, and it may be applicable to some folks, so i thought i'd chime in here.

the way i have been setting up sessions is i route everything to a bus, then print that bus to a new pair of tracks, usually with a master fader in the path to add inserts like a bus compressor or an eq. So this master fader is downstream of all the other routing i'm doing in a session (which can sometimes be two or three more levels deep).

what it seems is that having delay compensation on the master fader throws things off for any auxes upstream. to test this, i took identical program material on two tracks, sent one to my "stereo bus" that would feed the two track master, and had the other one take a detour by sending it through another aux bus before it hits the stereo bus. signals cancel 100%, until i put a plug in or a hardware insert on the master fader, and then i get a bunch of comb filtering.

when i disable ADC on the master fader, all is well. and when the tracks both feed directly to the stereo bus, there is no problem. its only when i need to maintain phase coherency when one one track is within a bus and one track is not. for instance, let's say i feed my kick, snare and toms to an aux bus, and leave my overheads out of it... in this scenario, a plugin on the master fader would change the phase relationship between the drums and the overheads.

i guess its sort of like the old joke: "It hurts when i do this..." "So don't do it!" but i was just curious... i found one reference that described problems when using two master (deleted the url, as it was mucking with the formatting of the post)


but this seemed slightly different, since something upstream is being affected by something downstream.

at the very least, i though it might help someone out having the same problem. or, maybe this is old news, in which case... oops.

cheers,

jw

[Zep Dude]

Hi, first, can you reformat your posting? It's not word wrapping so it extends way off the the right of the page and is amost unreadable.

I have found that if you have a track in your song that exceeds the maximium ADC buffer size it seems to throw everything else off. I always thought it would just mean that this one track in the red in the ADC display would just be delayed beyond everything else, but that's not the case.

I also recently found my Sonic Synth loses sample accuracy when a Bombfactory plugin is instanciated (but not Sony Oxford or some others I tried) with ADC. This occurred with the Bombfactory plug on a different unrelated track!

Also, although ADC will compensate if you use external inserts (going out to analog for a compresser etc) as long as you've entered the offsetts in the IO window, it does not compensate if you go to an external output from a send and then return that output on an Aux (for ex, a buss send to interface 1 output and then set up an aux channel with interface input 1).

I was curious about this very thing you've posted. I'm going to run some tests. Let me know if you find out any other oddities with ADC. I'll post back.

[wyman]

sorry about the word wrapping... the long URL was responsible for that... i ditched it.

here's the thing: in my tests, the maximum ADC buffer was never exceeded. Even a simple eq with 3 samples of delay caused the comb filtering, and there were no ADC red lights on.

i'd be happy to upload a session file that i tested with if you like. its a totally fix-able thing, but might not be immediately apparent to folks.

thanks again.

jw

[jeremyroberts]

jw:

is your "final" master fader across the hardware, or a bus?

Try this: INSTEAD of routing your final output to "analog out 1-2" or whatever you are calling your control room output, make a bus and call it: "2MIX". Call your Analog 1-2 hardware outputs "ControlRoom".

Your bussing should look like this:

1. FINAL stereo 2mix is on the "2MIX" bus
2. Use an aux to route 2MIX bus to ControlRoom output. Set this to unity gain. DO NOT put any plugs on this -- this is just to get the signal from 2MIX bus to an output. Disable automation on this track (you do not want to write a move in error) and solo safe it.
3. If you want to hear anything, all paths must be ultimately bussed to "2MIX".
4. OK, now -- put a master fader across "2MIX".
5. AND, now try the test again. You've now eliminated the hardware from the test... everything is bussing... and you only hit the hardware to monitor.

Does this change anything?

PS -- one bonus to this routing: you can put a plug or insert on the "2MIX to Control room" aux and it becomes a post-masterfader insert. If you want it that way.

If you want me to look at your session, I'd be happy to. Send it to my email in my profile.

[wyman]

jeremy,

this is exactly how i usually route things, except instead of the Aux Fader outputting to the CR monitor out, i use an audio track, so i can print the results (i like to have an instrumental, acap, voxup, etc in time in alternate playlists...).

I tried your suggestion, swapping the audio track for an aux input. when there was another bus before things hit the 2mix bus, there was definitely some comb filtering...

i know, hard to describe in words.... i'll try to send a sample session along...

cheers,

jon

[fman]

jon

i know exactly what your talking about. i set my sessions up the same as you and i find that bypassing ADC for the master buss (or master Auxillary) essential. i seem to notice the problem when i'm adding extra plugins on my drum buses.

for example: i route all my drums to 2 auxs. one with compression and one without. sometimes i'll want to put an effect on just one of those aux's, like a filter (i usually reach for Filterfreak for this) and as soon as i add it, i get crazy comb filtering. Its not as though the new plugin is unbalancing the sample delays, because it was already unbalanced in the first place by having a compressor plugin on just one of the auxs.

my ADC is not in the red anywhere and i can't seem to work out what the change is; ie. whether its adding more delay to that particular aux and not the other or something else? and its irrelevant whether its RTAS or TDM. I avoid the issue by just adding the same plugin to both Auxs but bypassing the one i dont want. but this can be tedious if your maxing out the system already.

its just one of those quirks that i doubt will ever be addressed, or rather theres probably an explanation but it doesn't help us get around the issue.

fman

[trombino]

I'm not sure which version of pro tools you guys are talking about, but I first noticed this when I upgraded to 7.4. I also route my tracks similarly to the way you guys do... but in 7.3 I never had any problems doing it this way. However, 7.4 made my sessions unplayable because of the comb filtering. I did some trouble shooting and discovered that whenever there was any kind of plug in or insert on the master fader, ADC broke - it would delay the left and right channels by different amounts, and neither of them by the proper amount. I tried it on 7.3 and discovered that, while the delay amount was still wrong, at least the left and right channels were delayed the same. Which is why I guess I never noticed the problem before.

I went over it with tech support and submitted a bug report but haven't heard anything about this since. They seemed pretty interested in it though.

I was unaware you could disable ADC on a particular channel.... how is that done btw?

[wyman]

mark,

i am on 7.3.1 and getting issues, though not quite like what you describe... i have experienced something similar... very rarely, when creating a new track or an aux, the entire mix gets that crazy, no-center-image, vertigo inducing sound that might be what you describe... shuffling a track around in the edit window (moving it up, then back) always remedied the problem for me... not a big deal, so i never thought much of it.

you can disable a tracks delay compensation by control-command-clicking the top number in the delay compensation window for that track... it will gray out. that's how i solved it; since it was on the master fader, it didn't really matter how much latency was on it.

cheers!

jon

[Vaporhead]

I have experienced the exact same symptoms as wyman. I am on 7.4cs2 but have had the same problem on 7.3 in the past.

[fman]

i experienced the problem on 7.2. I've just upgraded to 7.4 then cs2 and haven't yet experienced the issue (thats not to say it wont happen), but it occurred pretty sparsly when was mixing on 7.2. i always seemed to find it happened right at the end of a mix when your maxing out the system and trying to get creative with some drum effects. does this happen to anyone when they're using hardly any plugins?

fman