Topic: Are there any negatives when running 4 channels with two separate ADC's??

[yug du nord]

SSIA.

When running a 4 channel rig, would there be any negative effects if using two separate ADC's?  I know that the clocks need to be sync'd and whatnot.....  but I'm more curious what y'all think of that possibility.  To me, it makes sense to use the same brand/model ADC for all four channels....  but would it matter??  I run two HDP2's using the internal ADC's in each box and sync em via word clock to make a four channel rig....  what would happen if I added an external ADC to the mix?  I'm sure things would record just fine....  but what about the sonic structure??  Would any confusion between the two separate ADC's occur?  Thanks.

[runonce]

I sync my dbx386 and Behringer Ultramatch sometimes...

You'll be wed to analog sound of the extra AD - but the digits should go together just fine.

[DSatz]

A/D converters have a small but definite transit time, depending largely on how their filters are implemented. Two different models or brands of converter might have different transit times, so even if you are using a common word clock to synch them, they might be a sample or two apart from each other by the time your recorder receives their output.

On the other hand, as I was writing this, it occurred to me that my standard setup for 3- and 4-mike recording is vulnerable to the same effect. I've never noticed any problem--most likely because the third mike (or the second pair of mikes) isn't coincident with the main pair, and the different arrival times of sound at the microphones is far greater than any difference in latency that is likely to exist among different converters. Unless you're working with an ambisonic-type or "SoundField"-type microphone, this is probably not worth worrying about.

--best regards

[ghellquist]

With a stable sync, no problems whatsoever.
I have often recorded symphony orchestras with some channels through a really good AD and the rest through my sound card AD. No issues.
The two channels of a stereo setup, say ORTF, I would run through one chain of AD just in case.

// Gunnar

[JasonSobel]

as other people have said, it should be no problem.  I've done many 4-channel recordings (with two Tascam HD-P2's and a V3).  The V3 would be the A/D for two channels (S/PDIF out to HD-P2 #1).  The second HD-P2 would record from the analog inputs (i.e. using the internal A/D in the HD-P2).  The second HD-P2 was also synced via word-clock to the V3, so both had the same clock source.

[Shadow_7]

I can think of several potential issues (if only theoretical).  World clock probably keeps these in check.  And most are probably neglible in practice.  But ask yourself if the two channels would be better by themselves if not adjusting sync with world clock?  There is probably a reason why there's several sync options besides world clock, and it's probably not completely evolution / patent driven. 

I've had two 2 channel interfaces for 4 channels of input once.  With no sync option, and even if you compensated for the latency between devices, there was still a drift in the proximity of a full second over an hour.  Even though the devices were the same make and model.  On the flip side of that I do notice some L/R drift between the two channels of my Korg MR-1000 (one device).  But that's zoomed down to the 1/1000 of a second level.  And it's much more evident past that first hour of continuous recording.  It could just be two mics in two different locations, but it doesn't appear to be since it's always skewed with the same channel always falling behind the other.  It could also be something in audacity or not.

[DSatz]

I think the upshot of all these messages so far is that the sampling rate would be held together by the shared word clock, but the exact start time might be very slightly "off". Whether that matters audibly or not would depend largely on the microphone arrangement.

[Church-Audio]

SSIA.

When running a 4 channel rig, would there be any negative effects if using two separate ADC's?  I know that the clocks need to be sync'd and whatnot.....  but I'm more curious what y'all think of that possibility.  To me, it makes sense to use the same brand/model ADC for all four channels....  but would it matter??  I run two HDP2's using the internal ADC's in each box and sync em via word clock to make a four channel rig....  what would happen if I added an external ADC to the mix?  I'm sure things would record just fine....  but what about the sonic structure??  Would any confusion between the two separate ADC's occur?  Thanks.

If it was me I would make sure that both ADC's were in the same brand name. And in the same devices. Because just because the ADC is the same chip does not guarantee a sync.

[yug du nord]

^^^This is all great advice/knowledge....  thanks. 

[Brian Skalinder]

If it was me I would make sure that both ADC's were in the same brand name. And in the same devices. Because just because the ADC is the same chip does not guarantee a sync.

If the two ADCs are synced via word clock, why does brand name and ADC chip matter?