where does the AD conversion take place?

[rec-tec]

hi all

this is a question to which the answer is probably yes (or should that be no - i thnk i have confused myself!)

i want to get a new set of mic pre-s and the audient ASP800 looks just the ticket (i was considering the mico - but you get 6 xtra channels for only a bit more with the ASP800) with the well respected pre's and the hmx and iron settings - oooh nice. it really does look remarkable value - anyway that aside.

the question is - is the quality of audio signal affected in anyway by the sound card used? i am assuming that all the conversion is done in the ASP800 and the soundcard simply acts as a conduit to the computer so it only needs ADAT in (JFI i want to put it through the MOTU 828 mkIII; BTW i also want to be able to plug in a TC gold channel through spdif and an octopre through the other ADAT in - i think this will restrict the ADAT to 44100 or 48000??)

your thoughts would be welcomed < on both questions (ie signal quality and plugging in the other two bits of kit)

cheers

RT

[Exalted Wombat]

If you're looking for preamps that have a "sound", you're ASKING for the audio quality to be affected

Yes, the A>D conversion happens before the ADAT output.

It really isn't about the gear, you know. But bless you for keeping the industry ticking over.

[Jack Ruston]

Just be aware that while it's possible to connect all these different converters it makes for a complicated life. You can't multitrack across them easily because the latencies of the converters is different, so you wind up with phase problems between the different inputs - Now it probably won't matter if your drums are all on one, and your guitars are all on another etc...but it depends obviously how great the discrepancy is. You would't want kick and snare on one unit and overheads on another for sure.

Furthermore, you'd need to tell the software manually what the AD latency was in order for audio to be correctly positioned on the timeline...ie compensated for the delay between when it's played and when it appears in the DAW...Almost all DAWs except Pro Tools allow you to make those compensations manually to allow for delays which the audio device (ie the MOTU) can't 'see'. But this is not normally done on per channel basis. So you'd need to change that value every time you used a different converter if you wanted what people played to wind up on the timeline at the instant they actually played it.

J

[rec-tec]

Thanks for your replies

I usually adjust for latencies in ableton (yup a bit of a pain)

So, to the original question the answer is that the interface won't affect the sound and multiple units, tricky but possible with a little hassle.

Cheers

RT

[Exalted Wombat]

You've got plenty of analogue inputs on the MOTU. Why not use them? The selection of preamps you mention establish that you're not after squeaky-clean digital realism.

[Hugh Robjohns]

Just be aware that while it's possible to connect all these different converters it makes for a complicated life. You can't multitrack across them easily because the latencies of the converters is different, so you wind up with phase problems between the different inputs - Now it probably won't matter if your drums are all on one, and your guitars are all on another etc...but it depends obviously how great the discrepancy is. You would't want kick and snare on one unit and overheads on another for sure.

Furthermore, you'd need to tell the software manually what the AD latency was in order for audio to be correctly positioned on the timeline...

While I agree about the complexity issue -- but more from the perspective of clocking than anything else if the OP adds other digitally-connected preamps -- I don't think converter latency is a serious problem.

Yes, I absolutely concur that tracking closely related sources (especially stereo pairs) across different preamp/converters would be a bad idea from a phasing problem point of view, I think that practical converter latency differences between preamps will be of the order of less than 1ms which is the equivalent to moving a mic a foot closer of further away... and so won't really be relevant if we're talking about guitars or vocals on one preamp/converter and drums on another.

I certainly wouldn't bother faffing about with telling the DAW what the converter latencies were... just as I don't when I close mic some things and use more distant mics on others!

But, as I said, if you are thinking of using a second set of mic pre amps on the second motu ADAT input, and something else on the SPDIF, then look very closely and carefully at how you're going to synchronise the word clocks on all these different devices. That's what's going to trip you up...

H

[Jack Ruston]

Oddly enough I was just earlier hunting for that link to (I think) Martin's clocking article from an issue passed.

I agree and disagree on this one Hugh. Yes, I agree that no-one is going to worry too much about a few samples, but where do you draw the line? It's not important if you record everything in a pass or two, but what if you have hundreds of tracks of overdubbing, which is very common in a lot of pop production. If all these overdubs are slightly late, it comes down to what the talent is 'locking on' to. Call me picky but I want to know that the audio sits on the timeline where the artist played it. Not a millisecond, or two, or five later.

J

[Hugh Robjohns]

Yes, I agree that no-one is going to worry too much about a few samples, but where do you draw the line?

I draw it on whether the additional effort required will make any difference that can be heard.

Call me picky but I want to know that the audio sits on the timeline where the artist played it. Not a millisecond, or two, or five later.

You're picky!

Seriously, though, if we are just talking about the difference converter latencies caused by different manufacturers using different A-D parts in their boxes (which I think we are), then the latency variation will be much less than a millisecond. Most converters impose around 0.75ms delay -- the equivalent of about eight inches of air. The difference between two different units is never going to be anywhere near 2ms, let alone 5ms, and usually the difference between two preamp/converter boxes will be less than 0.25ms (equivalent to three inches of air between source and mic).

Putting that into context, your snare drum mic is probably within an inch or two of the snare head, but your overheads are probably about a foot above the cymbals and four feet above the snare... and your vocal mic might be anywhere form three-six inches from the vocalist. What distance (latency) do we use as the timeline reference?

So, unless you get the tape measure out to make sure that your mics are always placed at exactly the same distance from each and every one of your sound sources, the small converter delay variances in different preamp/converters really won't make any difference whatsoever provided you don't use different preamps for multiple mics that really do require phase-critical relationships (which really only means coincident stereo pairs).

Personally, I think there are far more significant, practical and audible things to worry about. Just don't connect the left mic of a pair into one box and the right into another -- the same advice we gave when syncing two 24-track machines back in the olden days.

H

[Jack Ruston]

Goddamit I KNEW I was picky

[Hugh Robjohns]

[rec-tec]

But, as I said, if you are thinking of using a second set of mic pre amps on the second motu ADAT input, and something else on the SPDIF, then look very closely and carefully at how you're going to synchronise the word clocks on all these different devices. That's what's going to trip you up...

H [/quote]

(how do you make the quote box??)

thanks for that hugh - this leads me to ask - how do i synchronise word clocks on all the different devices - do i need somekind of external clock and chain the devices together (or some such wizardry)?

(we are all very lucky to share the knowledge and experience of hugh - so thanks for your enduring support of basic questions)

cheers

RT

[Hugh Robjohns]

(how do you make the quote box??)

You place the word quote inside square brackets to start the quoted text, and /quote in square brackets to end it.

how do i synchronise word clocks on all the different devices - do i need somekind of external clock and chain the devices together (or some such wizardry)?

Ah.. yes... well that's where it gets interesting, isn't it?

Rule no. 1 is that there must be only one clock master for the whole system. Everything else has to be clock-slaved to that master.

Any device could serve as a master -- the interface itself, or any of the external preamp/converters -- but the choice will depend on the clocking capabilities of everything else and how you want the system (and DAW) to work as a whole. You could even use a dedicated external master clock device and run everything from that. However, not everything has an external clock input (or not in the most convenient format), or sufficient clock outputs, so the availability of physical clocking connections might well determine the most practical configuration.

These articles should help to point you int he right direction:

http://www.soundonsound.com/sos/apr03/a ... ocking.asp

http://www.soundonsound.com/sos/feb07/a ... facing.htm

http://www.soundonsound.com/sos/apr13/a ... 0413-2.htm

H

[rec-tec]

cheers hugh

RT