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R/E/P => Recording - Engineering & Production => Topic started by: dennis1127 on November 29, 2011, 07:41:04 pm

Title: starting out- some questions about mobile recording and jitter
Post by: dennis1127 on November 29, 2011, 07:41:04 pm
I'm a composition student at Cal State Northridge and I'm just getting involved in starting to record my compositions, so I have some questions.

But, I am an experienced audiophile with way the hell too much *Playback* equipment. Also I worked for a while as a programmer and have a degree in electrical engineering. So I know a lot about the technical aspects of digital, formats, some stuff about analog, etc.

I want to have a mobile laptop-based recording setup. I already have an M-Audio Fast Track Pro. It's not high-enough quality for me as an A/D, but it has a S/PDIF input, so I could get another higher quality portable A/D unit with S/PDIF output and send it to my laptop via the M-Audio.

I'm interested in the Apogee Mini-Me, for one.

As far as getting this stuff to work, I think I have the technical know-how, but I have some questions about jitter and digital performance.

First let me say that I have had good success with my playback equipment by using a battery for low-power circuits (such as the DAC) rather than power from the wall. Battery power is much cleaner. I also use a set of parallel capacitors to provide for instantaneous current delivery on transients.

So I want to do something similar with my portable setup.

For the M-Audio Fast Track Pro, I have already tested it like this: I cut the red line in the USB cable to cut the dirty power from the computer, and power it from its 9V input instead (using a 9V battery, or I can also use a 9V linear regulated supply). It works fine.

To consider the Apogee Mini-Me, it can run from a range of inputs from something like 7V to 16V. I already have this nice 12V SLA battery (with the capacitors) so I can use that to power the Mini-Me.

The only connection between the Mini-Me and the Fast Track Pro will be a digital coax (RCA) cable.

Okay, so my question is: is the jitter behavior primary determined by the accuracy of the clock in the Mini-Me? In other words, does the (rather cheap) Fast Track Pro have no influence on the jitter that affects the A/D process, and does it merely have to transcribe the bits?
Title: Re: starting out- some questions about mobile recording and jitter
Post by: Fletcher on November 30, 2011, 09:16:30 am
You probably want to pot this again in Bruno's forum... he's one of the better digital audio designers on the planet and can probably give you better insight on this question than the folks that hang on this forum.  I didn't move the post because there may very well be some folks that hang on this forum that might be able to shed some light... I just know I'm not one of them.

Title: Re: starting out- some questions about mobile recording and jitter
Post by: Hollis on December 13, 2011, 04:40:23 pm
Jitter in a transport (SPDIF) is not an issue. The rule of thumb for best-case jitter performance is the device doing the conversion should use it's internal clock (master)--not externally clocked. If jitter is induced at the ADC stage there is no way to remove it later. As Fletcher says, see Bruno's forum, Dan Lavry's old forum (http://recforums.prosoundweb.com/index.php/f/38/0/), or Bob Katz's digido.com.

Just found this, probably the only thing you ever need to read about jitter: http://recforums.prosoundweb.com/index.php/m/31331/0/?srch=Jitter#msg_31331