Computer Audio Card vs. External A/D Question

[Bob's Mods]

Has anyone who has recorded using both a decent sound card and a decent external box, ie. firewire, S/PDIF, noticed and difference in quality? I do not see how there could be a difference. If you have a sqeaky clean power supply in your computer it should not really matter.

Bob the Mod Guy

 

[pipelineaudio]

There are some measurments that you can make that will affect the audio you are recording:

1. noise

2. distortion

3. frequency response

Number one can be greatly influenced(but doesn't have to be) by having the converters inside the computer case. The other two have a lot more to do with the component choice and implementation.

 

[Bob's Mods]

What kind of noise? Baseline noise? A higher noise floor? I would think that as long as your power supply is clean there should be no difference. Once the analog signal is converted to digital, noise flying around the computer case should no longer matter. The analog path is generally very short from the input to the convertors and does not stay in that form long. Also if the analog path is properly shielded up to the convertors that helps a lot.

Bob

 

[pipelineaudio]

For instance, look at two different mic preamps, both with power supplies far better than anything you'd find inside a PC and notice that there may be a noise difference between them.

You are right to assume that once it becomes digital, the audio quality is locked and, except for dropouts, poor clocking implementation, driver issues and the like, the audio quality shouldn't change

But getting to digital is a big deal indeed. There are all sorts of ways to screw it up.

For some of us, all we want is to make our mics loud enough to get to the buffer amps or whatever of the A/D input (and hopefully the A/D itself wont have too much effect on those three biggies), but again, its not an easy thing to do.

A soundcard that has a stereo input where both left and right channels share a ground and are unbalanced probably also made a lot of other unfortunate choices elsewhere in the chain

 

[SonicAlbert]

There is a long thread about this already here at HomeRecording, but I notice that my MOTU PCI interface sounds better than my MOTU Traveler firewire interface. And that's on straight digital transfers/playback. Don't know why, and in the previous discussion I had at least one person emphatically tell me I was wrong. But nevertheless it is something I could hear. Which is why I'm still using my PCI interfaces in my home studio.

 

[dgatwood]

There is a long thread about this already here at HomeRecording, but I notice that my MOTU PCI interface sounds better than my MOTU Traveler firewire interface. And that's on straight digital transfers/playback. Don't know why, and in the previous discussion I had at least one person emphatically tell me I was wrong. But nevertheless it is something I could hear. Which is why I'm still using my PCI interfaces in my home studio.

If you mean capturing data via S/PDIF, there's no possibility that there is any difference in the data. A digital bitstream is a digital bitstream. If the data isn't identical on one end of an S/PDIF connection to the data sent on the other, the device is just plain broken.

If you mean listening to software play-through while you're capturing the digital data, I could believe that you'd hear a difference there, though. Any D/A conversion is going to be affected by clock quality and the quality of the reconstruction circuit.

If you're driving both a 2408 (for example) and a Traveler with the digital signal from a digital device, they're trying to lock their clock to the incoming S/PDIF signal. Depending on how good or bad their PLL circuit is, this can introduce significant jitter, which results in poorer sound quality. As soon as you yank the external clock, however, that goes away. A that point, the data you captured digitally should be identical regardless of which of the interfaces was used to capture it initially. (I'm not saying that the data will sound identical no matter which device you use to play it back---the analog sections of those interfaces are likely pretty different---but if you play signals captured with both interfaces using one interface, there should be precisely zero difference. You should be able to a bit-for-bit comparison and find no differences....)

BTW, to my knowledge, MOTU has never built any interfaces with converters "in the box". That 324 card is just a fat digital pipe. All the interesting stuff is happening in the outboard unit. Even their NuBus board was digital-only inside the computer....

 

[SonicAlbert]

Yeah, I can't explain it, but the PCI interface does sound subtly better than the firewire Traveler. I think it has to do with jitter somehow. And I am referring to a straight digital data pipe, yes.