Quality aspects of ADC, Does quality gear makes a difference? Options

[ktf]

Hi all!

Background information (safe to skip )
Currently I'm preparing a large archival operation: my musical society has a collection of recordings on compact cassettes from 1980 to 1990, about 140, which I think have to be digitalized to protect them from deteriorating. I have found a nice tape deck (Nakamichi DR-3) but it has to stay at the office of the executive committee. A friend of mine has a nice ADC for this job, a quality portable mixing console with USB out, but it cannot stay at the office as it is more or less public and he doesn't trust everyone there. However, it is too much hassle to bring it every time, as I think this operation will cost about a year: 3 recordings a week. So, I guess I'm stuck with my own ADC: a Creative X-Fi Surround 5.1 USB-card, which is not really made for the job, but I can at least leave it at the office, as it isn't that valuable.

On-topic
That friend of mine that offered me his portable mixing-console with ADC argued that with a cheap ADC all little nuances and details would vanish, so I should look for another solution than using my Creative ADC. That made me think: i'm much more a theoretician than he is, so I was wondering, what makes a high quality ADC? I know the working principle of several ADCs, I can't imagine which link in the chain would be capable of 'erasing details' anyway. How does a non-linearity in an ADC sounds like in practice? And jitter? I guess uncorrelated, random jitter could sound like 'more noise', as it is random. To me it seems a low-quality ADC doesn't add anything but noise and a non-linearity, let alone it could remove things from the signal.

In short: what could a low-quality ADC do with the signal except adding noise? Probably I'm far too short sighted, can anyone enlighten me?

[DVDdoug]

The only thing I'd even consider is noise. And, unless the ADC/soundcard is defective the noise will be very low compared to the background noise on the cassette tape. Most soundcards are superior to cassette in frequency response & distortion.

Probably the most important thing is to get your recording levels right. You want a strong enough signal so that the ADC noise remains insignificant, but you don't want to clip (distort) the ADC either.

If altering the sound doesn't violate any archiving principals, you might be able to use some digital noise reduction and/or EQ to make the digital copy sound better than the analog cassette! Or, you can keep both an unaltered archive and an enhanced version (something I've done). Just beware that noise reduction can introduce artifacts (depending on the source material) and of course any EQ "improvements" involve of taste & judgment.

I have no idea what jitter sounds like... I've never heard it. From what I understand, the amount of jitter in any normally operating device is inaudible. ... You have to intentionally design something with gross jitter before it becomes an issue.

If someone starts giving you a bunch of nonsense about jitter, or "high resolution" audio", or "nuance", Ethan Winer's website has a lot of useful information to bring you back to reality.

P.S.
When it comes to timing-related issues, I be more concerned about the analog side... Tape speed, wow & flutter, stretched tape, etc. These are likely to be at least 10x worse than digital jitter. But again, I've never heard any speed/timing problems from any properly working turntable or tape machine. (I have heard these kinds of defects when the unit had a worn belt, or something was bent, etc.)

This post has been edited by DVDdoug: Mar 4 2011, 00:10

[ktf]

The only thing I'd even consider is noise. And, unless the ADC/soundcard is defective the noise will be very low compared to the background noise on the cassette tape. Most soundcards are superior to cassette in frequency response & distortion.

Digitalizing those cassettes is the case now, but I was wondering about ADCs in general. So, the THD+N (for distortion and non-linearity) and SNR are the only specifications that matter for an ADC?

If altering the sound doesn't violate any archiving principals, you might be able to use some digital noise reduction and/or EQ to make the digital copy sound better than the analog cassette!

I will archive the sound unaltered, but of course, I could improve the recordings if necessary. I'll do that upon request to seperate files, not for the archive as a whole.

This post has been edited by ktf: Mar 4 2011, 16:27