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?

[Arnold B. Kruege...]

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.

According to this published test from an independent reliable source, your USB audio interface performs at a very adequate level for your intended purpose:

Link to iXBT Labs Audio Rightmark Test Results

"
Frequency response (from 40 Hz to 15 kHz), dB +0.10, -0.08 Very good
Noise level, dB (A) -96.1 Excellent
Dynamic range, dB (A) 95.9 Excellent
THD, % 0.0046 Very good
THD + Noise, dB (A) -82.7 Good
IMD + Noise, % 0.0077 Excellent
Stereo crosstalk, dB -96.1 Excellent
IMD at 10 kHz, % 0.0075 Excellent
General performance Excellent
"

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.

Frankly, he is talking out of the back of his neck. ;-)

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?

Depends what you call a low quality ADC. Back at the turn of the millennium, people were still putting ADCs and DACs on PC motherboards and PCI add-on cards whose performance with 16 bit data was essentially equal to 8 bits. They had dynamic range of less than 50 dB and clearly added audible noise. I even ran into some motherboard DACs that for all the world sounded like they were not only 8 bits or less, but were also undithered. The original SoundBlaster 16's ADC would switch into 8 bit mode when run full duplex (recording and playing at the same time).

A low quality DAC or ADC will add noise, and if it is undithered it will also add low-level distoriton. In addition there have been DACs that had no reconstruction filters, and actually output the stairstep waves that some analog advocates still use to characterize all digital equipment.

The DACs in CDROMs and DVD drives for your PC are still pretty poor to this day. Not only are they noisy, but they have simplified reconstruction filters that are either peaky or prematurely roll off the high end.

Even today some common consumer-grade audio interface cards will have relatively substandard ADCs that not only have poorer (now about 80 dB) dynamic range and may be 3 dB down at 16 Khz which might actually cause audible dulling or certain sounds. I've also encountered DACs with too-small coupling capacitors that rolled off the low frequencies into a headphone load, starting as high as 100 Hz or more.

To summarize, a poor converter can add:

(1) Audible noise
(2) Audible low level distortion
(3) Audibly poor frequency response, particularly but not limited to the high end of the audio band.
(4) Ultrasonic noise that may spread down into the audible range by poor-quality analog amplification.

That all said, there are sonically blameless converter chips that cost less than a dollar as parts, and are found in products costing a few dozen dollars. For example the CPU System chip in a Sansa Clip has 2 90 dB dynamic range DACs and 2 slightly lesser ADCs , and 2 output capacitorless headphone amps built into it, and costs less than $10 in quantity. The finished product incorporating it with 4 Gb flash memory, li ion battery, display and case sells in stores for as little as the $20 range.