Digital Audio: why cd players sound different

A friend commented that my stereo system sounded good, but the CD player lacked detail. I set out to learn why cd players sound different. Since music is stored digitally on a CD, and my CD player is attached digitally to my pre-amp, I wondered why my pre-amp wouldn't receive an exact copy of the recorded music.

One source of errors is reading the CD, since there are no checksums in the Redbook CD audio format. The other source of errors is data transmission between the CD player and pre-amp, since it also lacks checksums. Since consumer digital audio connections run about 1000 times slower than modern computer CPUs, and over 100 times slower than a good external SCSI bus, I figured disc read errors were the culprit. However, disc read errors usually result in popping sounds, because bad bits make sudden changes in volume, and that's much different than a lack of detail.

I did an experiment with a network audio player, the Momitsu v880n, to eliminate the effect of disc read errors. I did an overlapping-sector rip of a favorite music track on my desktop computer, creating a virtually error-free copy of the CD's information. I transferred the this error-free copoy to the Momitsu over ethernet via TCP, thereby eliminating undetected transmission errors. The Momitsu then sent this pristine, digital audio data over it's digital (spdif) output to my pre-amp. I expected to hear the best possible audio reproduction my stereo could produce. Instead, the result was lackluster, slightly worse than my main CD player (Rotel RCD-02), and much worse than a high-end player I borrowed (Lexicon RT-10).

I concluded that transmission of digital audio data over an spdif interface was the problem, or else the high-end CD player "touched up" the data it read from the CD. Both explanations seemed unlikely. Since Lexicon probably wouldn't reveal their audio secrets to me, I studied the spdif protocol and electrical specifications. If you are interested, I recommend http://www.epanorama.net/documents/audio/spdif.html, or else search Wikipedia. I began to believe that timing errors called "jitter" were responsible for the audible difference between CD players using digital connections. You can read a lot about jitter and CD players at http://www.stereophile.com/features/368/ and http://stereophile.com/features/396bits/. Those articles are somewhat old (mid-1990s), but they present convincing arguments that jitter is a real problem in digital audio systems.

That said, shouldn't modern CD players have addressed and eliminated jitter? The answer is no. Though an Ultra160 SCSI bus built in 1999 can move 160MB per second over several meters of cable with no errors, the spdif interface can't even move 1MB of audio over one meter of cable without errors. The spdif interface is old, and that's part of the explanation. Still, I didn't believe any of this until I conducted a few experiments of my own, using an oscilloscope. I've made a hasty write-up of my conclusions, which you can find at http://komarix.org/per/computers/spdif.

I will highlight the most interesting points and graphs from my experiments in a later post. The summary is that even a very primitive experimental setup (read "ghetto") can reveal errors in spdif data transmission, and those errors correspond directly to subjective assessments of audio reproduction quality. I tested four CD players with three listeners, one of whom let me conduct a single-blind a/b test. The measured error rates of the CD players corresponded perfectly with the listeners's preferences. Unfortunately, the error rates also corresponded to price, with the most expensive player (the Lexicon RT-10) have the lowest error rate (when using it's AES/EBU digital interface, instead of the lower-quality spdif).

The Lexicon was so good that my wife complained: "I don't appreciate how bad it makes our Rotel sound." In fairness to the Rotel, the Lexicon RT-10 retailed for over $3000 in 2003, versus $500 for the Rotel RCD-02 in 2004.

Why haven't we seen newer, better digital transmission protocols appear on consumer audio equipment? Why do $3000 CD players use inferior technology, when compared to $100 portable music players? Why hasn't firewire replaced spdif? My best guess is that the RIAA and music publishers are discouraging high-quality interfaces, because they worry consumers are thieves. With crappy interfaces like spdif, the RIAA can rest assured we'll rarely hear what is recorded on the CD. If this argument seems far-fetched, consider that Sony's high-end super-audio CD (SACD) specification prohibits digital audio transmission when using surround sound. This means we have to use analog transmission when listening to the highest-quality digital recordings.