Why do digital cables sound different?

1439bhr, I got a question I am sure you can answer. Does the low pass filter on the output of the DAC also handle any aliasing that may be present? Also, how much phase noise is acceptable in the system before there is any degredation and does phase noise closer to the carrier or farther away from the carrier seem to have the most effect. Thanks in advance for your response.

Using a FIFO and reclocking is not the end all and is not so simple. When the Gensis Digital Lens came out, Bob Harley assumed that all transports would sound exactly the same. He found that he was wrong. Rather than reclock, the best solution, given the current CD methodolgy is to have a custom data transfer mechanism like Lvinson and Spectral. I believe that the SPDIF interface is inadequate for high quality playback. I did some testing about 7-8 years ago to prove that there was no difference in transports or digital cables and published that dat on the net. At the time I had only worked with proprietary digital record and playback protocols and was not that familiar with the CD playback mechanism. My tests proved just the opposite of what I thought. Not only that but subjectively the tests seemed to show that jitter was not the most important of the digital flaws, both read errors from discs and signal loss of high frequencies seemed to be more of a problem. While discman buffering is universal and basically required for a device that bounces around, everyone I've ever owned always said to turn off the buffering when not needed because the sound was not as good as the direct signal. I still believe that the fewer components in between the transport and the DAC the better. Buffering of CD data has all sorts of other issues that need to be handled like skipping forward / back etc that make it impractical since you always have to handle the case when the buffer is empty. All the sysytems I have designerd, and most commercial systems with proprietary protocols in general send raw data to the DAC and the DAC interface has the only clock and jitter is at an absolute minimum. Jitter is a fact of life in any digital medium. The use of the clock has been there since digital technologies in the 60s. I always get a kick out of manufacturers who claim jitter reduction devices that produce 0 (zero) jitter. It's physically impossible to accurately measure jitter anyway. We can improve the accuracy, but there is always some small error in the measurement. This error will decrease as technology improves. By the way jitter errors have a much more detrimental effect than just causing pitch errors. Lack of perfect integrity of the audio signal effects soundstage and imaging and if jitter is so bad that the dac syncs incorrectly on the signal, severe static can be produced. See my previous postings.

I have read your post and can not dispute your claims with the exception with the exception of one. Jitter is not very difficult to measure accurately if you understand the measurement concept. Jitter and be made simply with a spectrum analyzer and a calculation. All that is needed to convert phase noise to jitter is a simple calculation. A spectrum analyzer can be used to measure phase noise if the device to be tested has little or no AM because the analyzer can not tell the difference between AM and phase noise.

The Levinson DAC provides SPDIF, AES/EBU, and Toslink input (and ATT, I believe) interfaces. It does not support other "custom" interfaces such as I2ES, which attempt to deliver accurate, jitter-free clock signals to the DAC. Problems with the transport such as dropped data bits, etc., is a separate issue from the effect of digital cables. A back-of-the envelope calculation reveals that clock jitter needs to be less than 100 picoseconds to ensure that all jitter artifacts are at least 96 dB down from a signal at 20 kHz (the most stressing case). Bob Harley notes in his book "Guide to High End Audio" that several popular clock recovery chips, based on PLLs, produce about 3-4 nanoseconds of jitter (about 16 dB worse than the 100 ps reference point). Delivering a 44.1 kHz clock signal to a DAC and assuring jitter less than 100 ps has a stability of about 4 parts per million, which is achievable with competent design. Devices such as a Digital Time Lens, which provide an SPDIF output, remain at the mercy of clock recovery jitter at the DAC. The best they can hope to do is to stabilize the transmit end clocking as seen by the clock recovery circuit.

Gmkowal 2 points here. It ought to be obvious that your test is inadequate, simply use a CD source. It should take about 4 seconds if memory serves me to fill a 1 meg buffer of SPDIF data. That's real time. When I say jitter can not be measured accurately I'm simply pointing out that any device that measures jitter has to have a clock, that clock has jitter, so your measurement has to be off by some amount. That amount may be small but it always exists. My point was that many claims of jitter reduction devices are total BS. 1439bhr I thought the Levinson also had a proprietary data transfer interface. My point was that some component combinations do and they seem to do the best job. My points were simply that jitter reduction devices are a poor substitute for a good implementation between a DAC and transport. They may even incrrease jitter. My second point is that the jitter on most "good" systems is low enough to not be a major factor in the sonic degredation. I believe jitter is third behind errors in the transport and signal loss of high Khz signals.