A "bit" of information


Besides word clock jitter, which some DACs are capable of almost eliminating (Benchmark DAC-1), what other things can make one transport sound different from another? Aren't they just machines that spit out a stream of ones and zeros? Is it all just cosmetics for more money?

Thanks
koestner

Showing 3 responses by audioengr

I am an engineer, with 30 years digital design experience, so I'll be happy to give you the technical explanations:

1) The S/PDIF output from a transport is specified at 75 ohms characteristic impedance. This means that the output impedance of the transport should be 75 ohms and the input impedance of the DAC should be 75 ohms, as well as the connecting cable and connectors. Mismatches in this impedance can cause reflections that cause jitter, and these mismatches are quite common in even the best transports. I have measured and fixed plenty of them.

2) The slew-rate or rise-time of the S/PDIF digital signal has a large effect on the jitter when the signal is detected at the DAC. All transports have quite slow risetimes by design in order to pass FCC emissions testing with the poorest S/PDIF cable attached and mismatched impedance. Therefore, they cause unneccessary jitter. Some Transports will have slower risetimes than others, but unless they are modded, they are all slower than desired.

This white-paper I wrote details the analysis of why the cable length, impedance matching and risetime are all critical to minimizing jitter:
http://www.positive-feedback.com/Issue14/spdif.htm

Steve N.
Empirical Audio
Manufacturer/modder
Rockvirgo makes an important point. This is why computer audio is intrinsically better than CD playback. The pits are jittery as written on the disk. The Transport also adds jitter when it reads these pits, because each read head has different accuracy of reading the pits. When a pit is actually detected on an inside track and on an outside track probably varies from one transport to the next.

Steve N.
Empirical Audio
Manufacturer/modder
"And the cheap CD drives in PCs don't add jitter when they
read the data on to the HD, and the noisy PC PSU doesn't effect the output signal?"

No. I'm talking about ripping the CD and playing it back from RAM on the computer. This is computer-driven audio. The CDROM drive is not involved in the payback. As for the noisy power supply and noisy ground, there are converters that go from USB to S/PDIF that have their own power supply or even a battery supply and they even isolate the computer and audio system grounds, so no noise of any kind gets injected from the PC. The USB interface does not affect the sound quality, no matter what the length.

This truly is the highest quality method for playback of CD images.

Steve N.
Empirical Audio
Manufacturer/Modder