Digital Source Comparison - An audible difference?

Al - I can understand the concept of reflections in a digial cable, but is there concrete evidence that these reflections will actually cause an error in the transmitted data? Can the reflections actually cause the "pulses" to be outside of the acceptable range? I would assume that the DAC is looking for specified ranges with some tolerance applied.

Hi Mceljo,

Assuming that the impedance mismatches are within limits that are not totally unreasonable (such as might occur if the connection of the shield in a coaxial cable were broken), the resulting reflections will not cause any errors whatsoever in the digital data as received by the dac.

The issue is that the waveform distortion caused by reflections, in combination with noise that will invariably be riding on the signal to some degree, will result in small but significant fluctuations in the TIMING at which the dac senses the transitions between bit intervals. That is what the term "jitter" refers to. Depending on the jitter rejection capabilities of the dac, that can and often will have audible consequences, even though all the bits will be correct. See this paper for further explanation.

Best regards,
-- Al

Al,

I passed the two main links along to an EE friend of mine that's my sounding board for all things electrical. Here's part of his response.

"Those are the first articles you've sent me where I completely agree with everything written. This guy is obviously a well-educated electrical engineer.

How noticeable corrections to these factors will be, I do not know. I don't believe that crystal oscillators used in today's computer equipment are much farther off timing-wise than a belt-driven turntable."

In any case, he generally give me a list of things he considers to be "bunk" in the things that I pass along.

Al,

On a somewhat related question to the information in the links. Do you think jitter is a significant contributer to the difference in sound that I get between my Onkyo iPod dock and Ethernet connected Pandora Radio? The other obvious factor is the difference in DACs that are being used. Pandora is using 128 MP3 files and it takes at least the Applelosses on the iPod to sound consistently and sometimes even that doesn't sound as good.

I find myself turning off the iPod because it just sounds subpar, but Pandora runs for hours at a time.

Hi Mceljo,

Thanks for providing the feedback from your EE associate!

I have no particular familiarity with iPods, docks, Pandora, etc. But assuming that the DAC function is being performed by the component into which you are connecting the dock's cable and "universal port" connector (as opposed to being performed by the dock itself), yes, it is certainly conceivable that jitter could be a significant factor. The magnitude and character of the resulting effects would be dependent on the iPod, the dock, the cable, the input circuit of the component containing the DAC, any noise that may couple onto the relevant signals, and the jitter rejection capability (if any) of the particular DAC.

Another thing to consider, though, especially when dealing with mp3 files, and considering the poor engineering of many recordings generally, is that increased reproduction accuracy can often result in less enjoyable sound. A traditional audiophile dilemma :-)

Best regards,
-- Al