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In Reply to: RE: digital or analog posted by Roseval on December 07, 2014 at 11:46:04
The 75 ohm impedance is only good at rf frequencies. At audible frequencies the impedance is all over the place. Also rca connectors do not spec there connectors at all . Only bnc connectors are true 75 ohm and again only at rf frequencies
Alan
Follow Ups:
The relevant factor is the length of the cable vs. the wavelength of the signal. At audio frequencies the wavelength is long, e.g. ten's of kilometers. No problem for audio interconnects, but of interest to telephone companies. As Roseval pointed out, SPDIF operates at RF frequencies and cable lengths need to be extremely short before impedance doesn't matter.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Extremely short S/PDIF cables whose impedance is NOT properly controlled to 75 ohms can create signal reflections between source and DAC. These confuse the DAC's clock and cause clock jitter. The sonic result is a loss of focus and, well, timing.
If one has no idea whether impedance is controlled properly in any of source, cable and the DAC itself, it's best to go with a coax cable 1.5m long. This gives any reflections time to travel down the cable and back slowly enough that they arrive at the DAC offset from the true signal step.
The issue here is the rise time of the signal, with fast rise times involving much higher frequencies than the base signaling rate. However, the rise time is not infinite and the frequencies involved are limited. If the cable is sufficiently short then there won't be an issue, i.e. the cable will not have to be analyzed as a transmission line. If the cable is short enough, reflections from previous transitions will have died out as well. This means that there won't be audio signal dependent jitter.
A DAC that uses SPDIF must have some means to deal with jitter on its input, e.g. some kind of reclocking or clock cleaning, typically done with some combination of buffers and PLL(s). If it doesn't have this then the DAC is junk. If it does jitter on SPDIF will have at worst a second order effect on sound quality (e.g. residuals sneaking pass a PLL) or a tertiary effect (e.g. ground bounce on DAC clock circuitry cause by noise generated by the SPDIF receiver circuitry bypasses reclocking).
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Ok. I fully understand. But, If i'm using the Huffman audio cable has a digital cable doesn't that mean I am getting all kinds of reflectionsand cable induced jitter yet this cable sounds better to me than the digital cables. Is it possible that the Huffman analog cable has been designed more like a digital cable so it is near 75 ohms at rf frequencies yet would still sound fine as an analog cable? Huffman does make a digital cable. Sorry to be such a pest.
Alan
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