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RE: we are talking termination

Fred,

Let us be clear, for both SPDIF and USB Cable impedance and termination (in the technical sense of transmission line termination http://en.wikipedia.org/wiki/Electrical_termination) matter greatly.

You can read more why here:

http://en.wikipedia.org/wiki/Reflections_of_signals_on_conducting_lines

Incorrect cable impedance or termination resistance will cause reflections that will corrupt the signal. A short or open circuit will reflect 100% of the signal. Less extreme mismatches reflect less signal.

Discontinuities in impedance at the connection points also can matter IF (and that is a big if) they are significant in length compared to the wavelength of the signal or to the equivalent in distance of the rise time of the signal.

So we need to look at the signals.

SPDIF - uses manchester code so appx. 3MHz (44.1K samplerate) to 12MHz (192KHz)

USB2.0 at high speed - uses a complex bidirectional coding scheme, 480MHz maximum frequency

So USB at high speed contains 40 Times the frequency of SPDIF at 192KHz and thus can tolerate different impedances along the line of 40 timesless than SPDIF.

For reference, at 12MHz we are dealing with a wavelength of 17.5 meters! The impedance discontinuity of an RCA plug is pretty much irrelevant at this frequency.

Measured on a TDR a 75R BNC connector can show that it is actually 66 Ohm and causes a glitch of appx 4.2mm total length.

RCA Connectors (plug & socket) show an impedance of 52Ohm (this is rather more variable) and causes a glitch of around 12mm total length.

This translates to a lot less than a 1nS glitch in either case with a 12% (BNC) and 30% (RCA) of the signal reflected.

Now the transition time for the LMV7219 Comparator (e.g. the SPDIF input often used with the ES9018) is on average around 10nS. This is much better than common SPDIF receivers, which are by far slower.

If such a glitch as described arrives close with the signal causing a transition on the receiving end it will simply be swamped by the comparators (or SPDIF receivers) slow transition. If the reflections are significantly delayed they simply do not matter.

So, bottom line, both BNC and RCA cause some reflections, both reflections are short enough in the time domain to not matter at SPDIF Data rates, objectively speaking.

If you have objective data that proves different, make it public.

Thor

At 20 bits, you are on the verge of dynamic range covering fly-farts-at-20-feet to intolerable pain. Really, what more could we need?

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