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RE: AES/EBU digital 110 ohm characteristic impedance

Posted by Thorsten (M) on March 9, 2016 at 19:38:09

In Reply to: AES/EBU digital 110 ohm characteristic impedance posted by Duster on March 3, 2016 at 18:40:57:

Hi,

The simple truth is:

1) An RCA plug will show a significant deviation from a nominal 75 Ohm characteristic Impedance.

2) A 75 Ohm rated BNC Plug will show a significant deviation from a nominal 75 Ohm characteristic Impedance (but less than RCA).

3) An XLR Plug will show a significant deviation from a nominal 110 Ohm characteristic Impedance.

4) Cables will show variance in characteristic impedance from that rated, which can be determined from official specifications.

5) Sources and sinks will vary in characteristic impedance from that rated, which is down to design and can be rarely (if ever) found in specifications of equipment and is non-trivial to measure..

With sources, Sinks, Connectors and Cables being substantially variant from "ideal" conditions for transmission lines what we get once we combine it all is somewhat unpredictable.

I always find it hilarious when people claim that fitting BNC connectors to something I have previously analysed and found to well outside formal specifications for source or sink impedance made huge improvements, because now the impedance is correct over the horrible RCA Plugs (talk about closing the stable door after the Horses have bolted, the Grooms have looted the till and burned down the stables).

The truth is that they simply changed the mix of different impedance deviations and have done zilch to correct the big and fundamental ones.

The most egregious offenders BTW are transformer coupled digital outputs that are done by "cookie-cutter" copying from some app note.

They simply do not work (ok, they pass signal, I guess to some that means "works") and most of the time levels are wrong, impedance is wrong and the leakage inductance of the transformer is not compensated, which leads to huge deviations from nominal characteristic impedance as frequencies go up.

I have seen one correctly designed transformer coupled output (and input) in commercial gear. This was designed by someone who got banned from all discussion boards because he would hurl abuse at people who talked about "75 Ohm RCA" and "75Ohm BNC" and debated the merits of heavily advertised SPDIF/AES-EBU transformers that are designed in a manner and fashion that absolutely maximises impedance mismatch with rising frequencies.

In fact, I have also seen people fitting such transformers and claiming huge sonic benefits, because these transformers are better, when objectively mismatch is worse, reflections are through the roof and jitter is up sometimes 10-fold over an unassuming industrial part actually better suited to the job.

What is the upshot?

There is no ideal passive SPDIF/AES Cable because there is no ideal connector and because even if they existed, the source/sink devices muck things up.

The ideal SPDIF Cable would be active, eliminate impedance mismatches at the source by having an active circuit build into the source side plug that ensures the signal from the source is received and corrected for waveform deviations, excessive noise etc. and ideally the clock would be extracted from the SPDIF stream (or even better provided by the source on a second connector!).

We then would then use LVDS or similar low skew, high speed, low jitter balanced connections between source side and output (can use Gigabit Ethernet cabling), preferably with a separate line for the clock and then re-clock our spdif signal just before it enters the output plug and include options for impedance and level adjustment's.

Then we can forget all about the whole cludgy mess called SPDIF or AES-EBU, as in practice all the relevant (bad) design issues would be cancelled respectively at source and sink.

Of course, we would still have awful PLL designs in the DAC's (read off the shelf Cirrus Logic / AKM receiver IC implemented "cookie cutter style) with to contend with, which would louse things up badly no matter how good a signal you would feed it.

Hence I never bothered productising this idea, but went USB instead. Give me USB and daisychained doohickies every day over that other mess...

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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