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Jon Risch - 75 ohm output termination

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Bod

i dont understand this. you say the digital out is designed to drive into 75 ohm loads, but actually this is not the case. 75 ohm coax cable isnt 75 ohms end to end is it? so how on earth is a 75 ohm resistor going to stop reflections? it doesnt make sense. 75 ohm coax is rated 75 ohm for more complex reason than it sounds. otherwise different lengths would need different thicknesses etc.

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Re: Jon Risch - 75 ohm output termination Jon Risch 18:54:23 04/25/00 (0)
I worked at a Video comapny for about two years, became quite familiar with Never Twice the Same Color, and RF/video impedance issues.
A properly designed digital output would have a 75 ohm (or approx. 75 ohms, they might allow for the output Z of the amp) build-out resistor, and look into a 75 ohm nominal Z cable, which was terminated by a 75 ohm load (resistor). Without a cable and load hooked up, the digital output stage is firing into an open circuit.
Classic RF impedance theory says that if a transmission line is open circuited, then it will have reflections at certain frequencies. Since the digital output signal has a wide range of signal content with regard to the frequency content, it will have some signal reflected.
This reflected signal can back feed into the input of the digital output circuit (no circuit is perfect, and none have the textbook infinite input/output isolation that the texts imply), and then back into the rest of the digital circuitry, causing jitter.
Terminate this unused digital output with a 75 ohm resistive load, and the reflections no longer occur, the transmission line has been terminated in it's characteristic impedance, and the backfeed has been reduced greatly or eliminated altogether. Less jitter, cleaner sound.
All I can say is, try it yourself, it is quite easy and inexpensive to do, and there is no reason to NOT try it.
If you want to learn the theory and practice behind this, I would seek out books on RF theory, and search for characterisitic impedance references on the web. I provide one in my Cable Esoterica note, see:
http://www.AudioAsylum.com/audio/cables/messages/2341.html
to get to:
http://www.belden.com/products/ciocahalf.htm
Jon Risch

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