Why do digital cables sound different?

Sean wrote:

If one could resonate a cable that introduced a perfect non-reactive 75 ohm load at that frequency, you would end up with no standing waves.

There may be no standing waves to a pure sine wave at that frequency, but you would certainly experience reflections with digital edges. There are only a couple of ways to eliminate reflections with digital signals, including:
1) series terminate (75 ohm driver) into a 75 ohm Zo cable - reflection from the end is absorbed at the driver.
2) parallel terminate a low-impedance driver into a 75 ohm Zo cable using a 75 ohm resistor at the end - no reflection.
3) Series and parallel terminate - (75 ohm driver) into a 75 ohm Zo cable with a 75 ohm resistor at the end - this cuts the signal in half, but eliminates all reflections.

Forgive me for coming late into conversation...

From what ive seen there is a major flaw in the spec itself The #1 contributer of EMI is the rise time. The quicker it
is the more EMI emitted. A common misconception is the Freq
is the culprit not true. Its seems to me that the rise times
are faster than needed for application. If some one knows why they used the 3mhz with a 12-15ns rise time... Please let me know? It doesnt make sense to me. Although those numbers are prehistoric it is asking for excessive EMI with
faster rise times than needed. Typically you would see a rise time more like 30 maybe as high as 80 for that type
of frequency.

Its like saying take 15min to drive to the store (a 5 minute drive) and then RUN in side and try and make up for lost time when you get there.

As far as termination I agree with Audioeng. You have have
termination in place at reciever end. Its the reflection back thats the killer... Backwards crosstalk is evil sh*t in any system.

Audioengr: Tara worked with terminating impedances years ago. Silver center conductor, Teflon insulation, Silver braided shield, 75 ohm resistor from center to shield at the load end. It is a very nice sounding cable and not bright in the slightest bit.

As far as having to use a resistor(s) to terminate the load, you would not need to do that IF the actual output impedance of the transport is 75 ohms, the cable is 75 ohms and the input of the DAC is 75 ohms. The problem is that none of these are exactly the correct impedance, so you have VSWR at the feedpoint, VSWR within the cable and VSWR at the load. If one could find out the actual output impedance of the transport and the input impedance of the DAC, it would be easy to make up a section of cable that would act as an impedance transformer. This would minimize standing waves / reflections and maximize power transfer. THAT was the point that i was trying to explain. Obviously, you could not market this cable as the specific values would change component by component.

As far as terminating the cable with a resistor at the load end, that would NOT solve an impedance mismatch at the source end. An impedance bump ANYWHERE along the signal / data path will result in reflections back to the source. Sean
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Gentlemen,

There are always Impedance mismatches.. the trick is
to minimize... usually we have tolerances at 10%...Even
for high speed stuff. Impedance mismatches do cause RF current problems BUT if terminated it will be minimized.

A 10% mismatch ( VSWR of 2.0 ) at RF frequencies ( which digital data is transferred via RF ) can begin to play MAJOR havoc with some circuitry. I know that text-book theory states otherwise, but you have to realize that much of that theory was based on tube circuitry that made use of matchingn networks and tank circuits. Most SS gear does not have this type of circuitry, so loading characteristics become far more critical.

The severity of how much the operation of a device would be affected obviously depends on the stability of the circuitry and whether or not the impedance mismatch was of a reactive or resistive nature. Not only would the sending unit have to deal with the reflections playing games with the output section, signal loss is increased. The resultant decline in detail is increased due to loading via the "back-pressure" within the transmission line. This is not to mention that line loss increases as VSWR increases, compounding the factors involved drastically. As such, ANY type of vswr / signal reflections tend to start a very drastic downward spiral effect once they come into play. This is FAR more apparent with SS gear, but then again, i don't know of any transport or CD player that uses a tubed digital output section.

As such, i would think that all of my experience with RF loading characteristics and impedance mismatches would directly apply to digital data transfer. I would suspect that the output section of most transports / cd players would not be very stable due to lack of a buffer circuit or complex impedance matching network. Therefore, ANY gains in terms of increased power transfer / minimization of reflections would be of multi-fold benefit for the aforementioned reasons. I have not verified this personally, but see no reason why this would differ from any other RF based SS circuit loading up. I really do want to start measuring and experimenting in these areas, but just have not had the time to do so. Sean
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