Why do digital cables sound different?

Actually the SPDIF signal is digital, not dual-phase sinewaves. It appears more like a "trapezoidal" wave, which is a square-wave with controlled slew-rate rising and falling edges. This is to minimize radiated emissions for FCC regulations. The signal is around 3 MHz with 12-15 nsec rise and fall times.

While it is strictly a matter of semantics, the actual digital out signal from a transport is sent at 2.822 MHz. 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. Bare in mind that terminating and feedpoint impedances are a LOT more complex than just selecting a 75 ohm cable ( we wish it was that simple ). This would allow the transport to load up easier due to a complete lack of reflections and a clearer signal path to transmit the data, which theoretically should drastically reduce jitter. This takes for granted that the transport and dac would both have nominal 75 ohm inputs & outputs though. This is something that i've been meaning to check into but just haven't gotten around to it. Sean
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Looks like you're right audioengr. I got into an argument with Monstrous Mike one time about Manchester Biphase encoding. He was trying to say there was no digital edge, only FM modulated sine waves. From what I can find, you are right -- the signal is still sent as square waves from what I can tell (but not with sharp edges -- the standard should likely work even if the signal were closer to a sine wave, but with more jitter).

As for who I am Drubin, just an audio enthusiast, who lurks on the audoasylum cable forum under "audioNeil".

Picking up on what Redkiwi said about coax vs. digital, I and a friend were able to A/B the new Audioquest Optilink 5 with Audioquest's digital RCA, the VSD-4. The result was truly surprising. The Optilink glass fiber Toslink was significantly better in every way; soundstage, bass, "openness," highs, "air," -- you name it, the glass toslink was better, even though the data is just the same 0's and 1's. I don't have a clue why it sounded better, technically, but this was not even close. We were startled, honestly, at the difference. Moreover, the toslink was significantly louder at the same volume level, requiring us to match volume for the two. So bottom line is, for whatever reason, digital cables sound different, no question whatsoever.

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.