Why do digital cables sound different?

I understand that the transmission is real time. I just do not believe a cable that is in good working order will cause a bit or two to be dropped. I agree with Blues_Man that the problem is probably in the transmitter or reciever if bits are being lost. I ran a quick test on my 3 digital cables at work today using a logic analyzer ( a scope is not the right tool for a real time test) with a pattern generator and deep memory (16 meg). I simply output a 16 bit burst every .5 seconds. The rep rate within the burst was set to 10KHz. I simply tied the pattern generator's clock to the logic analyzer's clock so that every time the pat gen's clock went high I captured the data until I filled up the memory and saved the data. I tried this with alternating 0's and 1's, all 0's , all 1's, a pattern of 0110111011001111 and it's complement. Once I had captured the data I saved them as ASCII files and I wrote a small visual basic program to look for missing bits in the patterns and found none. I also fed a repeating pattern of 0's and 1's into the cables and terminated the cable with what I approximated was the impedance of my D/A. I looked at the waveforms with a scope and looked for any droop, overshoot and undershoot. The risetime of the pulses appeared to be close on all 3 cables but I did notice some some other differences. I noticed one cable inparticular did cause more overshoot than the rest but when I varied the impedance used to terminate the cable I could minimize the overshoot (probably more capacitance in this cable causing an impedance mismatch). I marked this cable and gave all 3 cables to a another engineer who has a a separate DAC and transport to take home to see if the cables sound any different from one another. I am sorry but I did not hear very much of a difference between the cables to begin, with but I thought this would be a more subjective test. As for real time and loosing bits, the logic analyzer does not lie. I will let you know what he thinks. I can not think of a better way to test the real time data transmission characteristics of a cable. I burned up today's lunch time trying this, tommorrow I think I will have something to eat :-)Thanks for the post

One more thing, the logic analyzer used actually had 1 meg memory, not 16 meg. Someone borrowed my 16 meg aquisition module.

I no longer have an analyzer, but whebn I did it was oked up to CD players and Transport / DACs. You just can't send bit patterns with your equipment because its probably far more sophisticated than what's in a CD playback system. Remember you also have to send 44,100 samples per second. A scope works fine at the output of the DAC to determine differences in the analog output of test signals. If you use the same transport, same DAC and same source, any differences must be from the cable. Also disconnect the destination end of the cable and put a volt meter on it, notice the large differences.

OOps almost forgot, in your test how did you verify that all samples sent were received and stored? If you sent 500,000 samples, did 500, samples get stored?

All I did was set up a burst of each pattern and a signature pattern (to be used in the little vbasic program I wrote) and had the pattern generator resend the the sequence over and over until I filled up the aquisition memory of the analyzer (It took some time so I did some real work also). However many samples it took to fill up the aquisition memory is how many samples I got. I did not count how many patterns I captured but it was alot! I used a Tektronix TLA704 analyzer so I stored the results on the analyzers hard disk. Tek's analyzer also runs windows so I wrote the Vbasic program right on the analyzer. I did a simple compare of the 5 bursts I sent (16 bits of all 1's, all 0's etc.) until I hit the signature burst and I started the compare over again and repeated the process until the software could not find the signature burst. If I got a situation where it did not compare I incremented a counter in my program. The counter never incremented so there were no errors. To test my software I buggered up my captured data file a bit and the program caught it. It was really an easy experiment but I probably spent more time on it than I should have at work but I trust nobody will tell my boss. As a designer of these types of systems I am sure you have run similar tests. While this is not my forte ( I do DSP design) it sounded like a valid test and you challenged me to the task.