Regarding the quality of optical digital transmissions

[Amphiprion]

Previously on this forum there was a discussion regarding optical vs. coaxial digital interconnects. I personally favor optical for the isolation factor, but there was some discussion as to whether cheap optical receivers/transmitters could cause errors.

I had the pleasure this last week of sitting down at an Audio Precision 2700 series measurement system with an evaluation board for an AES3/SPDIF reciever + sample rate converter + AES3/SPDIF transmitter chip. The I/O to the board was available with either coax or Toslink connectors. Using the TOSLINK connectors for both input and output from the board, putting the sample rate converter in pass through mode, and using the Audio Precision as a generator and analyzer, I ran a 1KHz signal through the setup (96kHz PCM). There were no bit errors for the several minutes I watched.

The connectors were TORX170 series toslink connector/receivers, TOTX170 series transmitter/connectors, and the cable was a TOCP172. The Rx and Tx parts cost about 67 cents from Digikey in 10,000 unit quantities. The connection on these connectors was not great, if I pushed on the connector so that it was angled and not seated properly I would totally lose everything. But connected and left alone there were no errors, with very inexpensive parts. I hope this helps put to rest a small issue in the debate between optical and coax.

Now the AP has some death grip TOSLINK connectors, those dudes never let the cable jostle or wiggle loose. I think people would kill me if I opened it up to find out the part numbers of them though

[Reet]

Generally speaking, TOSlink is a more jittery transmiossion than a coaxial transmission, and isolation for this type of transmission is simply not necessary. Jitter affects the ability for the DAC to recreate the analog signal.

[littlesaint]

Generally speaking, TOSlink is a more jittery transmiossion than a coaxial transmission, and isolation for this type of transmission is simply not necessary. Jitter affects the ability for the DAC to recreate the analog signal.

Any digital transmission is going to have a certain amount of jitter unless you have perfect clocking on both ends. Optical has an additional "hazard" due to the conversion of electrical signals to light, however there's a big difference between measurable jitter, and audible jitter. With decent cabling over relatively short distances, there shouldn't be any audible jitter in the signal.

[Amphiprion]

I've offered no data on jitter, and thus will reserve judgment on that point until I can take more measurements.

[Dennis H]

I've offered no data on jitter, and thus will reserve judgment on that point until I can take more measurements.

I look forward to that, Mark.

[jquin]

Any digital transmission is going to have a certain amount of jitter unless you have perfect clocking on both ends. Optical has an additional "hazard" due to the conversion of electrical signals to light, however there's a big difference between measurable jitter, and audible jitter. With decent cabling over relatively short distances, there shouldn't be any audible jitter in the signal.

My understanding of jitter was that it lead to distortions during the DAC stage. I'm pretty sure DAC distortion performance will be quoted for a given jitter or (phase noise).

The effect of jitter needs to be considered for each receiver as it's effects can be reduced in the receiver in the phase locked loop used to extract bits from the S/PDIF stream.

Of couse the killer with all this is that the output amplifiers from most CD players are not very good and can easily be the dominant source of distortion.

[littlesaint]

My understanding of jitter was that it lead to distortions during the DAC stage...

Yes, but if the distortion is below the noise floor, it doesn't really matter. You can split hairs over these things for theoretical debate (and there's nothing wrong with that), but in the end they don't have all that much to do with the quality of sound.