192 kHz capability and S/PDIF handling
finally I found the time to answer you. Because I thought, this topic could the of public interst, I answer you here. Maybe, it would be a good idea to create an own thread for the S/PDIF stuff at least, though.
I've checked the speed of the crystal as you adviced. The clock is running at 28.322 MHz. However, I don't understand what you mean by "your's may or may not be upgraded". "Upgraded" sounds like changing devices after you already sold them instead of directly manufacturing new models. Except that, I thought that *all* DAC1s were capaple of 192 kHz - at least via the electrical input. So there were some even not accepting 192 kHz through the other inputs?
After all, if the crystal (and the rest of the parts) are able to support 192 kHz, you would just have to exchange the optical receiver by a newer model, right?
However, I doubt if the whole 192 kHz is worth the fuss since the DAC1 is going to convert everything down to ~ 110 kHz, anyway. The only (technical) advantage for 192 kHz material to be played back would be to avoid the conversation to 96 kHz in software (which could be worse) and the theoretical benefit of about 14 kHz Samplerate (the difference between the internal rate and the 96 kHz, resulting from the input's limitation).
Since the DAC1 is able to recognize sample rates over a wide range (not only the common steps 44.1, 48, 96, etc.), I wonder if even the older optical inputs aren't able to receive 110 kHz SR. Thus, playing a file, sampled 192 kHz from the computer and resampling it to the internal sample rate of the DAC1 before sending it via Toslink could work and if the resampling is of the same quality like the one of the DAC1, no quality would be lost (in theory, I doubt that I could hear any difference, anyway).
In regard to S/PDIF TTL from PC drives, I did additional tests and also tried the DAC1 in conjunction of a Yamaha CD-Player which is said to handle the "valid flag" in the S/PDIF-stream correctly.
Although this conforms to the standard, I had to recognize that in the case of the DAC1, this is even a disadvantage to the (strange) usage of the "non pcm" flag, like pc drives seem to use it instead. Why? Because the DAC1 doesn't display invalid S/PDIF samples. :-(
This is a real pity because this way I don't see anything. When using the PlexWriter as source (you could see it on the picture I had linked, hehe), on most (unfortunately not all) errors, the "non pcm" led lights up at least. This is unusual but still better than nothing.
After thinking about it, this is clear. Even when (uncorrectable!) C2-errors occur, the S/PDIF stream itself stays valid, so of course the "error" led of the DAC1 doesn't show this. "Non pcm" isn't the case either (at least when using common CD-players as source). So the result is that the DAC1 simply doesn't have a led to show standard conform C2-errors. This would be a real feature for people who want to know in which conditions their CDs are (C2 errors can indicate a soon death, so one better makes a copy before it's too late).
I wonder also how the DAC1 handles samples, flagged as invalid (VALID=true - the boolean logic is inverted here)? It doesn't show them, that's for sure. But is there a stage which performs interpolation like built into every CD-player or are they simply ignored? Because you have to distinguish between errors on the distance source --> DAC resulting in loss of sync ("error" led lights up) and erroneous content. The S/PDIF standard also mentions parity bits used for error detection. Does the DAC1 make use of them?
Some time ago I had a soundcard, featuring two Toslink interfaces (in and out). When using a pretty long (and cheap) cable between it and the CD-Player, the sound from the sound card was distorted and noise. The amazing thing was that it even gaves a sound when holding the plug near the input. The more the distance, the more the distortion. The DAC1 however either gives a perfect sound signal or no at all. The is no in between. I wonder how it detects that the data is errorfree before converting it to analog. For the standard, here you can find more information:http://www.epanorama.net/documents/audio/spdif.html
It would be very interesting to know how the DAC1 handles and processes the whole S/PDIF data (interpolation, parity checks, etc.). If someone can organize such an overview, then YOU. :-)
Thanks again, Elias!