I don’t dispute that people have heard differing results from different optical cables on other DACs. The thing is - why would that make a difference, and does that effect apply to the DirectStream DAC?
The answer to the second question, as far as I can ascertain, is no!
An optical cable simply guides light from the transmitter to the receiver. The receiver is watching for transitions between bright and dim states, which it then mimics with voltage transitions between high and low on its electrical back end. The cable needs to be clear enough that a bright state can be unambiguously detected, and it needs to minimise internal reflections so that when the transmitter stops shining the cable becomes dark as quickly as possible.
The SPDIF signalling protocol is cheap and easy to implement but has a huge flaw when it comes to clocking. In most cases the sender uses its own clock and the receiver is forced to sync itself to the arrival timing of the incoming high/low or bright/dim transitions. Just about every DAC prior to the DirectStream uses phase-locked loops or asynchronous sample rate conversion to process the incoming signal stream. PLLs try to make the DAC follow the timing of the signal as if it’s a conductor, while ASRC uses mathematical approximation to map the incoming stream with reference to a local clock at a higher resolution. Both of these approaches are impacted in audible ways by variations in the time between each successive signal pulse seen from the electrical output of the optical receiver. That is to say, input jitter matters in these DACs.
To reduce input jitter, you need a good stable clock at the source end to start with. That’s one main reason why different transports or other digital sources sound different with SPDIF connections. Then with optical you need the receiver to be noticing and reacting to the bright/dim transitions with precise regularity. If the light doesn’t shine brightly enough it’ll be slow to respond to a bright state, and if there’s too much scattered reflection it’ll be slow to respond to a dim state. That’s why the quality of your optical cable matters with those DACs.
The DirectStream is different to anything which came before it. It has a single ultra high stability clock of its own at a frequency which is a high integer multiple of all the supported digital audio sample rates - it’s up in the MHz somewhere, I forget the exact value. The stability is not about long-term average but about consistency from one clock pulse to the next, and in fact the overall rate is adjustable by varying the voltage applied to the clock. That clock directly drives the output stage but it also controls the FPGA and all the input processing.
The FPGA input processing watches the incoming signal line for transitions between high/low states. That doesn’t mean it reacts to such changes, rather it’s making millions of observations every second to see whether the signal is currently high or low. Based on the pattern of changes it sees, and the spacing between them, it can identify SPDIF bi-phase mark encoding and start to extract the data. But here’s the most beautiful thing: it does this all so fast that it can notice if it’s drifting away from sync with the incoming data stream, and it gently adjusts the voltage going to the ultra stable master clock so that it matches the overall pace of the source.
What this means is that the kinds of jitter which make Toslink generally the worst option for other DACs and which make Toslink cable quality an issue for those DACs are basically irrelevant for the DirectStream. A really unstable source will see the DS having to make repeated adjustments, impacting the sound. But given reasonable quality sources and cables which aren’t right on the edge of the functional envelope, optical is a fantastic and cheap route to great quality sound from your DS.
