S/PDIF, or Sony/Philips Digital Interconnect Format (also SPDIF), is widely used for consumer digital audio connections. It is similar to the professional audio AES3 standard, both of which were developed in the early 1990’s. For basic applications, S/PDIF and AES are entirely adequate, but they contain some limitations that become increasinbly visible in today's high-end playback systems.
The fundamental problem is that S/PDIF and AES both "fold" digital audio data and clock signals into a single bit stream. The clock doesn’t have a cable of its own, and so must be “extracted” from the incoming bits by logic in the receiving device. Conceptually, this approach is reasonable enough, but in higher-end systems, it creates several difficulties, each of which the Audiophilleo addresses.
During playback, the master clock should ideally be inside the DAC, because optimal reconstruction of the analogue waveforms is highly sensitive to clock noise or Jitter. When the master clock comes from “upstream”, as it is in the case of computer audio, it must be extracted “on the fly” as the bits show up at the DAC.
In the case of a CD player, the transport and DAC are in close proximity, usually just inches apart, so clock jitter between the two is minimized. But,, when using an external DAC with a CD transport, the master clock could be several feet away; the same holds true computer audio, when using S/PDIF stream to drive an external DAC. In addition, the computer itself generates considerable amounts of electromagnetic interference (EMI) which further degrades the accuracy of the clock. The results are often less than ideal.
Furthermore, the clock extraction process inside the DAC itself is somewhat imperfect, adding still more jitter to the conversion process. The resulting analogue waveforms are slightly distorted in ways that may create objectionable, audible artifacts that are difficult to remove through electronics or software.
Truly satisfying digital audio via S/PDIF or AES has been somewhat
of an elusive goal. Fortunately, the advanced design of the Audiophilleo address
all these issues, and finally makes it possible to enjoy both the convenience of
digital audio and state-of-the-art sound.