The challenge of scaling the digital threshold
Over the weekend I’ve been involved in some forum discussions regarding whether USB cables can make a sonic difference when connecting a computer to a DAC. I went into the discussion thinking it couldn’t possibly matter since USB sends CRC-signed data packets, but changed my mind as I researached the topic deeper. In short: With most current USB dac’s, the clock for the D/A is derived from the 12 MHz frequency of the USB 1.0 spec that the stream is locked to. Since this means that jitter can be a factor, suddenly the cable can make a difference, at least theoretically.
I think the deeper insight from all this is how much a consumer needs to know to make informed decisions. To confidently judge whether to spend money on a USB cable or dismiss it as placebo and a waste of money, the following needs to be understood:
- The fundamental difference between analogue and digital audio
- Pulse Code Modulation data structure, with bits and sample points
- How a clock pulse is used in D/A conversion
- The concept of jitter and its detrimental effect in the D/A stage
- Jitter supression using a Phase Locked Loop, or PLL
- Jitter supression using a memory buffer, and the inherent problems of buffer overflow or underrun
- The principles of packet data, with handshakes, tokens, data and status
- The difference between bulk and isochronous modes in USB
- How adaptive isochronous mode is fundamentally different from asynchronous isochronous mode
- Understanding the physical principles of electromagnetic interference, EMI, as it applies to cables
That’s just the basics for whether a USB cable can theoretically make a difference. No wonder the old hifi dinosaurs get it wrong.