Today you’ll get a glimpse at life inside of inSync. We always get all kinds of interesting feedback to inSync from our readers. In response to last week’s tip on S/PDIF cables (see inSync 4/25/00) we had one response that came dangerously close to questioning our motives. See for yourself.
“Please explain how a cable can affect jitter. This makes no sense to me. According to all of the literature I’ve read on the subject, jitter is a clocking problem. I hope you are not just repeating another myth of the audio industry that is a sea of snake oil and fraud. Excuse my suspicions, but since you sell cables, you have a vested interest in supporting their claims.”
Ouch!
While I was responding to this guy I received the following message:
“On our digital products that support S/PDIF, our most frequent customer service problem is people using the wrong type of cable. In general, S/PDIF will not run reliably on audio cables over three feet, and even then it is introducing jitter that could be a problem for you. Over and over we have run into people trying to use 10, 20, or even 30 foot audio cables and then complaining about pops in the audio. Please, please, everybody, buy the correct cables.”
Rick Kreifeldt
Director of Engineering
dbx Professional Products
The main reason there is so much confusion around this topic is because in practice you can use a standard audio cable to connect S/PDIF devices and it will often appear to work. When people get clicks, pops, and other more subtle anomalies in their audio it doesn’t occur to them that it could be the cable. Our Technical Support department reports that 50% of all the problems they encounter are cable related. Hint: When you have trouble, always check your cables.
We don’t have the space to get into an in-depth discussion of all the ways in which cable quality and type can affect signals, but with regard to the digital cable question one should always try to use 75 ohm cable on S/PDIF connections (as we said last week). These cables are designed to carry much higher frequency information than a typical audio cable, which is critical for digital (or video) signals. Further, the waveforms (square waves and pulses) found in digital signals are much more difficult to transmit accurately at those high frequencies. The tolerances of all components must be radically different than what you’d find in an analog phono plug I/O circuit (you usually don’t encounter pure square waves in audio). So just because the connector is the same, and just because it often appears to work, don’t let yourself be fooled into thinking it doesn’t matter. Think about it. A clock signal (which is a critical component of transferring digital audio) is a square wave (or at least a waveform resembling a square wave). If distortions and reflections along the transmission line cause that waveform to get rounded off, then you compromise the precision of the clock because the device that must read it has to wait for the voltage to ramp up to a point where it triggers as a clock pulse. We’re talking very small amounts of time here, but nevertheless when you have garbage on the line, or allow the introduction of distortion to the waveform shapes you are compromising the data transfer in a number of ways (not just jitter).
Or, put another way, if you think it doesn’t matter for S/PDIF, why not try 25 little RCA cables to transfer information between your SCSI drive and computer? Not suggesting that S/PDIF and SCSI are the same thing, but there are some similarities.