FWIW, S/PDIF Cables Do Matter

    I spent about an hour yesterday, trying to get my KPA to connect via S/Pdif, to my Roland Quad Capture audio interface. The problem, as it turns out, was the S/Pdif cable that I used, or even more importantly, the fact that it was not actually a S/Pdif cable at all, but instead, a simple RCA audio cable. They both look the same for the most part, but they are not the same.


    When I connected what I thought was a S/Pdif cable, it did work, but only at 44k, turning the sample rate up any higher, and the audio interface and KPA would not sync with each other, at all.


    It seems that you can use a generic RCA audio cable to connect your KPA if, the sample rate is low, and/or the cable is very short. The minute you start increasing the sample rate and/or the cable length, the sync signal quality will start to suffer due to the cable.


    I'm no expert, but I've read that impedance changes with the length of a regular audio cable, so i assume that at some point, the impedance is out of the range required for successful sync between digital audio devices, especially when you start increasing the sample rate.


    The fix? Make sure to use a 75ohm RCA cable to connect your KPA and digital audio interface. These cables are designed so that a 5 foot, 10 foot, or even 25 foot rca cable length, will still have 75ohm impedance, end to end. This will allow for a solid sync connection at any sample rate. I actually used an AV cable that I received with my TV, I think. Those are the red, white and yellow cables that you probably have in a drawer somewhere, around the house. The red and white connectors are for analog audio, Left and right. The yellow connector is 75ohm, for video.


    Again, regular RCA audio cables (short ones), may work fine, but why risk it?

    Correct weight, Dave.


    I used to use 1ft RCA cables back in the day, but couldn't run 3ft or longer ones to and from my digital Yamaha desk. The solution was custom-made, heavy-duty (thick and stiff!) S'PDIF cables with mega-quality connectors.


    I no longer use any digital interfacing, but I learned the lesson you speak of well back then through trial and (many!) errors.

    An Spdi cable is similar to a television cable that you use for your digital tv.
    Does not need to be high quality but it must meet the simple 75 Ohm.
    Otherwise the data get corrupted, when the cable is only a few cm long.

    It is instructive to look at the waveform of an S/PDIF signal on a CRO.


    http://www.jacquesstompboxes.com/oscsiemens.JPG


    [Blocked Image: http://www.jacquesstompboxes.com/oscsiemens.JPG]


    It is - more or less - a stream of "square" pulses. The cable acts as an RC filter with R in series and C in parallel - so it is a low pass filter. The sharp corners on a square wave will get progressively more rounded off as the higher frequencies are rolled off by the filter - attenuate them too far and you cannot read the digital data any longer => no sync.

    If you have the old yellow/red/white av cable, the yellow video cable works fine for spdif, I used one for years before I bought a real spdif cable.

    (PEDALBOARD): Kemper Stage, Shure GLXD-16 wireless, JHS A/B Switch, SoloDallas Storm, Keeley Halo, Mission Engineering EP-1 KP, Voodoo Labs Pedal Power 2, Pedaltrain-3 board