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Cable Asylum Interconnects, speaker wire, power cords. Ask the Cable Guys. |
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In Reply to: Digital Cable Length posted by geezerrocket on April 7, 2014 at 06:58:59:
Yes, depending on the impedance matching accuracy of your transmission system. Longer cables are fine as long as your transmitter and receiver are as close as possible to being perfectly matched with the cable over the entire bandwidth required by the signal. In the real world this rarely happens with consumer grade equipment so you want the shortest cable you can get away with without introducing sharp bends in the cable - particularly near the strain relief of the connector as this will put a torque on the connector in the socket and can cause poor connections and potentially signal distortions due to the dielectric being deformed. It also depends on the primary sample rate which defines the propagation time (and the bandwidth of the signal). Basically you don't want standing waves messing with the transmitted pulses. If the reflected pulses are of a sufficiently low amplitude and/or out of phase with the transmitted pulse, you should have no problem. The receiver PLL should be able to lock on to the primary signal without problem and be largely unaffected by low amplitude reflections - the jitter tolerance of the SPDIF spec is of the order of nano-seconds. To put this in perspective jitter levels within the DAC at the reconstruction stage need to be of the order of pico-seconds. Good receivers are designed to handle such signals and recover the clock and data correctly with minimum output jitter. From memory the standard (IEC60958-3) defines something like 50ps for receivers. Old devices prior to this standard such as the CS8414 have an intrinsic jitter of 200ps which is still an order of magnitude less than the transmission jitter. The problem comes when the transmitted pulses become distorted by the reflections when the geometric length of the cable is related to the transmission frequency. The receiver then has difficulty establishing the transitions from the bi-phase coding.
It's not just the cable, the connector needs to be 75 ohm matched as well as the source and receiver - the best cable in the world won't fix that problem! Any problems in impedance matching will cause reflections to be set up. RCA connectors are notoriously poor for RF terminations as they are not generally impedance matched for 75 ohms, so make sure the connectors are cleaned first and try and minimise the number of times you insert and remove. If you can, I recommend switching to BNC connectors on your transport and DAC.
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
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- RE: Digital Cable Length - flood2 01:02:31 04/22/14 (0)
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