opaqueice;194637 Wrote: > Sorry, I thought my statement was suffciently clear, but apparently not. > The good approximation is that the voltage is the same everywhere along > the wire *at a given time*. Obviously it changes with time, but it > doesn't depend on position along the wire.
This is one of those (rare) phenomena that can be easily quantified. The formula for the wavelength of a signal propagating along a linear conductor is v=fl, where v is the wave velocity, f is the frequency, and l is the wavelength. In a coaxial cable, v is about 80% the speed of light, or 240M m/s. The raw data rate (fundamental) on an S/PDIF cable for 44.1kHz sample rate is around 5.6 Mbit/s, which gives a wavelength of 43 metres. So, a 1 metre cable is short compared to the wavelength, but not totally insignificant. That's not the whole story, though, because it's not the data rate that matters, but the edge speed. Fast edges are desirable in this case because the faster the edge, the more accurately its position can be determined - and the lower the jitter. Data edges can easily contain harmonic components well over 100MHz, which have a wavelength comparable with the length of the cable. Transmission line effects aren't just significant - they completely describe the propagation of the signal. The Squeezebox does have a very clean digital output, with rise and fall times in the order of 10ns or so. Depending on the length and impedance of the cable, and how it is terminated, the receiver may see a nice, clean, monotonic edge, or it may see a waveform which is quite different. S-shaped edges which hover around the 1<>0 threshold are surprisingly common, as are 'edges' which cross the threshold one way, then go back across it the other way, before finally crossing it again on the way up to the final voltage. All these cause problems recovering a low jitter clock. -- AndyC_772 ------------------------------------------------------------------------ AndyC_772's Profile: http://forums.slimdevices.com/member.php?userid=10472 View this thread: http://forums.slimdevices.com/showthread.php?t=34406 _______________________________________________ audiophiles mailing list [EMAIL PROTECTED] http://lists.slimdevices.com/lists/listinfo/audiophiles
