On Fri, Feb 17, 2012 at 07:23:12PM -0500, gene heskett wrote: > > The concept of cable impedance makes sense only if the lenght > > becomes a non-trivial fraction of wavelength. For audio that > > means that for anything shorter than a few hundred meters it's > > only capacitance that matters. > > But at a 60 ohm impedance, that capacitance is considerable. I could put > 20 volts P-P into a cable headed for our news dept, at say 15 kilohertz, > and 200' away, I could only see 6 or 7 volts.
Sure, no discussion about that. But at that lenght and frequency, this is not due to the cable's characteristic impedance but only the result of the line amp's output impedance (600 ohm) and the the cable capacitance forming a simple RC lowpass circuit. The C in this case is proportional to length, while a cable's characteristic impedance is not. At audio freqencies the cable's series resistance dominates the impedance of its inductance, which means that the classic equation for impedance, sqrt(L/C) is no longer valid. It still has an impedance which turns out to be something like sqrt (R/wC). In theory you could try and match that at both ends but since it depends on frequency that would be very complicated. The practical solution for long lines, and what Ma Bell does, is to increase the cable's apparent inductance by inserting series inductors at regular intervals. These combine with the cable's capacitance to produce a purely resistive impedance even at audio frequencies. The distance between the inductors determines the bandwidth. Lines used for full bandwidth audio (e.g. broadcasting) require more inductors per unit lenght. The practical solution for audio connections up to a few hundred meters is to ignore cable impedance completely, accept the capacitive load and just drive it with a very low impedance. Anything longer than that will be digital these days. Ciao, -- FA Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl. _______________________________________________ Linux-audio-dev mailing list [email protected] http://lists.linuxaudio.org/listinfo/linux-audio-dev
