On 17/01/2010, Richard L. Hess wrote:
> Calculating maximum cable length is a complex task and is never
> accurate. There really is no brick wall in analog--it is just how
> much degradation is acceptable--but digital is another story and it
> has brick walls.
> Two parameters affect analog signal transmission in voltage audio
> systems: (a) The capacitive reactance of the cable creates a voltage
> divider with the source impedance of the microphone, creating
> high-frequency roll-off (b) The capacitive loading of the cable may be
> beyond the current drive capability of the source's output driver.
> This creates high frequency distortions as well as roll-off.
> With normal professional balanced audio connections and voltage
> distribution, especially if the driver source impedance is around 60
> ohms, driving 100 m or more of typical cable is easily possible.
> Higher impedance circuits, and especially unbalanced ones, should be
> kept short -- typically in the neighbourhood of 3 m or less.
> Phono cartridge connections and Nakamichi cassette machine unbalanced
> outputs are especially sensitive to cable loading.
I think this is a problem with most consumer and "hi fi" equipment using
unbalanced connectors, and accounts for much of the subjective
description found in magazines.
A buffer amp with high input impedance and low output impedance can
> For some additional insight into cable effects and a definition of
> voltage audio distribution, please see my paper on the subject:
> You might be better off leaving the A-D converters in the control
> room and running digital signals to the computers (AES/EBU either on
> 110 ohm twisted pair or 75 ohm coax). If I were re-building my
> transfer suite, I would consider MADI and local converters for each
> machine. While it is more expensive than wires and patchbays, the
> price differential is narrowing.
Or one could run digital on optical cables.
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