[GreenKeys] Far OT: audio inputs

[Craig Sawyers]

>  I always wondered
> where the
> 600-ohm standard evolved from -- anybody?

Telephones.  When people were connected by many miles of wire to the
exchange and simple old telephones, good power transmission was essential.

The 600 ohms is not the characteristic impedance (which is resistive),
however.  It is the magnitude of the complex impedance at the centre of the
voice band at 1.6kHz.  A phase angle of 45 degrees at that frequecy was the
design norm.  So twisted pair was used, chosen to have the correct R,L and C
per unit length to give 600 ohms at 45 degrees at 1.6kHz.

Craig