600 Ohms balanced line with 1mW set as 0dB is standard in electroacoustics
and broadcasting equipment. Also the 3 pin XLR connectors used exclusively
for passing this type of signal.

Most other 'line' levels are 'flexible' adaptations of this standard. 1mW
into 600 Ohms unbalanced is 0.775V rms or about 2.17V pkpk. This is close
to what someone else said. The input impedance on nonstandard equipment
will vary wildly from 600Ohms to 100k (!). Common is 47k and 200mVpkpk on
consumer equipment, also 1k, 5k and 10k , all with 200mVpkpk. Consumer
equipment that accepts 2Vpkpk without distortion into a line input is rare
in my experience.

Peter

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