Measuring RMS noise...

[ppppenguin]

I've known this method for many years. Pretty much standard for analogue video measurement. It's certainly described by Weaver in 1971 "Television Video Transmission Measurements". He calls it "Tangential noise measurement" and his technique involves adding a calibrated square wave to the signal under test but the principle is the same. Likely to pre-date Weaver's book - though he does not refer to it in BBC monograph 24 from 1959
http://downloads.bbc.co.uk/rd/pubs/archi...aph_24.pdf

Usual caveat that you need to define the bandwidth over which you're measuring the noise. 100kHz noise is irrelevant for audio.

Won't work on a digital scope. Even the best "digital phosphor" ones probably won't be much good.