There was no standard in the boatanchor days but a 6 to 20 dB SNR was often used in ads. For AM that was with a 30% 400Hz modulation. I use the 10dB in my receiver testing certification and on 10M; thats enough signal to be good copy on a quiet band with sufficient modulation.
Later MDS became a quasi standard but even that is open to interpetation. Some refer it to 3dB above the noise and others to what they can reliably copy in whatever bandwidth, neither is scientific
Carl
