R-S-T system

[8] For a quantitative assessment, quality HF receivers are calibrated so that S9 on the S-meter corresponds to a signal of 50 μV at the antenna standard terminal impedance 50 ohms.

On VHF and UHF receivers used for weak signal communications, S9 often corresponds to 5 μV at the antenna terminal 50 ohms.

With modern technology, imperfections in the quality of transmitters’ digital modulation severe enough to be detected by human ears are rare.

Exceptionally strong signals are designated by the quantitative number of decibels, in excess of "S9", displayed on the receiver's S meter.

The Q replaces "Tone" with "Quality" on a similar 1-9 scale indicating presence or number of unwanted 'sidebar pairs' in a narrow-band digital mode, such as PSK31 or RTTY.