On digital modes (particularly PSK31) RSQ (Readability, Strength, and Quality) is usually used for signal reports, instead of RST (Readability, Strength, CW Tone). There is some interesting history and background information on the RSQ system available at psb-info.net a site dedicated to the RSQ signal report.
R = RSQ Readability (Percent characters correctly received):
- R 1 – 0% Undecipherable
- R 2 – 20% Occasional words distinguishable
- R 3 – 40% Considerable difficulty, many missed characters
- R 4 – 80% Practically no difficulty, occasional missed characters
- R 5 – 95%+ Perfectly readable
S = RSQ Strength (Signal over Noise rather than S-Meter):
- S 1 – Barely perceptible trace
- S 3 – Weak trace
- S 5 – Moderate trace
- S 7 – Strong trace
- S 9 – Very Strong trace
Q = RSQ Quality:
- Q 1 – Splatter over much of the spectrum
- Q 3 – Multiple visible pairs
- Q 5 – One easily visible pair
- Q 7 – One barely visible pair
- Q 9 – Clean signal – no visible unwanted sidebar pairs
Here is a minified table version I found:
Another way signal reports are exchanged on the digital modes is by giving the Intermodulation Distortion (or IMD).
A quote from the RSQ website about what IMD is:
Inter-Modulation Distortion (IMD) is a report often exchanged during a PSK QSO as a figure of merit for the received signal. It is widely
assumed that a very good IMD report for an idling signal is around
-30db, a poor report around -20db with the worst possible at -10db.
Howard (Skip) Teller KH6TY who developed the first panoramic PSK31
transceiver and Digipan software advises that great care must be taken
when attempting to measure IMD at the receiving station. The following
advice was received from KH6TY:
"...IMD is the measurement of the first pair of unwanted sidebands to
the desired ones, and if the signal to noise ratio is not around 26
db, you will start to measure the noise instead of the unwanted
sidebands, which will be under the noise threshold. Also, if there is
any distortion in the receiver, you create a false IMD reading.
Whenever I measure a station's IMD, I have to be sure the S/N is good,
and then I reduce the RF gain of my transceiver until the IMD stops
falling and starts rising. At this point, I believe I have almost
eliminated IMD caused by the receiver distorting the signal.