The R-S-T system

The R-S-T system is used by amateur radio operators, shortwave listeners, and other radio hobbyists to exchange information about the quality of a radio signal being received. The code is a three digit number, with one digit each for conveying an assessment of the signal's Readability, Strength, and Tone.

For non-CW (non Morse code) signals, the Tone (i.e. last) digit is normally omitted.

Readability

R1 - Unreadable

R2 - Barely readable, occasional words distinguishable

R3 - Readable with considerable difficulty

R4 - Readable with practically no difficulty

R5 - Perfectly readable

Strength

S1 - Faint—signals barely perceptible

S2 - Very weak signals

S3 - Weak signals

S4 - Fair signals

S5 - Fairly good signals

S6 - Good signals

S7 - Moderately strong signals

S8 - Strong signals

S9 - Extremely strong signals

Tone

T1 - Extremely rough hissing note

T2 - Very rough AC note, no trace of musicality

T3 - Rough, low-pitched AC note, slightly musical

T4 - Rather rough AC note, moderately musical

T5 - Musically modulated note

T6 - Modulated note, slight trace of whistle

T7 - Near DC note, smooth ripple

T8 - Good DC note, just a trace of ripple

T9 - Purest DC note