RST codes
RST Codes	Readability R1 Unreadable R2 Barely readable, occasional words distinguishable R3 Readable with considerable difficulty R4 Readable with practically no difficulty R5 Perfectly readable Signal strength S1 Faint, signals barely readable 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 Tl Extremely rough hissing note T2 Very rough ac note, not musical T3 Rough, low-pitched ac note, mod. music T4 Rather rough ac note, mod. 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