The Readability-Strength Exchange
The RSx exchange (readability, strength, and a mode-dependent third metric) is a two- or three-digit number that conveys the readability and strength of a radio signal, along with (on other modes) other aspects of the signal. It is used most often on HF in SSB and CW contacts, and it also forms a part of the exchange of several contests. The first number indicates the subjective readability of a signal from 1 (unreadable) to 5 (perfect). The second number indicates the signal strength in S-units. The third is mode-dependent: on most modes, it is unused; on CW, it indicates the tonal quality of the CW signal using a number from 1 (harsh buzz) to 9 (pure, undistorted sine wave); and on SSTV, it indicates the video quality from 1 (heavy distortion; parts unreadable) to 5 (perfect). An RSx report of 00 or 000 is often used during a pileup, DX, or contest situation to tell a particular operator that their operating practices are not in order and that they should stop calling.
CW
On CW, the RST (readability, strength, and tone) system is used. Readability and strength are equivalent to other modes, but tone measures the tonal quality—the signal's closeness to a pure sine wave—of the received signal using a number from 1 (harsh buzz) to 9 (pure, undistorted sine wave). Tonal quality was more variable when the RST system was formalized in the early days of CW. Lower values indicate distorted, wide-bandwidth, or harsh tones, and/or tones with a significant power-line frequency component. Because of the advent of modern electronics, the tone part of a signal report is often ignored and pegged at 9. The suffixes K and C are appended to signifiy key clicks and chirp, respectively. Key clicks occur when a CW signal is switched on and off rapidly, causing modulation sidebands to form and a clicking noise to be heard at the beginning and end of a CW signal. When especially fast operation is desired, numbers can be abbreviated as letters with a relatively shorter CW length, to wit: 5 (·····) is often abbreveated as E (·); 9 (----·) as N (-·); 1 (····-) as A (·-); and 0 (-----) as T (-).
SSTV
On SSTV, the RSV system is used as text overlaid on the image being sent (as is standard). Readability and strength are again equivalent, but an additional number measures video quality. A V of 1 indicates heavy distortion or interference; a V of 5 (the highest) indicates a perfect image.
WSJT-X modes and JS8
On several digital modes, including FT4, FT8, and JS8, the RSx exchange is replaced by the SNR (signal-to-noise ratio) in dB.