Test signals and reference tones (1kHz, -20dBFS) broadcasters transmit before programming — calibrate transmission chains and receivers. Also called tuning tones.
Before a broadcaster starts its program, standardized reference tones are transmitted – these test signals calibrate the entire transmission chain from the studio to the receiver. The 1-kHz tone at −20 dBFS is the classic example: a pure sine wave that sound engineers and technicians in control rooms immediately recognize and align with their measuring instruments. The function is pragmatic: it's used to check if the levels are correct, if distortions have occurred, and if the frequency responses remain intact.
In classic linear television and radio, these on-air tones were standard. The process was ritualized – they were played about 30 to 60 seconds before broadcast began, sometimes accompanied by test patterns (test slides, color bars). Technicians at broadcasters and distributors could check live if everything was in order. The tones also served as a warning signal: if the sound suddenly disappeared, everyone immediately knew that something was wrong with the transmission. Some viewers still remember this characteristic beep from their childhood.
Today, on-air tones in their classic form have become rarer – streaming services and digital TV employ less analog-influenced calibration routines. However, in professional broadcast environments, for outside broadcasts, and at broadcasters that still transmit terrestrially, they remain the gold standard. They function as silent quality control: a sound engineer can quickly check during a live show if the transmission path is still stable. During editing or the digitization of archive material, we use these reference tones to adjust old recordings – the 1-kHz tone immediately shows us whether the old tape deck is still calibrated or if time and wear have shifted the level.
Related to these are burst tones and pink noise references, which are used in more complex test signals. Modern standards such as the EBU Test Tone or AES reference signals build on this concept but are significantly more finely tuned. Anyone working professionally with audio should add these test signals to their arsenal – not just for understanding broadcast history, but as a practical tool for quick system checks.