Pilot tone
In telecommunications, a pilot tone is a signal (usually a single frequency ) that is transmitted outside and independently of the actual useful signal via a communication channel. It is used for monitoring, control, reference or monitoring purposes.
Stereo radio
With FM - stereo radio transmissions a 19-kHz pilot tone is also sent to indicate that a stereo signal is present. This pilot tone is used to obtain the subcarrier required for demodulating the stereo information at 38 kHz (2 * 19 kHz) by doubling the frequency, which is not transmitted. The stereo information itself is sent in the 23–53 kHz range (see pilot tone multiplex method ).
The pilot tone itself does not carry any information (apart from the presence of the stereo signal and the phase position of the carrier , which is important for demodulation ). However, a pilot tone can also be used for the transmission of narrowband data, e.g. B. by being weakly amplitude modulated .
If the receiver does not remove this pilot tone itself, some cassette recorders offer a switch that allows a so-called MPX filter to be switched on, which filters out this pilot tone.
Sound film and tape recordings
In the case of sound films, a pilot tone supplied by a sound generator built into or flanged to the film camera is used to synchronize separate image recordings and sound recordings. Alternatively, a pilot tone supplied by a quartz pilot tone generator can also be used, provided the camera is also quartz-controlled. The sync cable between the film camera and tape recorder is not required.
With the classic pilot tone method, the 50 Hz signal recorded on a separate track is amplified with the aid of a pilot tone amplifier to such an extent that it can be used to drive the synchronous motor of a performance rotor. This is used to transfer the original sound recording from the "Senkel" (1/4 "tape) to Perfoband / magnetic film . With this method, the synchronization between the film recording and the sound recording is automatically guaranteed without any external control. The recorded magnetic film can then be connected to a film cutting table synchronously be processed to the image.
Analog tape recorders such as Nagra or Stellavox also work with a pilot tone process. With analog sound recording, the tape material is transported at a fixed tape speed , but the tape material itself can stretch or shorten slightly due to climatic factors. In order to enable playback at exactly the same speed as when recording, the tape recorder generates a pilot frequency of exactly 50 Hz during recording. This pilot tone is recorded together with the audio signal. The sound represents a kind of electronic perforation, with the help of which it is possible to precisely regulate the tape speed during playback, regardless of any tape slippage or the expansion / shrinkage of the tape material.
Wireless microphones
Wireless microphones transmit with a pilot tone (switchable pilot tone monitoring) etc. a. the following information to the recipient:
- Battery status control,
- MUTE switch status of the transmitter,
- Tone Code Squelch .
These data transmitted by means of the pilot tone are evaluated in the receiver's pilot tone decoder and shown on the receiver's display.
In addition, a wireless receiver that is able to evaluate "its" pilot tone automatically switches off its audio output ("mute") as long as it receives a signal that does not contain a pilot tone. In view of the relatively short range of such a radio link, it is assumed here that it is a foreign or interference signal in such a case.
Baby monitor
Baby monitors have a pilot tone to avoid interference from other radio-operated devices.
Some battery-operated receivers switch to an energy-saving mode if no sound is picked up by the transmitter for a long period of time. The pilot tone of the transmitter also signals to the receiver that the transmitter is ready for operation and is within range.
Commercial radio
Pilot tones in mobile radio signal other units of the same company that this program is for them. If the pilot tone is missing, the broadcast is from outside companies and is automatically hidden. Depending on the manufacturer, the procedure is called CTCSS , subaudio, Privat Line etc. Technically, however, it is always the same and the procedures are compatible with each other.