Main page These signals fall into two broad classes which comprise one or more tones which shift between a number of fixed frequencies within the channel or an ensemble of independent sub-channels. Members of the first class frequently have a distinctive musical quality to the sound they make whereas the second class are harsh sounding.
There are numerous systems that use this principle including automatic link establishment (ALE) signals (such as MIL-STD-188-141A), PICCOLO, aircraft SELCALLs etc.
Some signals are very short in duration such as the ALE types and will be very difficult to hear, while other short duration multi-tone signals such as SELCALLs are easier because they occur together with lots of voice traffic.
PICCOLO is an example of a long duration multi-tone signal (invented for use by the British Diplomatic service) which is used as a teleprinter. The PICCOLO system transmits ITA No 2 characters at 10 cps (equivalent to 75 baud on a conventional 2-level FSK system). The tone duration is 100 ms on one of 32 frequencies (in a voice grade channel), one for each character supported by the system. PICCOLO has a characteristic musical quality which leads to its name.
Voice Frequency Transmissions (VFT's) are often heard on the HF bands. These comprise a number of slow speed teleprinter channels (or canals) combined together to create an ensemble which is transmitted over a conventional voice grade channel. Typical parameters used are as follows:
| Speed | Frequency | Canal separation |
| 50 baud | 60 or 70 Hz | 120 Hz |
| 100 baud | 80 or 85 Hz | 170 Hz |
| 100 baud | 120 Hz | 240 Hz |
| 200 baud | 170 Hz | 360 Hz |
| 200 baud | 220 Hz | 480 Hz |
Various international standards are used in which 6-24 canals are supported.
It is also possible to send the same data over each canal in a delayed fashion so that the signal is robust to fades or brief periods of noise. This works by combining the individual channels together in a voting process so that those lost or corrupted can be ignored.