I. Field of the Invention
This invention relates generally to the electronic signal processing art and, in particular, to an improved digital tone detection method suitable for tone control squelch systems.
II. Description of the Prior Art
As a result of the extensive use of radio communications channels presently available, it has become common, especially in heavily populated metropolitan areas, to have several communication systems simultaneously operating on a common radio channel. Operators whose radios are not equipped with some form of tone squelch must listen to all conversations on the channel from transmitters that are within range. This is both annoying and fatiguing for the operators. The use of tone squelch to alleviate the annoyance is a common solution to the problem. A standard for subaudible continuous tone control squelch systems (CTCSS) is covered in an Electronic Industries Association (EIA) Standard RS 220-A, published in March of 1979.
Basically, such systems operate by equipping each radio transmitter in the particular radio system with a tone encoder operating on one of the 33 standard EIA channels between 67.0 and 250.3 HZ. When the transmitter is keyed, the operator's voice modulates the transmitter in the 300-3000 HZ range and simultaneously with the voice, the subaudible tone modulates the transmitter as well.
Receivers within the radio system are equipped with a frequency selective device (tone decoder) tuned to the same tone frequency as the tone encoder within the transmitters of the system. This tone decoder normally holds the receiver in the squelched mode. If a received signal is not modulated with the tone or is modulated with a tone that is not on the decoder frequency, then the receiver will remain squelched and the operator will not hear the interference. If a transmission is received having a tone corresponding to the frequency of the tone decoder, the receiver will be unsquelched and the operator will receive the message.
Numerous other applications are possible using subaudible tone, such as, control of radio repeaters, identifying a particular vehicle out of a plurality of vehicles, alarm and control signal applications, switching radio receiver or transmitter frequencies, switching communication circuits, etc.
Many of the tone squelch systems now being manufactured employ vibrating mechanical resonant reeds. Resonant reed systems suffer from a number of problems. In order to reprogram such a system the resonant reed must be replaced by a reed of another frequency. Long lead times are involved as service organizations rarely stock the 33 types of encoding and decoding reeds required for this type of change. In addition, vibrating mechanical resonant reeds suffer from short life and false operation when subject to vibration encountered in vehicular two-way radio service.
Until very recently it was difficult to employ electronic means for generating and decoding these low tone frequencies. One related approach to tone detection is referred to as a quadrature correlation detector for a signal of unknown input phase. This prior art electronic approach suffers from lack of flexibility in that it must be designed for a single frequency in a predetermined bandwidth. It is also difficult to integrate a circuit of this type with other circuits of a using system.
Thus, it is desirable to provide an electronic tone detector which can meet EIA standards and which is easily programmable. In addition, it is desirable that such a system be easily integrated with other circuits in a radio system and which minimizes sensitivity to vibration in mobile units and to noise in noisy transmission environments.
Solutions for these and other problems are provided for by the instant invention which is a digital pseudo continuous correlation tone detector. The frequency of the tone to be detected is programmable and can be specified with sufficient accuracy to meet EIA standards for subaudible continuous tone controlled squelch systems. Related subject matter is disclosed in a patent to Backof et al., U.S. Pat. No. 4,216,463.