Patent GB-1253400-A (GB)
Title: Remote control system for operation over same audio channel providing voice signals between remote station and base station
IPC: H04B1/40
Filed: 1969-12-17
Abstract: 1,253,400. Voice frequency signalling; radio signalling. MOTOROLA Inc. 17 Dec., 1969 [20 Dec., 1968], No. 61419/69. Headings H4L and H4R. In a voice frequency signalling system, particularly for controlling the operation of a radio transmitter at a base station from a remote station connected to the base station by a line, a signal from the remote station is preceded by a high level signal of a first frequency for a predetermined duration followed by a signal at a normal level at a different frequency to convey the control information. At the base station the high level signal is detected and prevents the application of signals to the line from the base station. Fig. 1 shows one embodiment in which transmission from the transmitter at the base station of speech originating at the remote station is provided by operating one of the keys 12 to 15, to select the transmitted frequency and then operating the " push to talk " switch 10. Switch 10 triggers 100 m.sec timer 25 which disables a 30 dB attenuator 28 and a muting circuit 30 to allow the output of guard tone oscillator 26 to pass at high level, via mixer 32 and line driver 37, to the line. At the end of 100 m.secs. the attenuator 28 is reinserted, to reduce the guard tone level by 30 dB, and the 25 m.sec. timer 36 is initiated which mutes the guard signal at muting circuit 30 and enables function tone oscillator 34 to transmit a signalling frequency characteristic of whichever of the keys 12 to 15 has been operated. At the end of 25 m.sec. The muting circuit 30 is switched out of operation so that the guard tone is transmitted at a low level, the function tone oscillator is disabled, and speech can be fed over the line from source 27, a filter 29 being provided to remove frequencies at the guard tone frequency e.g., 2175 Hz, to prevent faulty operation of the guard tone detector at the base station. To provide a function other than speech transmission, e.g. changing receivers, aerials or power supplies, controlling transmitter power of receiver squelch level &c., one of the keys 18 or 19 is operated, this energizes the 100 m.sec. timer and causes transmission of the guard tone and a selected function tone in a similar way to the " press to talk " switch, except that transmission of the guard tone ceases after the 100 m.sec. period since the mute circuit is not held off by the manually operated button as it is in the " talk " position. At the base station the received guard tone is detected by detector 40 and causes inhibition of the receiver 46, to prevent the receiver feeding audio signals to the line, and also operates 100 m.sec. timer 42. The timer output enables " and " gate 48 for 100 m.sec. and allows function tones to pass to the detectors 50 to 55 to cause the appropriate function change at the transmitter. Each of the outputs of function detectors 50 to 53 is fed to an " or " gate, the output of which inhibits " and " gate 48 to prevent false operation of a second detector after one has responded. An activity checker 58 is provided which maintains the receiver inhibited and the transmitter functioning while any tones are being received. An automatic gain control system is provided on amplifier 39 which increases the gain thereof on detection of a guard tone so that the guard tone detector remains operated even when the 30 dB attenuator is switched into operation in the guard signal channel at the remote station. In a modification, Fig. 5, a disconnect signal is generated on release of the " push to talk " key or the function key. The disconnect signal is provided by enabling the function tone generator and switching its frequency determining capacitor at 20 m.sec. intervals to frequency modulate the output. The signal is detected at the base station and used to return the system to its original state. Circuits for the signalling system are described with respect to Figs. 3 and 4 (not shown).