RADIO RECEIVES WITH MORE THAN ONE RADIO-FREE CHANNEL FOR DIGITAL TALE TRANSMISSION

摘要

1476578 Radio transmitter-receivers INTERNATIONAL STANDARD ELECTRIC CORP 10 July 1975 [10 July 1974] 29041/75 Heading H4L A radio transmitter-receiver of the kind employing delta modulation for digital speech transmission in one of a plurality of channels, the modulator producing 0/1 sequences during silent intervals with a scrambler and an unscrambler adjusted to the transmitter-receiver's own address in the ready-to-receive state and switchable to a desired address for transmission, includes a timing circuit 17 which automatically switches the set to the ready-to-receive state if for a predetermined time the set has neither transmitted, nor received 0/1 sequences from another such set. In the receive mode the output of unscrambler 4 is connected via a switch 10 to an 0/1 sequence recognition unit 11. An output signal corresponding to the presence of an 0/1 sequence passes via a connection 15 to an OR gate 16 and starts timing circuit 17 producing logic "1" which then prevents channel switching unit 19 from switching local oscillator 21 to another channel. Timing circuit 17 also has connections to an AND gate 27 and to a control circuit 29, and if a push-to-talk button 30 is depressed while 17 is still on control circuit 29 keeps 17 on via a connection 39, and the output of AND gate 27 causes the set to switch to the transmit mode. If timing circuit 17 is off when button 30 is depressed a channel idle test unit 32 searches for an idle channel, and if it locates one it restarts the timing circuit and thus stops channel switching unit 19. At the same time a call timing circuit 33 is operated which first connects an 0/1 sequence generator 35 to scrambler 5 and then switches the latter over to a delta modulator 38 and microphone 37. When timing circuit 17 turns off, logic "0" applied over a connection 46 resets address input unit 36 to the station's own address. To insert 0/1 sequences into silent intervals during the transmit mode, a first embodiment includes connections 42, 43 to switches 10, 12 respectively, so that in the transmit mode the output of delta modulator 38 passes into 0/1 sequence recognition unit 11, the output of which passes into a further timing circuit 44 having a shorter time constant than timing circuit 17. After a silent interval of predetermined length, 44 switches 0/1 sequence generator 35 into the input of scrambler 5, for a period so brief that intelligibility is unaffected. In a second embodiment (not illustrated) the output of delta modulator 38 is applied to unscrambler 4 which is permanently connected to the input of 11.