Patent GB-1334912-A (GB)
Title: Transmission control device for telephone signal transmitters
IPC: H03G7/00
Filed: 1971-09-08
Abstract: 1334912 Line activity detector THOMSONCSF 8 Sept 1971 [17 Sept 1970] 41998/71 Heading H4R In a transmission system the transmitted signal is sampled at regular intervals and each time a sample is above a predetermined maximum level the attenuation in the transmission path is increased a predetermined increment until, after a number of increments a maximum value is reached, while if none of a predetermined number of signal samples exceeds a predetermined minimum value the attenuation is reduced one step, as long as a predetermined minimum value of attenuation has not been reached. In addition an inactivity detector is enabled at the commencement of each period of transmission to control the end of the transmission period, and an activity detector is enabled at the start of each period of interruption of transmission, to control the resumption of transmission, both detectors being controlled by the attenuator stepping signals. As described with respect to Fig. 1, each time a signal sample exceeds the maximum level a pulse is applied to terminal 16 and on the following clock pulse from 50 a pulse is fed through gate 17 to step on the attenuation counter 13, to increase the loss in the attenuator (not shown), and to step on the inactivity detector 15. Each time the signal samples have failed to reach the minimum level over a predetermined number of samples, equal to a duration of about 12À5 msec., a pulse is applied to terminal 14 and the following clock pulse enables gate 19 to feed a pulse to step back counters 13 and 15. Assuming transmission has just commenced state counter 60 has output 61 activated, allowing transmission of telephone signals between terminals 11 and 6 via gate 55. In addition, the inactivity detector 15 is at a count of 6, due to resetting to this value. During transmission the counters 13 and 15 will be stepped up and down according to the signal level until the signal level drops and remains at low level for such a time as to allow sufficient pulses on input 14 to reduce counter 15 to a count of 5, the number of pulses required depending on the previous signal level history. Reduction to a count of five is regarded as a detection of inactivity and enables "and" gate 51 and the succeeding clock pulse feeds through "and" gate 51, "or" gate 83 and steps on state counter 60 to count 62 which causes enabling of gate 56 and of gate 58 so that the succeeding clock pulse causes generator 3 to transmit an "end of transmission" signal to line 6 via gate 56, and to enable gate 52. The following clock pulse passes through gate 52 and steps state counter 60 to its next state, with line 63 enabled. Resumption of activity is indicated when two samples of the speech signal exceed the predetermined maximum level within the time duration of eight clock pulses. The activity detector comprises a counter 36 which initially is in a rest state with output 30 energized. The first sample to exceed the maximum level causes a pulse to pass through gate 35 and, via gate 82, step on counter 36. Off zero count in counter 36 causes gate 35 to be disabled and gates 34 and 36 to be enabled. Clock pulses pass through gate 46 to step counter 36 through its six remaining states until the counter reaches its zero count again where, due to disabling of gates 34 and 36, the counter holds until a succeeding sample exceeds the maximum level. If, however, a maximum level is exceeded again while the counter 36 is off zero, the resulting pulse will pass through gates 34 and 53 to step on state counter 60 to position 64. Enabling of output 64 causes a start of transmission signal to be generated by generator 67 and fed to the line via gate 57. The generator 67 also enables gate 54 so that the succeeding clock pulse is fed to state counter 60 to shift it to state 61, where gate 55 is enabled to feed telephone signals to the line 6 from terminal 11, while the output pulse from gate 54 also resets the attenuation counter 13 to give zero attenuation and the inactivity detector counter 15 to a count of 6 ready to commence stepping as transmission proceeds. The arrangement is said to be used for echo suppression or control of transmission in a TASI system.