| 摘要 |
847,626. Radio transmitting and receiving systems. PHILIPS ELECTRICAL INDUSTRIES Ltd. April 26, 1957 [April 26, 1956], No. 13379/57. Class 40(5). A single sideband radio system for the transmission of speech, of the type described in Specification 797,696 in which a limiter follows the single sideband generator at the transmitter and a limiter precedes the demodulator at the receiver, is modified in that instead of the transmission of a voltage representing the signal envelope, a switching voltage is transmitted which varies between fixed values in accordance with the presence or absence of letter sound signals, which at the receiver controls a blocking switch which opens or closes the speech signal channel in accordance with the presence or absence of letter sound signals. The system provides improved working with low signal-to-noise ratios. At the transmitter, Fig. 1a, speech signals from the microphone 1 are fed via a pre-emphasis network 2 and amplifier 3 to the single sideband modulator 4 which also receives an input from the carrier wave generator 5. The output of the modulator 4 is passed via a filter 6, which selects the upper sideband, a limiter 7, the make contact 8 of a relay 9, e.g. an electronic relay, a further limiter 10, and a single sideband filter 11 which also passes the carrier frequency, to the frequency changer 17 and aerial 19. An amplitude detector 12 is supplied from the amplifier 3 via a filter 13 and the envelope voltage derived from the detector 12 is applied via a low-pass filter 14 having a cut-off frequency of from 100 to 200 c.p.s., and a D.C. amplifier 15, to the relay 9. In the absence of speech signals the contact 16 of the relay 9 is closed and the carrier wave from the oscillator 5 is transmitted. As soon as speech frequencies representing letter sounds are present the relay 9 is operated, and the transmission of the carrier wave ceases, a single sideband speech signal being supplied to the limiter 10, and when the letter sound signal stops the relay 9 is released again. At the receiver the incoming signals are applied via a frequency-changer 21, filter 23, intermediate frequency amplifier 24, filter 25, limiter 26, and contact 27 of a relay 28, to a single sideband demodulator 29, the demodulated signal being supplied via a low-pass filter 31, and amplifier 32 having a de-emphasis network, to the loudspeaker 33. A filter 34 is connected to the output of the amplifier 24 to select the switching frequency which is applied to a frequencychanger 35 whose output is applied through a filter 39 to control the relay 28. When the switching voltage is received the relay 28 through the contact 27 interrupts the speech signal channel, thus ensuring that interference signals which occur during the intervals between letter sounds cannot reach the loudspeaker 33. Details of the transmitted circuit are given in Fig. 3a (not shown) and of the receiver circuit in Fig. 3b (not shown). |
