| 摘要 |
1506517 Data transmission PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 12 Sept 1975 [16 Sept 1974] 37644/75 Heading H4P A transmitter 2 for use in a vestigial side band data transmission system constituted by the transmitter 2, a transmission channel 5 and a receiver 4 comprises a data signal source 1; a clock signal source 6 for synchronizing the source 1 at a data rate, for example, of 2400 bits/second; a carrier source 8; and a filtering and modulating circuit 7 arranged for generating a VSB channel signal which both at the location of its carrier frequency, e.g. 2100 Hz, and at the location of a frequency, e.g. 900 Hz which in the complete sideband is spaced from this carrier frequency by a distance, e.g. 1200 Hz, equal to one half of the clock frequency, is a double sideband modulated signal within a frequency band, having a width, e.g. 120 Hz, smaller by one order of magnitude than the frequency of the clock signal. With such an arrangement, the Specification explains, with reference to Figs. 2b to c (not shown), how the reference carrier and the reference clock signal can be recovered in the receiver 4 by the comparatively simple methods associated with DSB transmissions, even though the VSB signal is used for transmission through the channel 5 which may be a telephone line. As shown, the filtering and modulating circuit 7 includes a pre-modulating low-pass filter 16, an amplitude modulator 17 and a post-modulating filter 18, the whole being designed to produce the required vestigial sideband signal for supply to the transmission channel 5. The receiver 4 includes circuits 12, 15 to recover the reference carrier and clock signals, and suitable arrangements for these circuits are described with reference to Figs. 3 and 4 (not shown). To ensure that sufficient energy is available under all circumstances for recovery, low level pilot signals at the appropriate frequencies may also be added to the transmitted signal. Modifications of the filtering and modulating circuit 7, permitting the required VSB signal to be produced by phase-shifting techniques, are described with reference to Figs. 6 and 7 (not shown). Dibit encoding may be employed to increase the data rate. |
