| 摘要 |
An improved pulse code modulation (PCM) data transmission system whose pulse amplitude modulation (PAM) stage contains a sampled data or scanning filter for the analog data which is scanned at a frequency fp which is a whole number multiple of the frame frequency fR of the PCM system, and in which quasi four-wire transmission is provided between the PCM system and the connected users or parties without using a four-wire terminating set, whereby only one sampled data filter need be provided both for receiving and transmitting data. According to the invention circuit means are provided for coupling the received transmitted PCM signals, at the scanning frequency fp, to the same sampled data filter utilized to filter the sampled analog signals from the respectively connected users, and further means are provided at the output of the sampled data filter for shifting the center frequency of the carrier-frequency bands utilized for the transmission of data from the PAM stage to the respective connected users so that after further scanning of these carrier-frequency bands at the scanning frequency fp the resulting frequency bands will not fall into the periodic pass bands of the sampled data filter. If a digital sampled data filter is utilized, the received PCM data signals may be fed directly thereto without decoding. Alternatively, if an analog sampled data filter is utilized, then the received PCM data signals must be decoded into PAM signals before they are fed to the sampled data filter.
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