PCM Principle

For multi-channel digital telephone systems, two standardized formats have been internationally recommended, namely the PCM 30/32-channel (A-law companding characteristic) format and the PCM 24-channel (μ-law companding characteristic) format. It is stipulated that for international communications, the A-law companding characteristic shall prevail (i.e., the 30/32-channel format shall prevail). Wherever conversion between the two formats is required, the equipment interface shall be the responsibility of the country adopting the μ-law characteristic. Therefore, China has stipulated the adoption of its frame and multiframe structure. In the PCM 30/32-channel format, a multiframe consists of 16 frames; a frame consists of 32 timeslots; and a timeslot is an 8-bit code group. Timeslots 1 to 15 and 17 to 31, totaling 30 timeslots, are used as speech channels to transmit voice signals. Timeslot 0 (TS0) is the "frame alignment code group," and timeslot 16 (TS16) is used to transmit signaling codes for each speech channel.

In terms of time, since the sampling repetition frequency is 8000 Hz, the sampling period is 1/8000 = 125 μs, which is also the frame period of PCM 30/32. A multiframe consists of 16 frames, so the multiframe period is 2 ms. Within one frame, 32 channels are time-division multiplexed, so each channel occupies a timeslot of 125 μs/32 = 3.9 μs. Each timeslot contains an 8-bit code group; therefore, each code element occupies 488 ns.

In terms of transmission rate, 8000 frames can be transmitted per second, and each frame contains 32 × 8 = 256 bits. Therefore, the total bit rate is 256 bits/frame × 8000 frames/s = 2048 kb/s. For each speech channel, 8000 timeslots are transmitted per second, and each timeslot is 8 bits. Thus, the information transmission rate after digitization of each speech channel is 8 × 8000 = 64 kb/s.

To ensure strict synchronization between the transmitting and receiving ends, each frame transmits a specific set of frame alignment code groups or monitoring code groups. The frame alignment code group is "0011011," occupying bits 2 to 8 of TS0 in even frames. Bit 1 is reserved for international use and transmits "1" when not in use. In odd frames, bit 3 is used for frame loss alarm; it transmits "0" when synchronized and "1" when out of synchronization. To prevent false alignment code groups from appearing in bits 2 to 8 of odd TS0, bit 2 is designated as a monitoring code, fixed at "1," and bits 4 to 8 are for domestic use, currently tentatively set to "1."