Using Digit Statistics to Word-Frame PCM Signals

When a signal is transmitted by PCM, the receiver must be able to group the serial pulse train into code words before it can properly recover the original signal. This process is called "framing." It is also called "word synchronization," as distinguished from bit synchronization where the time base of the individual pulses is sought. When the pulse train contains several POM signals multiplexed together, there is also the task of multiplex framing or frame synchronization whereby 2985 2986 T H E B E L L SYSTEM TECHNICAL J O U R N A L , NOVEMBER 1964 the individual channels must be identified. Word synchronization can be derived from frame synchronization if the words are always arranged in a definite order within a multiplex frame; otherwise, word synchronization is acquired independently. This article will consider only the problem of word synchronization, hereafter simply called "framing." Framing is ordinarily accomplished by using supplementary framing pulses inserted among the information-bearing pulses at predetermined intervals. The receiver will then find these framing pulses by searching and testing for the unique pattern of these pulses. If the framing pulses are inserted between every word, a substantial loss of channel capacity will result; 011 the other hand, if framing pulses are inserted only occasionally, the PCM words will not be uniformly spaced, which is inconvenient for a sampled-data system. When the PCM signal contains known redundancies, it is possible to accomplish framing without the use of supplementary pulses.