Patent GB-1511546-A (GB)
Title: Reducing the length of digital words
IPC: H03M7/30
Filed: 1975-10-20
Abstract: 1511546 Methods of coding; compression WESTERN ELECTRIC CO Inc 20 Oct 1975 [25 Oct 1974] 43028/75 Heading H4P A method of reducing the bit rate of non- uniformly coded digital words each containing first bits identifying a range of values and second bits identifying a value in the range comprises identifying the maximum value of first bits in a group of input words and processing each word in the group in dependence on difference between maximum value of first bits in the group and first bits of each respective word to produce words having fewer bits which are transmitted together with the maximum value. In the embodiment 8 bit words are translated into 6 bit words by determining the maximum value of a particular segment i.e. first bits in each word relative to groups of input words and processing each word according to the difference between group maximum value and the actual value of each segment of each word. The segment in each word may be replaced by a single bit of one type preceded by a selected number of bits of other type dependent on the difference. Each 8 bit word is clocked in parallel form by C2 pulses into 9 stage registers 112-118, 112 storing most significant bit (m.s.b.) and 114 storing least significant bit (l.s.b.) of the segment values, and a 10 stage register 111 storing sign bits, the additional stage allowing for processing delays within the arrangement. Registers 115-118 contain 4 bits representing one of 16 intervals into which each segment is divided. Registers 112-114, receiving A-C segments are connected to latch circuit 121 which accepts bits only if enabled by a C3 clock pulse through OR 124 or a C2 clock pulse + comparator 122 output; compparator 122 only gives an output if segment value of A + B + C is greater than the previously stored A + B + C into 122; a C3 pulse enables 121 at the beginning of a new word and previous maximum is transferred into 126, output (designated M2-M0, M2 being m.s.b.) from which is coupled to down counter 133 enabled by C2+C1 pulses, also to a serializer 190 for subsequent transmission. C2 pulses also shift register 111 output being passed direct to 190 and 112-114 which are coupled to OR gate 149 and thence to first stage of 5 stage register 160; 112-114 are also coupled to gates 152-154 receiving counter 133 outputs on their inhibit terminals. Registers 115-118 outputs pass into register 160 at C2+C1 time if 160 is enabled together with counter 133 by AND 157 which also permits each C1 pulse to reduce 133 by 1 and register 160 to shift one stage hence permitting a 0 from 158 to enter the first stage, all provided that C2 inhibit inputs are not present. Positive going transitions of C1 shift register 160 by an amount dependent on difference between maximum segment value initially stored in 133 and the segment value presented by signals on lines 142-144. When difference = 0, gate 156 has 0 inputs hence 157 is disabled; 157 is also disabled when either of significant bit outputs of 133 = 0. Output of register 160 is passed to serializer 190 also receiving square clock pulses through line 184. Sync words may be transmitted to indicate that a new group has begun followed by maximum segment bits and processed digital words. At the receiver (Fig. 2, not shown) transmitted bits are received into a deserializer (290) having a clock pulse extraction arrangement, the bits M2-M0 passing through a latch circuit (226) into a counter (233) and thence to output and N0-N4 through a register (260) of similar configuration to 126, 133, 160 of the transmitter, the sign bit S with one bit delay being passed direct to output. In the embodiment described, C2 pulses are derived by ¸ 6 division from C1 and C3 by ¸ 8 division from C2.  The 8 to 6 bit and back to 8 bit translation described is illustrated tabularly in Fig. 4 (not shown).