| 摘要 |
1,201,967. Electric calculator; telephone system. INTERNATIONAL STANDARD ELECTRIC CORP. 9 Oct., 1968 [16 Oct., 1967], No. 47844/68. Heading G4A. [Also in Division H4] In a pulse code modulation system providing conference facilities a companding circuit comprises coding and linearizing systems operating according to an approached logarithmic law. The graph, Fig. 1, relating amplitude X as a proportion of permissible maximum to binarycoded compressed signal Y, is made up of a number of straight lines with gradients successively related as 2: 1 on either side of the origin. The highest digit of Y is 1 or 0 according to whether Y is +ve or -ve. In the latter case the code is in complementary form. The amplitudes of the speech pulses of the parties in conference are expressed in linear form in binary-decimal code in register 3, line I, Fig. 3, where the most significant 1 represents the fractional point. At the same time register 4 receives binary 7. Compression then takes place by stepping register 3 leftwards and register 4 down until the position F is reached with the code and mantissa in registers 4 and 3, respectively, the initial 1 in register 3 still representing the fractional point. The value is now expressed in binary-decimal code approximating the logarithm to base 2. Linearization is effected by the reverse process (Fig. 4, not shown). Addition of amplitudes. Fig. 5.-A central control CC receives 7-digit codes a, b of speech values obtained from two parties in a threesided conference. CC then applies a, b to a logical circuit Le in true or complementary form according to whether the highest digit is 1 or 0. Lc and Li from comparison of a and b decide whether the values are to be added or subtracted and whether the result is to be +ve or -ve. The values a, b are then linearized in translator T under control of sequencer S, the resultants being added (or subtracted by complementary addition) in AD and passed back to T for compression. The resultant code is directed to Ls and back to central control CC, which directs the answer to the third party in the conference. |
