| 摘要 |
<p>1344312 Digitally encoded colour television POST OFFICE 22 Aug 1972 [27 Aug 1971] 40309/71 Heading H4F An analogue signal including a base band and a modulated subcarrier is differential pulse code modulated by comparing the instantaneous and a previous value of the signal at each sampling instant and encoding the difference; both values having the same subcarrier phase. The invention is particularly directed to the encoding of colour television signals with the previous value being either on the same line or at about the same horizontal position on a previous scan line. A sampling frequency of three times the subcarrier frequency is preferred and is used in all the examples. As shown, (Fig. 5) the signal to be encoded is fed to subtracting element 2. The resulting difference signal is sampled and quantized at 3 and encoded in coder 4. The difference is also applied to adding element 5 and after delay in D1 and D2 the difference signals are recombined at 6A and fed to subtractor 2. The output of 6A is also fed back to adder 5. Delay D1 is the period of the subcarrier frequency, i.e. three times the sampling frequency and gives the "same line" comparison (i.e. RP on Fig. 6, not shown) and delay D2 is the time for one line scan plus or minus half the period of the subcarrier frequency for an NTSC signal, i.e. PU on Fig. 6. Xa 1 and Xa 2 weight the signals to give the best resulting picture which is obtained after transmission by a complementary decoding system 8-11. Either the same line or the previous line delay may be used on its own (Fig. 4, not shown). For PAL signals with line by line phase reversal a similar system using same line comparison (RP on Fig. 7, not shown) and/or two line delay comparison (VP on Fig. 7) may be used or else a different system is used (Fig. 8) in which delay D1 gives "same line" comparison as with the NTSC signal (RP on Fig. 9). D2 produces the delay on the preceeding line and the output signal is modulated at 12 by a carrier of twice the subcarrier frequency, low pass filtered 13 and passed to the subtracting element 2. Further algebraic methods are described for PAL encoding in which for example D1 is the delay PQ (Fig. 9), D2 is the delay PS and D3 (in Fig. 8) is the delay SU.</p> |
