Adaptive time compression
The drawback with time-compressed SF drive is the reduction in panel brightness as a result of reducing the total sustain period of each SF. This is especially significant for darker images. To overcome this, adaptive time-compressed drive is used in which the compression ratio is changed with varying picture brightness. For a bright image, a high compression ratio is used and vice versa for dark images. In this way, images with a low average picture level (APL) have a longer sustain period and increased level of brightness. Figures 10.20 and 10.21 show the scan and sustain wave- forms for low and high APL pictures respectively.
Sub-field splitting
In addition to time-compression, one or two highly weighted SFs may be split into two lower-weighted SFs. With the conventional binary coding, the maximum intensity weighting is provided by SF8 with 128. If this SF was split into two SFs, each with a weighting of 64, then if a false contour described earlier is to take place, the visualised intensity would be greatly
reduced to 127 + 64 = 191. In practice, using this SF splitting, nine SFs are employed with a weighting distribution shown in Figure 10.22. SFs corre- sponding to 128 and 64 are divided into four SFs of weighting 48 each and positioned at either end of the SF set. In the figure, the SF with weighting 1 is neglected to reduce the number of SFs and maintain the total level of brightness. The effect of removing the LSB SF is compensated for by an error diffusion circuit.
Non-binary sub-field coding
Non-binary coding is similar to SF splitting in that it removes the very high-weighted SF8 and introduces a larger number of lower-weighted SFs that can be selected for any greyscale level to reduce the effect of false con- tours. Figure 10.23 shows a typical non-binary 12-SF drive system.