Information Technology Reference
In-Depth Information
Figure 3.67:
A de-interlacer needs an interpolator which can operate with input samples which are positioned
arbitrarily rather than regularly.
3.20 Compression and requantizing
In compression systems the goal is to achieve coding gain by using fewer bits to represent the same information.
The band-splitting filters and transform techniques decribed earlier in this chapter
do not
achieve any coding gain.
Their job is to express the information in a form in which redundancy can be identified. Paradoxically the output of a
transform or a filter actually has a longer wordlength than the input because the integer input samples are
multiplied by fractions. These processes have actually increased the redundancy in the signal. This section is
concerned with the subsequent stage where the compression actually takes place.
Coding gain is obtained in one simple way: by shortening the wordlength of data words so that fewer bits are
needed. These data words may be waveform samples in a sub-band-based system or coefficients in a transform-
based system. In both cases positive and negative values must be handled.
Figure 3.68
(a) shows various signal
levels in two's complement coding. As the level falls a phenomenon called
sign extension
takes place where more
and more bits at the most significant end of the word simply copy the sign bit (which is the MSB). Coding gain can
be obtained by eliminating the redundant sign extension bits as
Figure 3.68
(
b) shows.
