Information Technology Reference
In-Depth Information
3.
In scaleable compression, a number of different pixel array sizes can be decoded, depending on how much
of the compressed bitstream is decoded. Decoding at high resolution involves upsampling the lower-
resolution image before adding detail to it.
4.
Changing between sampling formats may be necessary in order to view an incoming image on an available
display. This technique is generally known as
resizing
and is essentially a two-dimensional sampling rate
conversion, the rate in this case being the spatial frequency of the pixels.
5.
In MPEG-4 a compression tool known as warping is introduced. This allows the changes in perspective as
objects turn to be coded as a few vectors instead of a large picture change. The warping process is a
twodimensional interpolation.
There are three basic but related categories of interpolation, as shown in
Figure 3.16
.
The most straightforward (a)
changes the sampling rate by an integer ratio, up or down. The timing of the system is thus simplified because all
samples (input and output) are present on edges of the higher- rate sampling clock. Such a system is generally
adopted for oversampling convertors; the exact sampling rate immediately adjacent to the analog domain is not
critical, and will be chosen to make the filters easier to implement.
Figure 3.16:
Categories of rate conversion. (a) Integer-ratio conversion, where the lower-rate samples are always
coincident with those of the higher rate. There are a small number of phases needed. (b) Fractional-ratio
conversion, where sample coincidence is periodic. A larger number of phases is required. Example here is
conversion from 50.4 kHz to 44.1 kHz (8/7). (c) Variable-ratio conversion, where there is no fixed relationship, and
a large number of phases are required.
Next in order of difficulty is the category shown at (b) where the rate is changed by the ratio of two small integers.
Samples in the input periodically time-align with the output. Such devices can be used for converting between the
various rates of ITU-601.
The most complex rate-conversion category is where there is no simple relationship between input and output
sampling rates, and in fact they may vary. This situation, shown at (c), is known as variable-ratio conversion. The
temporal or spatial relationship of input and output samples is arbitrary. This problem will be met in effects
machines which zoom or rotate images and in MPEG-4 warping.
The technique of integer-ratio conversion is used in conjunction with oversampling convertors in digital video and
audio and in motion estimation and scaleable compression systems where sub-sampled or reduced-resolution
versions of an input image are required.
Figure 3.17
(a) shows the spectrum of a typical sampled system where the sampling rate is a little more than twice
the analog bandwidth. Attempts to reduce the sampling rate by simply omitting samples, a process known as
decimation, will result in aliasing, as shown in
Figure 3.17
(
b). Intuitively it is obvious that omitting samples is the
same as if the original sampling rate was lower. In order to prevent aliasing, it is necessary to incorporate low-pass
filtering into the system where the cutoff frequency reflects the new, lower, sampling rate. An FIR type low-pass
filter could be installed, as described earlier in this chapter, immediately prior to the stage where samples are
omitted, but this would be wasteful, because for much of its time the FIR filter would be calculating sample values
which are to be discarded. The more effective method is to combine the low-pass filter with the decimator so that
the filter only calculates values to be retained in the output sample stream.
Figure 3.17
(c) shows how this is done.
