Information Technology Reference
In-Depth Information
Figure 3.21:
In 4 oversampling, for each set of input samples, four phases of coefficients are necessary, each of
which produces one of the oversampled values.
Fractional ratio conversion allows interchange between different images having different pixel array sizes.
Fractional ratios also occur in the vertical axis of standards convertors.
Figure 3.16
showed that when the two
sampling rates have a simple fractional relationship
m
/
n
, there is a periodicity in the relationship between samples
in the two streams. It is possible to have a system clock running at the least-common multiple frequency which will
divide by different integers to give each sampling rate.
[
6
]
The existence of a common clock frequency means that a fractional- ratio convertor could be made by arranging
two integer-ratio converters in series. This configuration is shown in
Figure 3.22
(
a). The input- sampling rate is
multiplied by
m
in an interpolator, and the result is divided by
n
in a decimator. Although this system would work, it
would be grossly inefficient, because only one in
n
of the interpolator's outputs would be used. A decimator
followed by an interpolator would also offer the correct sampling rate at the output, but the intermediate sampling
rate would be so low that the system bandwidth would be quite unacceptable.
