Analog Devices SHARC ADSP-21368 User Manual

ADSP-21368 SHARC Processor Hardware Reference 10-7
Asynchronous Sample Rate Converter
Hardware Model
The output rate of the low-pass filter of Figure 10-2 is the interpolation
rate, 2
20
× 192 kHz = 201.3 GHz. Sampling at a rate of 201.3 GHz is
clearly impractical, not to mention the number of taps required to calcu-
late each interpolated sample. However, since interpolation by 2
20
involves zero-stuffing 2
20
– 1 samples between each f
S_IN
sample, most of
the multiplies in the low-pass FIR filter are by zero. A further reduction
can be realized by the fact that since only one interpolated sample is taken
at the output at the f
S_OUT
rate, only one convolution needs to be per-
formed per f
S_OUT
period instead of 2
20
convolutions. A 64-tap FIR filter
for each f
S_OUT
sample is sufficient to suppress the images caused by the
interpolation.
The difficulty with the above approach is that the correct interpolated
sample needs to be selected upon the arrival of f
S_OUT
. Since there are 2
20
possible convolutions per f
S_OUT
period, the arrival of the f
S_OUT
clock
must be measured with an accuracy of 1/201.3 GHz = 4.96 ps. Measuring
the f
S_OUT
period with a clock of 201.3 GHz frequency is clearly impossi-
ble; instead, several coarse measurements of the f
S_OUT
clock period are
made and averaged over time.
Another difficulty with the above approach is the number of coefficients
required. Since there are 2
20
possible convolutions with a 64-tap FIR fil-
ter, there needs to be 2
20
polyphase coefficients for each tap, which
requires a total of 2
26
coefficients. To reduce the amount of coefficients in
ROM, the SRC stores a small subset of coefficients and performs a
high-order interpolation between the stored coefficients. So far the above
approach works for the case of f
S_OUT
> f
S_IN
. However, in the case when
the output sample rate, f
S_OUT
, is less than the input sample rate, f
S_IN
,
the ROM starting address, input data, and the length of the convolution
must be scaled. As the input sample rate rises over the output sample rate,
the anti-aliasing filter’s cutoff frequency has to be lowered because the
Nyquist frequency of the output samples is less than the Nyquist fre-
quency of the input samples. To move the cutoff frequency of the

Analog Devices SHARC ADSP-21368 Specifications

General

Brand	Analog Devices
Model	SHARC ADSP-21368
Category	Computer Hardware
Language	English