Digital Signal Processing Reference
In-Depth Information
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0
1089
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Input Sequence Sample
Figure 3.36:
Original samples (at 8 kHz sample rate) and interpolated values (at 11025 Hz) of a 250 Hz
cosine, same marker system as for the previous figure. Note that at this lower frequency there are many
more samples per cycle of the signal, and the difference between linear and sinc interpolation is much
less. The interpolated samples are shown prior to post-interpolation lowpass filtering.
we must read the ROM at 32 kHz. This is very straightforward. If we happened to have 2 cycles of a
sinusoid in the ROM, over 32 samples, we would only need to have a sample readout rate of 16 kHz.
3.17.2 CONSTANT SR, INTEGRAL DECIMATION
Another method of readout from a ROM containing a sinusoid is to maintain a fixed sample rate
SR
,
and to read out every
n
-th sample.
A formula expressing the net output frequency of a sinusoid produced by reading each address
sequentially from a ROM containing a sinusoid would be
SR
(N / F
ROM
)
=
SR
ยท
F
ROM
N
F
Out
=
where
SR
is the rate at which samples are read from the ROM,
N
is the total number of samples in the
ROM and
F
ROM
is the number of cycles of the sinusoid contained in the
N
samples of the ROM. Most
usually,
F
ROM
= 1, but that need not be the case.
