Figures
4.17 and
4.18 show
amplitude response and
phase delay, respectively, for 5thorder
Lagrange interpolation,
evaluated over a range of requested delays between
and
samples
in steps of
samples. Note that the vertical scale in
Fig.
4.17 spans
dB while that in
Fig.
4.15 needed less than
dB, again due to the
constrained zero at half the
sampling rate for oddorder interpolators
at the halfsample point.
Figure 4.17:
Amplitude responses,
Lagrange interpolation, order 5, for the range of requested delays
, with and included as well (see
next plot for why).

Figure 4.18:
Phase delays, Lagrange
interpolation, order 5, for the range of requested delays
, with and included as well.

Notice in Fig.
4.18 how suddenly the phasedelay curves
near 2.5 samples delay jump to an integer number of samples as a
function of frequency near half the
sample rate. The curve for
samples swings down to 2 samples delay, while the curve for
samples goes up to 3 samples delay at half the sample rate.
Since the gain is zero at half the sample rate when the requested
delay is
samples, the phase delay may be considered to be
exactly
samples at all frequencies in that special case.
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