June 26, 2015
Hello! Just got the book the other day and I'm loving it so far - I've run into a road block though.
On page 76-77 we note that the one-sample delay added 180 degrees of phase shift at the Nyquist frequency. Then we move through the 1/2 nyquist and 1/4 nyquist frequencies (I think I understand this part).
The problem comes when I get to page 85, Figure 4.19. The Phase graph for the filter coefficients we'd been using the whole section show a -90 degree phase shift at the Nyquist rate, not a 180 degree shift.
I was confused by this, so when I programmed the filter in the next section I set both coefficients to 0.5 and sure enough the phase plot agreed with what I thought - it shows -180 degree shift at ~22kHz, -90 degree shift at ~11kHz and ~-45 degree shift at ~5.5 kHz.
Essentially it looks like my filter response is off by a factor of 2 from what the book shows.
So to summarize:
Page 76-77 and my filter response graph (in linear mode) say that the phase shift is -180 degrees at 22kHz (the Nyquist rate in this case)
Figure 4.18 and 4.19 say that the phase shift is -90 degrees at 22kHz
What am I missing?
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