TOPIC:

I have read a the link on bi-quads, I have downloaded the nice zip spreadsheet from a community member here but I still confused on the all pass filters.

The spreadsheet has three changeable values Freq, Q and Fs, is there some details on what each of these mean?

Fs is the sample frequency. Minidsp works with a fixed 48kHz.
Freq. is the centre frequency of the phase shift.
Q is the "steepness" of the phase shift.

Some plots to give you an idea.

Phase 1000Hz, Q=1:


Phase 1000Hz, Q=2:


Group delay 1000Hz, Q=1:


Group delay 1000Hz, Q=2:

As usual, Peter stepping up to the plate to share his knowledge on biquad programming. Thanks Peter!

For all other members, we may recommend that you have a look at Robert Bristow-Johnson's work (RBJ), which will give you a better idea on how to build Biquad and equations behind it. His EQ cookbook is a typical resource for any beginner:

www.musicdsp.org/files/Audio-EQ-Cookbook.txt

Hope this helps,

PDF version: www.musicdsp.org/files/EQ-Coefficients.pdf