Chapter 1
"Constant Q" means the ratio of each frequency bin's centre frequency to its bandwidth is constant. Low notes get wide analysis windows (good frequency resolution); high notes get narrow windows (good time resolution).
This mirrors how musical pitch works. On a piano, the distance from C3 to C4 spans the same perceptual interval as C5 to C6, even though the latter covers four times the Hz range. The CQT spaces its bins logarithmically to match, typically placing 12, 24, 36, or more bins per octave.
Q factor: Q = fk / Δfk. In a standard FFT, Q varies — low-frequency bins are narrow (high Q), high-frequency bins are wide (low Q). In the CQT, Q is the same for every bin. This is why musical notes look equally spaced in a CQT spectrogram.