# What is the maximum allowed transition band of the low-pass filter used in the reconstruction kernel for a CD with maximum speech frequency of 4 kHz?

My question is if the transition band in the question below would be from 4Khz to the Nyquist frequency of 22.05Khz or from 4Khz until 44.1Khz-4Khz=40.1Khz?

If you don't want DAC noise near 40-44 kHz to alias down into your audio, you want the narrower/lower transition band reconstruction filter.

• So, upper limit is at 22.05kHz? – Dokams May 1 '16 at 12:19
• If the signal bandwidth is $B$, the LPF response should be flat up to $B$, then go to 0 as steeply as feasible. – MBaz May 1 '16 at 18:32