Frequency response of the fractional delay component


I observe the frequency response of the fractional delay line in the ADAU1787 as a function of it's configured delay. The project and spectrum screens are attached.

The blue line: the spectrum at the output of the fractional delay

The red line: the reference spectrum

Is it the expected behaviour of this SigmaStudio component ?

Thanks for help!

  • +1
    •  Super User 
    on Apr 8, 2021 11:51 AM

         Hello larmac,

         Yes, it's the fractional delay's normal response.  The fractional delay works by splitting the difference between the two adjacent whole-numbered delays.  Given a requested delay of N.x, where N is an integer and x a decimal remainder, it produces an overall delay of (N+1) and takes a weighted average:  x of  delay N+1 and (1-x) of N.  We can model this as a delay of N (which of course doesn't affect the magnitude response) and a two-tap FIR filter providing the remaining portion of the delay. (this graph was made at Fs = 48K; the audio effect is far less at higher sample rates):

         As you have observed, the worst case is for a delay of fractional part 0.5 -- where the fractional delay becomes a two-tap, moving average FIR filter.  It loses about 12 dB at 20 kHz.  It looks terrible by audiophile standards.  Yet in a typical application where the delay is part of a variable effect, it may get masked rather nicely.  In my case I can't hear the difference (sometimes old age comes in handy).

         Best regards,