Hi, I'm wondering about the RLB filter coefficients of ITU1770. They are clearly specified for a samplerate of 48 kHz. For other samplerates ITU expects the same shape of curve.
So for computing the coefficients for other samplerates of course a good way is to reconstruct a IIR filter from the given coefficients. An example can be found at http://www-public.tu-bs.de:8080/~y0035293/ITU-R-BS.1770-1-filters.pdf The method decribed here works very well for the prefilter. But it seems that the RLB filter cannot perfectly be described by a biquad IIR filter. You can simply check this: ITU1770 defines the coefficient values a0 = 1, b0 = 1, b1 = -2 and b2 = 1 Indeed the parameters high-pass gain factor Vh = 1, band-pass gain factor Vb = 0 and low-pass gain factor Vl = 0 fit to the equations for b0, b1 and b2. But the equation a0 = 1 = 1 + K/Q + K^2 requires that K^2 = -K/Q With K <> 0 (K=0 would not make sense, fc would be 0) we get K = -1/Q but a negative value for K also does not make sense. Indeed using the data Vh, Vb, Vl, Q, Omega=K as published do not exactly lead to the published filter coefficients, we get deviations of up to 0.5%. You may check out by yourself the formulas of the Audio EQ Cookbook. You will step into the same problem. So IMO the given RLB filter cannot be described by a biquad IIR filter under aspects of perfect accuracy. Which IIR filter will exactly fulfill the conditions specified by the ITU 1770 RLB filter coefficients? Uli -- dupswapdrop -- the music-dsp mailing list and website: subscription info, FAQ, source code archive, list archive, book reviews, dsp links http://music.columbia.edu/cmc/music-dsp http://music.columbia.edu/mailman/listinfo/music-dsp
