Forums

RLB Weighting

Started by ikerr April 24, 2010
Greetings,

I have been reading through the ITU-R BS.1770-1
(http://webs.uvigo.es/servicios/biblioteca/uit/rec/BS/R-REC-BS.1770-1-200709-I!!PDF-E.pdf)
document and have been trying to reproduce the frequency response of the
RLB weighting using the supplied filter coefficients.  However, when I use
the supplied filter coefficients to construct a Bode plot in octave, the
resulting plot attenuates more quickly than the one shown in Figure 4.  At
10 Hz, the attenuation is about -55 dB in my octave plot whereas the
attenuation is about -24 dB in Figure 4 of the ITU-R BS.1770-1 document. 
Here is the sequence of octave instructions that I am using:

octave:84> fs = 48000;
octave:85> a = [1 -1.99004745483398, 0.99007225036621];
octave:86> b = [1 -2 1];
octave:87> sys = tf(b, a, 1 / fs);
octave:88> sysout(sys) 
Input(s)
      1: u_1

Output(s):
     1: y_1 (discrete)

Sampling interval: 2.08333e-05
transfer function form:
1*z^2 - 2*z^1 + 1
--------------------------
1*z^2 - 1.99*z^1 + 0.99007
octave:89> bode(sys)

I'm trying to figure out what I'm doing wrong, however, thus far it is
eluding me.  Can someone point me in the right direction?

Thanks,
--Ian


>Greetings, > >I have been reading through the ITU-R BS.1770-1 >(http://webs.uvigo.es/servicios/biblioteca/uit/rec/BS/R-REC-BS.1770-1-200709-I!!PDF-E.pdf) >document and have been trying to reproduce the frequency response of the >RLB weighting using the supplied filter coefficients. However, when I
use
>the supplied filter coefficients to construct a Bode plot in octave, the >resulting plot attenuates more quickly than the one shown in Figure 4.
At
>10 Hz, the attenuation is about -55 dB in my octave plot whereas the >attenuation is about -24 dB in Figure 4 of the ITU-R BS.1770-1 document. >Here is the sequence of octave instructions that I am using: > >octave:84> fs = 48000; >octave:85> a = [1 -1.99004745483398, 0.99007225036621]; >octave:86> b = [1 -2 1]; >octave:87> sys = tf(b, a, 1 / fs); >octave:88> sysout(sys) >Input(s) > 1: u_1 > >Output(s): > 1: y_1 (discrete) > >Sampling interval: 2.08333e-05 >transfer function form: >1*z^2 - 2*z^1 + 1 >-------------------------- >1*z^2 - 1.99*z^1 + 0.99007 >octave:89> bode(sys) > >I'm trying to figure out what I'm doing wrong, however, thus far it is >eluding me. Can someone point me in the right direction? > >Thanks, >--Ian > > >
Indeed it was a simple mistake. The octave bode plot's x-axis has units of radians/second while the plot published in ITU-R BS.1770 has units of Hertz along the x-axis. Converting to the appropriate units gave the desired results. --Ian