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Entry  Wed Jul 25 16:55:01 2012, Den, Update, digital noise, notch, lowpass filters 6x
    Reply  Wed Jul 25 23:03:22 2012, rana, Update, digital noise, notch, lowpass filters 
       Reply  Thu Jul 26 10:22:03 2012, Den, Update, digital noise, notch, lowpass filters same_bw.png16384_bw0p8.pngcorners_bw0p1.pngcorners_bw0p01.png
Message ID: 7031     Entry time: Wed Jul 25 16:55:01 2012     Reply to this: 7034
Author: Den 
Type: Update 
Category: digital noise 
Subject: notch, lowpass filters 

 Direct Form 2 is noisy in the first test. This is the one similar to Matt's in his presentation. Input signal was a sine wave at 1 Hz with small amplitude white noise x[n] = sin(2*pi*1*t[n]) + 1e-10 * random( [-1, 1] ). It was filtered with a notch filter: f=1Hz, Q=100, depth=210dB. SOS representation was calculated in Foton. Sampling frequency is 16kHz.

iir_psd.png        iir_time.png   iir_coh.png

DF2 output noise level is the same if I change white noise amplitude while DF1, BQF, LNF can follow it. Time series show quantization noise of DF2. I've plotted coherence of the signals relative to DF1, because non of the signals will be coherent to it at low frequencies due to fft calculations.  

In the second test the input was white noise  x[n] = random( [-1, 1] ) It was filtered with a 2 order low-pass butterworth filter with cut-off frequency f = 0.25 Hz. SOS representation was calculated in Python. Sampling frequency is 16kHz.

iir_psd_lowpass.png         iir_time_lowpass.png      iir_coh_lowpass2.png

In this test all implementations work fine. I guess dtt works with single precision and for that reason we see disturbance in coherence when we do the same test online.

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