Re: [digitalradio] Noise Reduction and the digital modes
Hello Andy and all, I don't think NR must be a so good idea for digimodes. Because, it can be seen as non-linear filter. In that type of filter, the next sample is calculated, knowing the previous symbols and guessing what is the most probable symbol if nothing change (a sort of no more set of information condition)... You are surely going to produce interference between symbols: the decoding will be not so good and the necessary synchronization will be more difficult because the difference between one symbol and the following will be reduced (i.e the difference between two successive symbols will be softened). But it would be interesting to experiment on calibrated signals and different speeds (from the PSKAM10 to ALE or PSK220F). 73 Patrick - Original Message - From: Andrew O'Brien To: digitalradio@yahoogroups.com Sent: Sunday, March 04, 2007 4:01 AM Subject: [digitalradio] Noise Reduction and the digital modes Noise reduction on my old rig never did much. Now I have a new rig the NR buttons actually appear to do some things. What about noise reduction and the digital modes, is it really helpful? I did just notice that with a Hell QSO NR made the print more blurred. For other modes I have not noticed much other than a lot less speckles in the waterfall. Maybe a graphically empty waterfall helps when a weak signal comes along but i worry the weak signal may get zapped by the NR. Comments ? -- Andy K3UK Skype Me : callto://andyobrien73 www.obriensweb.com
Re: [digitalradio] Noise Reduction and the digital modes
Patrick Lindecker wrote: Hello Andy and all, I don't think NR must be a so good idea for digimodes. Because, it can be seen as non-linear filter. I disagree on the transfer function. It is an adaptive linear filter. Since it does not mix two tones in the passband, it can't be nonlinear. However, it does indeed introduce serious phase and amplitude distortion on the signals. This is not the way to better copy. These Widrow type/ LMS adaptive filters, in single sample update, or block adaptive form are intended TO AID THE HUMAN FATIGUE FACTOR in listening to noise or interfering tones. We agree that they are no good for digital modes. In that type of filter, the next sample is calculated, knowing the previous symbols and guessing what is the most probable symbol if nothing change (a sort of no more set of information condition)... You are surely going to produce interference between symbols: the decoding will be not so good and the necessary synchronization will be more difficult because the difference between one symbol and the following will be reduced (i.e the difference between two successive symbols will be softened). But it would be interesting to experiment on calibrated signals and different speeds (from the PSKAM10 to ALE or PSK220F). 73 Patrick Bob N4HY -- AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair Taking fun as simply fun and earnestness in earnest shows how thoroughly thou none of the two discernest. - Piet Hine
Re: [digitalradio] Noise Reduction and the digital modes
Hello Robert, TKS for the correction. I returned to my books. LMS filters are in general linear (LMS-FIR), however they can also be in a recursive structure (LMS-IIR). We agree that they are no good for digital modes. Yes the a priori is not very favourable. 73 Patrick - Original Message - From: Robert McGwier To: digitalradio@yahoogroups.com Sent: Sunday, March 04, 2007 6:15 PM Subject: Re: [digitalradio] Noise Reduction and the digital modes Patrick Lindecker wrote: Hello Andy and all, I don't think NR must be a so good idea for digimodes. Because, it can be seen as non-linear filter. I disagree on the transfer function. It is an adaptive linear filter. Since it does not mix two tones in the passband, it can't be nonlinear. However, it does indeed introduce serious phase and amplitude distortion on the signals. This is not the way to better copy. These Widrow type/ LMS adaptive filters, in single sample update, or block adaptive form are intended TO AID THE HUMAN FATIGUE FACTOR in listening to noise or interfering tones. We agree that they are no good for digital modes. In that type of filter, the next sample is calculated, knowing the previous symbols and guessing what is the most probable symbol if nothing change (a sort of no more set of information condition)... You are surely going to produce interference between symbols: the decoding will be not so good and the necessary synchronization will be more difficult because the difference between one symbol and the following will be reduced (i.e the difference between two successive symbols will be softened). But it would be interesting to experiment on calibrated signals and different speeds (from the PSKAM10 to ALE or PSK220F). 73 Patrick Bob N4HY -- AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair Taking fun as simply fun and earnestness in earnest shows how thoroughly thou none of the two discernest. - Piet Hine
Re: [digitalradio] Noise Reduction and the digital modes
Patrick Lindecker wrote: Hello Robert, TKS for the correction. I returned to my books. LMS filters are in general linear (LMS-FIR), however they can also be in a recursive structure (LMS-IIR). Patrick, thanks for your reply. An IIR filter (with an FIR component and a feedback component) is still a linear filter. Even if you are adapting it, it is still a linear transfer function for each and every sample. The transfer function is Sum(Outputs * Feeback_coeffients) = Sum(Inputs * FeedForward_ Coefficients is linear on both sides and this is an IIR. The adaptation is funky and may be a nonlinear adaption, but at each sample instant a linear filter is applied to all samples in its delay lines and so no mixing can occur. Now, if you are thinking about decision directed LMS equalizers, where you make a hard decision as the output, that is most decidedly nonlinear. You do not hard limit the output of your NR filter! The typical AGC circuit in a receiver is more nonlinear than any LMS based NR filter, FIR, IIR, etc. could ever be. Bob We agree that they are no good for digital modes. Yes the a priori is not very favourable. 73 Patrick 73's Bob N4HY -- AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair Taking fun as simply fun and earnestness in earnest shows how thoroughly thou none of the two discernest. - Piet Hine
