On Jul 31, 9:58�am, dbd <d...@ieee.org> wrote:> On Jul 31, 5:27 am, Dirk Bell <bellda2...@cox.net> wrote: > > > > > > > On Jul 31, 3:42 am, dbd <d...@ieee.org> wrote: > > > > On Jul 30, 10:57 pm, Dirk Bell <bellda2...@cox.net> wrote: > > > > > On Jul 31, 12:42 am, robert bristow-johnson > > > > > <r...@audioimagination.com> wrote: > > > > > On Jul 31, 12:20 am, Dirk Bell <bellda2...@cox.net> wrote: > > > > > > > On Jul 30, 1:47 pm, robert bristow-johnson <r...@audioimagination.com> > > > > > > wrote: > > > > > > > > On Jul 30, 1:09 pm, Dirk Bell <bellda2...@cox.net> wrote: > > > > > > > > > I was reading a chapter in the DSP tips and tricks book on using comb > > > > > > > > filtering after two-level saturated sampling for FSK demod > > > > > > > > preprocessing. The text said a comb filter equivalent could be made > > > > > > > > with a direct path and a delayed path input into an XOR gate because > > > > > > > > the inputs were binary. �However, for the binary inputs, the outputs > > > > > > > > of an XOR are normally 2 level, while the outputs of the comb filter > > > > > > > > are 3 level. > > > > > > > > yeah, i would think the output would be a 2-bit number from the > > > > > > > following truth table: > > > > > > > > I1 � I2 � � OUT > > > > > > > ----------------- > > > > > > > 0 � �0 � � �10 � � �aka "-1" > > > > > > > 0 � �1 � � �00 > > > > > > > 1 � �0 � � �00 > > > > > > > 1 � �1 � � �01 � � �aka "+1" > > > > > > > > so, i think i understand what you're questioning here, Dirk. > > > > > > > > r b-j > > > > > > > Hi r b-j, > > > > > > > With the DSP algorithm provided, the comb filter is specified as > > > > > > follows, and compared to the output with the XOR replacing the adder: > > > > > > > (second column is delayed bits) > > > > > > + Bit � - Bit � Out �XOR > > > > > > -------------------�-------------- > > > > > > 0 � � � �0 � � � �0 � � � 0 > > > > > > 0 � � � �1 � � � -1 � � � 1 (problem here) > > > > > > 1 � � � �0 � � � �1 � � � 1 > > > > > > 1 � � � �1 � � � �0 � � � 0 > > > > > > yeah, i didn't realize i had a + and - bit here. �i treated them as > > > > > having identical polarity, but the analog stream was (-1)^a[n] where a > > > > > [n] is a member of {-1, 1} like MLS is. �then you add (and XOR) the > > > > > bits when signals are multiplied (as in correlation). �either way we > > > > > look at it, 3 levels (and a single bit) ain't enough. > > > > > > r b-j- Hide quoted text - > > > > > > - Show quoted text - > > > > > r b-j, > > > > > The "Plus bit" gets added, and the "minus bit" gets subtracted in the > > > > normal linear filter implementation with the bit values being in > > > > {0,1}. > > > > > Later in the chapter (appendix?) they talk about the binary signals > > > > being represented by +-1 into the XOR. Still need 3 levels out though > > > > to reproduce the same results as the normal linear filter > > > > implementation. > > > > > Dirk > > > > In the publication of the tip in: > > > > �Filtering tricks for FSK demodulation > > > Shiung, D. � Huei-Wen Ferng � Lyons, R. > > > Signal Processing Magazine, IEEE > > > �May 2005, Volume: 22, �Issue: 3, page(s): 80- 82 > > > > the figures show the comb filter output truncated to zero at those > > > times when the output is negative. So try including this limiter for > > > the comb filter. > > > > Dale B. Dalrymple- Hide quoted text - > > > > - Show quoted text - > > > Hi Dale, > > > The limiter in the article is applied in the the same manner as the in > > the chapter, applied prior to the comb filtering, They get 3 level > > outputs in the article too. > > > Dirk > > > Dirk Bell > > DSP Consultant > > Doesn't the comb filter -output- limited to 0,1 give the desired > frequency at a constant duty cycle? > > Dale B. Dalrymple- Hide quoted text - > > - Show quoted text -Hi Dale, It seems the objective of the comb filtering after clipping is to produce a symmetric output with minimal DC component and reduced harmonics. For a constant clipped sine wave, at the output of the comb filter it spends as much time high as low (+1, -1) and the rest of the time at 0. It has less harmonics than the original clipped waveform where the duty cycle depends on the DC offset going into the limiter; less harmonics desirable for follow-on processing. If you wanted a two- level constant (but not necessarily predetermined) duty cycle waveform, you have that coming out of the limiter without any comb filtering. Dirk Bell DSP Consultant
QUESTION FOR Rick Lyons Re: tips and tricks book
Started by ●July 30, 2009
Reply by ●July 31, 20092009-07-31
Reply by ●July 31, 20092009-07-31
On Thu, 30 Jul 2009 10:09:26 -0700 (PDT), Dirk Bell <bellda2005@cox.net> wrote:>Hi Rick, > >I was reading a chapter in the DSP tips and tricks book on using comb >filtering after two-level saturated sampling for FSK demod >preprocessing. The text said a comb filter equivalent could be made >with a direct path and a delayed path input into an XOR gate because >the inputs were binary. However, for the binary inputs, the outputs >of an XOR are normally 2 level, while the outputs of the comb filter >are 3 level. That makes it look like the equivalent XOR circuit >described isn't possible. > >What am I missing? > >Dirk > >Dirk Bell >DSP ConsultantHi Dirk, You're not missing anything. Your question is super-sensible and points out an unfortunate source of confusion. The software modeling of the comb filter, used to generate Figures 4-5(a) and 5-5, used the multi-bit comb shown in Figure 5-6(b). That's why there are three amplitude levels in Figures 4-5(a) and 5-5." I apologize for not detecting that confusion years ago when I was working with D. Shiung and H. Ferng on the article. Thanks for your post Dirk. I'll add a note to the book's errata in an attempt to clarify this issue. Later tonight I'll post, here on comp.dsp, my current version of the book's errata for you. Regards, [-Rick-]
Reply by ●August 3, 20092009-08-03
On Jul 31, 7:33 am, Dirk Bell <bellda2...@cox.net> wrote:> Hi Dale, > > It seems the objective of the comb filtering after clipping is to > produce a symmetric output with minimal DC component and reduced > harmonics. For a constant clipped sine wave, at the output of the comb > filter it spends as much time high as low (+1, -1) and the rest of the > time at 0. It has less harmonics than the original clipped waveform > where the duty cycle depends on the DC offset going into the limiter; > less harmonics desirable for follow-on processing..>If you wanted a two- .> level constant (but not necessarily predetermined) duty cycle .> waveform, you have that coming out of the limiter without any comb .> filtering. .> .> Dirk Bell .> DSP Consultant Yes, but the duty cycle changes from the simple limiter due to DC offset changes or other low frequency noise appear as timing jitter. The comb filter reduces this. Since the 0->1 and 1->0 transitions contain the information for the FSK application, the 'trick' is useful to simplify getting this information. Dale B. Dalrymple
