Now you speak my language! In PAL and SECAM it was 64us exactly and it is a quartz delay line, not glass, and it was in every color TV since there was no other way how to do it in the early days. (the little box had a whole TV line in sonic transit! :D) - so it was not only delay, it was a FIFO with resolution of about 1000-2000 entries, what we would call it in the digital plane.
But doing a simple "comb" FIR filter as you propose is a good idea anyway and very, very easy to implement in few lines of asm or C. Only thing we really need is to supply exact schematic drawings, exact coefficients to multiply to counteract the errorneous output. We do not want do only do 'some' filtering, ideally we want exact counterfilter for our aches. And we need someone with good math skills to show us precisely how. On 11/6/07, Gene Heskett <[EMAIL PROTECTED]> wrote: > On Monday 05 November 2007, Jon Elson wrote: > >Kenneth Lerman wrote: > >> Steve, > >> > >> The problem with a filter is that it introduces a delay. So a two-tap > >> filter would introduce a delay of at least one servo cycle. Is that better > >> or worse than just halving the servo cycle? Could feedforward compensate > >> for that? > > > >No, if you set up the order of the processes correctly in the > >script that installs the hal components, there is actually NOT a > >total delay of all frequency components. You would set it up so > >that the servo thread reads the encoders, then does the PID, > >then the filter, then the selected servo output. This is in the > >addf commands in ..../configs/univpwm/univpwm_load.hal, for > >instance. > > > >The filter takes the current input times a coefficient, then > >adds several previous samples times their coefficients. > >Depending on how much each sample is weighted, there would be > >more delay on the lower frequency components with a low-pass > >filter. I'll have to see how this works. Since it is filtering > >the command OUTPUT of the PID, it shouldn't increase error that > >much, IF you can make the loop stable. > > > >I'll have to think about the notch filter vs. low-pass filter > >thing. Maybe I really want a notch filter at 1/2 the sample rate. > > > >Jon > > Some food for thought Jon, although I don't expect it to be called new. > > In tv, we use, in the better tv's, a 'comb' filter to separate the luminance > from the chroma. It works as a 3 tap delay line where the first and last tap > each supply 25% of the output signal, and the center tap supplies 50%. The > two mixed signals are then obtained by adding the first and last tap, and > subtracting the center tap to recover the 3.58mhz chroma signal, and adding > the center tap to recover the luminance signal. > > There are two ways to do this in a tv. If the delay is 63.333 u-secs for each > delay, then a very good vertical resolution enhancement can also be done, but > if the delays are only 135ns, or half a color subcarrier cycle, only the > horizontal resolution is enhanced. The longer delay was at one time done in > very expensive glass acoustic delay lines, so that was only available in high > end studio monitors. I believe that may be available in silicon now but have > no actual knowledge to back that up. > > This same idea done digitally by a parallel transfer between 3 8 bit latches, > clocking it by the same clock as the sample clock, could do a very good job > of removing the 1/2 sample noise if the output was from an 8 bit digital > adder. Much faster that way than in cpu cycles although it could be done if > the sample rate was low enough. 1khz at 32 bits resolution is certainly > doable by the cpu, maybe 10khz with good coding. > > Just throwing the idea out there as its easier to code than hack a board full > of TTL stuff. > > -- > Cheers, Gene > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > You will be run over by a bus. > > ------------------------------------------------------------------------- > This SF.net email is sponsored by: Splunk Inc. > Still grepping through log files to find problems? Stop. > Now Search log events and configuration files using AJAX and a browser. > Download your FREE copy of Splunk now >> http://get.splunk.com/ > _______________________________________________ > Emc-developers mailing list > Emc-developers@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-developers > ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Emc-developers mailing list Emc-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-developers
