The DSP in the original Omni 6 was a low pass filter intended to eliminate white noise from the IF stage. Later, the Omni 6 Plus used the DSP as a CW Peaking filter. It really sounded great without the typical ringing so common in narrow analog audio filters. Steve N4LQ n...@carolina.rr.com ----- Original Message ----- From: "David Woolley (E.L)" <for...@david-woolley.me.uk> To: <Elecraft@mailman.qth.net> Sent: Sunday, August 16, 2009 1:43 PM Subject: Re: [Elecraft] APF vs 10 Hz DSP, why they don't sound the same
> Merv Schweigert wrote: > >> APF does not distort the audio at all, its extremely simple, all it >> does is peak >> an audio freq by X amount. In older radios its done with a analog >> circuit of > > So how does that differ from the existing filters in the K3. What > people seem to be saying is that it actually boosts the audio, but that > is just equivalent to turning up the AF gain, so I was speculating that > the gain is actually turned up to the point where later stages overload, > and that is why people think it differs from a simple filter. > >> one or two chips and resistor capacitor sets to determine the freq of >> the peak. >> I assume there is some feedback loop to increase the audio gain at the >> peaked >> freq. > > I.E. is a band bass FILTER! > >>> >> It does nothing as far as filtering, years ago it was used in cheap > > If it boost one frequency with respect to another, it is, by definition, > filter. This is what I mean by people claiming magical properties. > It is just a linear filter (although possibly followed by non-linear > output stages). > >> receivers as a CW filter > >> Since APF is totally an audio function the gain and AGC is not effected >> by its use > > The gain is influenced. The audio gain is part of the overall gain of > the receiver. People seem to be claiming that the "peak" part of the > name means that there is excess audio gain at the filter centre, over > that without the filter. > > The reason people introduced AGC was to point out that, being post AGC, > the subjective effect of tuning through a signal was a much greater > change in amplitude than pre-AGC, where the AGC would keep the amplitude > more or less constant, although the signal to noise ratio would peak as > you tuned through the signal. > >> at all, unless you have a radio that has audio derived AGC. >> As I stated before APF is for very weak signal detection, one would not >> usually >> use it on signals that are copyable with normal filters etc. It is >> used for copying >> signals you normally tune across because they are too weak to copy or you >> perhaps dont even hear them. > > It's just a normal filter and if it is better than the existing filters, > that is a matter of working out what it is about the filter shape > (considering the overall effect of all filters in the receiver), in > phase and frequency, that makes it subjectively better, and adjust the > DSP filter to reproduce that characteristic. If one understands exactly > why it seems better, the flexibility of a DSP filter may mean that by > designing a first principles solution, one can do even better. > >> It will take a signal that is at the noise level, and peak it to the >> point of being >> able to copy. > > As does any narrow band pass filter. > > The only difference between filtering at audio and at IF, is that > distortion products will remain in band, and it is more likely that it > will be done post-AGC. > > > > -- > David Woolley > "we do not overly restrict the subject matter on the list, and we > encourage postings on a wide range of amateur radio related topics" > List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm> > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
