Jerry Avins <jya@ieee.org> writes:> That won't demodulate either without some way to beat one of the > sidebands to baseband (with inversion if it's the lower one. *A filter > alone* can't do that.That's true. I originally stated "a filter and a decimator." I got a little lazy recently and have not been saying "filter and decimator" but simply "filter," but what I meant was "filter and decimator.">> I looked up synchronous demodulation in [haykin], p.95 (which is >> apparently more commonly known as "synchronous detection") and the >> method I proposed is NOT the same. Synchronous demodulation requires a >> multiplier; bandpass sampling does not. > > A diode bridge is usually used instead of a multiplier. It simply > inverts the negative part of the signal. (Harmonics of the square wave > can make aliases.) A linear multiplier driven by a sinusoid suppresses > them.It sounds to me like you're talking at the implementation level. I'm speaking at the ideal, block-diagram level. In any case, you should argue the point with Haykin, as he is the one who put the "product modulator" in the block diagram for synchronous demodulation, and he earlier defined a "product modulator" as a device with two inputs x(t) and w(t) and one output y(t) which operates according to the mathematical relation y(t) = x(t) * w(t), where "*" is multiplication. That is what I call a multiplier. -- % Randy Yates % "With time with what you've learned, %% Fuquay-Varina, NC % they'll kiss the ground you walk %%% 919-577-9882 % upon." %%%% <yates@ieee.org> % '21st Century Man', *Time*, ELO http://home.earthlink.net/~yatescr
AM demodulation using DSP
Started by ●June 3, 2007
Reply by ●June 5, 20072007-06-05
Reply by ●June 5, 20072007-06-05
Randy Yates wrote:> Jerry Avins <jya@ieee.org> writes: > >> That won't demodulate either without some way to beat one of the >> sidebands to baseband (with inversion if it's the lower one. *A filter >> alone* can't do that. > > That's true. I originally stated "a filter and a decimator." I got a > little lazy recently and have not been saying "filter and decimator" > but simply "filter," but what I meant was "filter and decimator." > >>> I looked up synchronous demodulation in [haykin], p.95 (which is >>> apparently more commonly known as "synchronous detection") and the >>> method I proposed is NOT the same. Synchronous demodulation requires a >>> multiplier; bandpass sampling does not. >> A diode bridge is usually used instead of a multiplier. It simply >> inverts the negative part of the signal. (Harmonics of the square wave >> can make aliases.) A linear multiplier driven by a sinusoid suppresses >> them. > > It sounds to me like you're talking at the implementation level. I'm > speaking at the ideal, block-diagram level. > > In any case, you should argue the point with Haykin, as he is the > one who put the "product modulator" in the block diagram for > synchronous demodulation, and he earlier defined a "product modulator" > as a device with two inputs x(t) and w(t) and one output y(t) which > operates according to the mathematical relation > > y(t) = x(t) * w(t), > > where "*" is multiplication. That is what I call a multiplier.Yes. It's easy to analyze, it has tame properties, and it's bad engineering. Can you imagine how much more complex a linear multiplier is than a conditional inverter? Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●June 5, 20072007-06-05
On Tue, 05 Jun 2007 11:58:21 -0400, Jerry Avins <jya@ieee.org> wrote:>Randy Yates wrote: > > ... > >> I can't believe we're having this discussion. One of us (and it >> could be me) is missing something very basic. > >According to theory, I'm wrong. If I build what I prescribe, it will >work because all the filtering needed is had simply by not sweating a >broadband design. > >JerryWhich works consistent with theory because all systems are, after all, bandlimited. So I don't agree that you're wrong according to theory. Your scheme wouldn't work in a conceptual, ideal system that had aliasable energy at specific frequencies. One would have to go out of their way to build such a system, though, as the ADC and a host of other components would need to support bandwidths way beyond the range useful for the application. Once the effects of practical devices are taken into account, your system would seem to hold up to theory quite well. One doesn't need to avoid aliasing entirely, only enough to get the job at hand accomplished. It is, after all, impossible to completely eliminate aliasing, theoretically, but one can get well within the range of practicality. FWIW, I followed your description and the relevance to the OP topic well enough, and even got an Aha! out of using the carrier forced into limiting as the sampling clock to make an automatic synchronous demod. Presto...filterless AM demodulation. I think Randy just had a different model or pov in mind. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org
Reply by ●June 5, 20072007-06-05
Eric Jacobsen wrote:> FWIW, I followed your description and the relevance to the OP topic > well enough, and even got an Aha! out of using the carrier forced into > limiting as the sampling clock to make an automatic synchronous demod. > Presto...filterless AM demodulation.This method is commonly used for the analog video demodulation in the ICs for the TV. Its virtue is in the good linearity, especially if the carrier frequency is comparable to the modulation frequency. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●June 5, 20072007-06-05
Eric Jacobsen wrote: (snip)> One doesn't need to avoid aliasing entirely, only enough to get the > job at hand accomplished. It is, after all, impossible to completely > eliminate aliasing, theoretically, but one can get well within the > range of practicality.And sometimes not at all. People seem to live with the wagon wheel in the movies effect, possibly due to the difficulty of making a filter to remove it. -- glen
Reply by ●June 5, 20072007-06-05
Eric Jacobsen wrote: ...> FWIW, I followed your description and the relevance to the OP topic > well enough, and even got an Aha! out of using the carrier forced into > limiting as the sampling clock to make an automatic synchronous demod. > Presto...filterless AM demodulation.I'm delighted to have been able to contribute an aha. Sort of like an unexpected bit of candy.> I think Randy just had a different model or pov in mind.Randy likes things neat and clean. The real world can be sloppy. Sometimes, like sex, it can be more fun that way! :-) Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●June 5, 20072007-06-05
Jerry Avins <jya@ieee.org> writes:> Eric Jacobsen wrote: > > ... > >> FWIW, I followed your description and the relevance to the OP topic >> well enough, and even got an Aha! out of using the carrier forced into >> limiting as the sampling clock to make an automatic synchronous demod. >> Presto...filterless AM demodulation. > > I'm delighted to have been able to contribute an aha. Sort of like an > unexpected bit of candy. > >> I think Randy just had a different model or pov in mind. > > Randy likes things neat and clean.I generally approach design by first formulating a theoretical approach, then iterating between practical considerations and corresponding modifications to the theory. This is based on the basic idea that if it isn't doable in theory, it isn't doable at all. So when you start bringing up things like diodes when we don't even have a clear indication that we understand or agree on the ideal block diagram level, I get a little worried that we're not on the same page. Not to mention that you never acknowledged that my statement was indeed correct and spoke, at least in spirit (since it's a filter AND a decimator, not just a filter), to the OP's initial inquiry.> The real world can be sloppy. Sometimes, like sex, it can be more > fun that way! :-)I like my sex sloppy, but I like my engineering clean. ;) -- % Randy Yates % "How's life on earth? %% Fuquay-Varina, NC % ... What is it worth?" %%% 919-577-9882 % 'Mission (A World Record)', %%%% <yates@ieee.org> % *A New World Record*, ELO http://home.earthlink.net/~yatescr
Reply by ●June 6, 20072007-06-06
Randy Yates wrote:> Jerry Avins <jya@ieee.org> writes: > >> Eric Jacobsen wrote: >> >> ... >> >>> FWIW, I followed your description and the relevance to the OP topic >>> well enough, and even got an Aha! out of using the carrier forced into >>> limiting as the sampling clock to make an automatic synchronous demod. >>> Presto...filterless AM demodulation. >> I'm delighted to have been able to contribute an aha. Sort of like an >> unexpected bit of candy. >> >>> I think Randy just had a different model or pov in mind. >> Randy likes things neat and clean. > > I generally approach design by first formulating a theoretical > approach, then iterating between practical considerations and > corresponding modifications to the theory. This is based on the basic > idea that if it isn't doable in theory, it isn't doable at all. > > So when you start bringing up things like diodes when we don't even > have a clear indication that we understand or agree on the ideal block > diagram level, I get a little worried that we're not on the same page.The diodes came into the discussion in the guise of of a ring modulator, which I put forward as a practical alternative to the linear multiplier that you brought up. I wasn't trying to sandbag you; I hope you're not aggrieved.> Not to mention that you never acknowledged that my statement was > indeed correct and spoke, at least in spirit (since it's a filter AND > a decimator, not just a filter), to the OP's initial inquiry.You may have intended filter and decimator, but you didn't write that. I don't quite see how decimation helps, but I didn't argue that. (A decimator works with existing samples, but special samples are needed for my scheme; they must be synchronous.)>> The real world can be sloppy. Sometimes, like sex, it can be more >> fun that way! :-) > > I like my sex sloppy, but I like my engineering clean. ;)Seriously, when the bandwidth of the IF or the sampler itself is narrow enough to exclude images, adding a filter to remove what isn't there shouldn't qualify as clean engineering. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●June 6, 20072007-06-06
Jerry Avins <jya@ieee.org> writes:> You may have intended filter and decimator, but you didn't write > that.Oh really? Do you remember this post? From: Randy Yates <yates@ieee.org> Subject: Re: AM demodulation using DSP Newsgroups: comp.dsp Date: Sun, 03 Jun 2007 14:29:31 -0400 Organization: The Universal Church of Christ Jerry Avins <jya@ieee.org> writes: > c1910 wrote: >> hi! >> i want to demodulate AM signal digitally... >> i want to use a digital filter to separate the carrier and the information >> signal. i've searched a lot...and i haven't found the answer. >> can anyone help?please... >> thx... > > The last time you asked this question, you were told that you can't > demodulate with a filter. The laws of physics haven't changed in th > last week or so. Well, actually you could with a filter and decimator, if your sample rate is just right... -- % Randy Yates % "She has an IQ of 1001, she has a jumpsuit %% Fuquay-Varina, NC % on, and she's also a telephone." %%% 919-577-9882 % %%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO http://home.earthlink.net/~yatescr ??? -- % Randy Yates % "Maybe one day I'll feel her cold embrace, %% Fuquay-Varina, NC % and kiss her interface, %%% 919-577-9882 % til then, I'll leave her alone." %%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO http://home.earthlink.net/~yatescr
Reply by ●June 6, 20072007-06-06
Randy Yates wrote:> Jerry Avins <jya@ieee.org> writes: > >> You may have intended filter and decimator, but you didn't write >> that. > > Oh really? Do you remember this post? > > From: Randy Yates <yates@ieee.org> > Subject: Re: AM demodulation using DSP > Newsgroups: comp.dsp > Date: Sun, 03 Jun 2007 14:29:31 -0400 > Organization: The Universal Church of Christ > > Jerry Avins <jya@ieee.org> writes: > > > c1910 wrote: > >> hi! > >> i want to demodulate AM signal digitally... > >> i want to use a digital filter to separate the carrier and the information > >> signal. i've searched a lot...and i haven't found the answer. > >> can anyone help?please... > >> thx... > > > > The last time you asked this question, you were told that you can't > > demodulate with a filter. The laws of physics haven't changed in th > > last week or so. > > Well, actually you could with a filter and decimator, if your sample > rate is just right...You didn't write that *consistently* and I took you at your word? Do I detect animus? Why? Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
