Reply by robert bristow-johnson●May 17, 20112011-05-17
On May 7, 4:47�pm, David Kirkby <drkir...@gmail.com> wrote:
> On May 6, 6:23�am, Randy Yates <ya...@ieee.org> wrote:
>
>
> > I've watched, with some amusement, all the responses and I don't
> > think anyone has really hit the mark.
>
> Yes, I get that feeling too.
>
> > FM discrimination using the formula you show above, or by using
> > "standard" differentiation of the phase, involves differentiation.
> > Differentiation is an inherently noisy process.
isn't that what the pre-emphasis and de-emphasis (they do for FM
broadcast) for?
BTW, David, that derivative formula you started with
V = (I*dQ/dt - Q*dI/dt)/(I^2 + Q^2)
still has to be implemented in discrete time. if you define dt as the
sample period (essentially just a scaling issue), you might get
V[n] = (I[n]*(Q[n]-Q[n-1]) - Q[n]*(I[n]-I[n-1]))/(I[n]^2 + Q[n]^2)
= (Q[n]*I[n-1] - I[n]*Q[n-1])/(I[n]^2 + Q[n]^2)
this would be thought to be the phase difference between adjacent
samples of the analytic signal:
I[n] + j*Q[n]
where Q[n] is the Hilbert transform of I[n].
but i think a more straight-forward approach is to directly compute
the phase difference between adjacent samples:
V[n] = arg{ (I[n] + j*Q[n])/(I[n-1] + j*Q[n-1]) }
or
V[n] = arg{ (I[n] + j*Q[n]) * conj(I[n-1] + j*Q[n-1]) }
when you do that, you get a slightly different result:
V[n] = arctan((Q[n]I[n-1] - I[n]Q[n-1])/(I[n]I[n-1] + Q[n]Q[n-1]))
to use arctan(Im/Re) instead of arg{ Re + j*Im }, we are assuming the
abs value of the phase difference between adjacent samples is less
than pi/2. if it's *much* less, the arctan function is pretty linear
(arctan(x) ~= x) and the two formulae can be directly compared (and
there is a small difference in the denominator). if you *do*
implement an arctan() function, a short power series with odd-powered
terms will do quite well for angles less than pi/4.
but the latter formula gets you the exact phase difference between
samples (which, if you get enough of these phase difference samples,
you can construct a filter that will better approximate the continuous-
time derivative, but at a slightly delayed time) and does not
approximate the derivative with the finite difference of the most
recent two samples. this filter can be teamed up with the de-emphasis
filter necessary in FM demodulation.
r b-j
Reply by Joel Koltner●May 11, 20112011-05-11
"tomcee" <tomcees_pc@yahoo.com> wrote in message
news:2af521ac-96a7-490c-9ef6-0e57a89b5c75@a18g2000yqj.googlegroups.com...
>This is a common misconception - if you study the actual performance
>of 'modern' radio - that is, Satellite or HD radio, the audio
>bandwidth is
>about 15 kHz and the SNR is about 70dB - just like conventional/
>current
>'analog' FM.
Yes, although this is now a constraint driven more by the cost of bandwidth
rather than the technical abilities of the receiving circuitry, as was once
the case -- and of course the fact that the human ear's performance hasn't
improved in the past umpteen millennia either.
Sirius satellite radio was, in its early years, going to be called "CD Radio."
While that arguably was changed because it wasn't thought to be very
marketable, it seems to me that they fact that they would have needed 3-4x as
much bandwidth per music station to put them within striking distance of true
CD quality might have influenced it as well.
And of course the "HD" in "HD radio" officially stands for nothing whatsoever.
:-(
---Joel
Reply by Randy Yates●May 8, 20112011-05-08
On 05/07/2011 09:53 PM, tomcee wrote:
> On May 6, 4:27 pm, Mark<makol...@yahoo.com> wrote:
>> On May 6, 11:00 am, Vladimir Vassilevsky<nos...@nowhere.com> wrote:
>>
>>> Mark wrote:
>>>> I've always wanted to see how a good threshold extended demod would
>>>> work for narrow band voice FM.
>>
>>> The ultimate threshold extended demodulator makes the efficient input
>>> bandwidth equal to the bandwidth of the demodulated signal. Hence the
>>> gain is input BW/output BW.
>>
>>> Vladimir Vassilevsky
>>> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com
>>
>> conventional "communications narrow band voice FM" uses +/- 5 kHz
>> deviation with an RF BW of about 13 kHz and a demodulated BW of about
>> 3 kHz so some worth while "extension" might be possible.
>>
>> It would be interesting to see how it works in practice.
>>
>> I thought it would be interesting to combine a very narrow band
>> threshold extended demod that was as narrow as possible to optimize
>> recovered SNR at the expense of some distortion with a conventional
>> demod that would automatically switch in whenever the signal was
>> above threshold.
>>
>> Of course the world has passed analog FM by...
>>
>> Mark
>
> This is a common misconception - if you study the actual performance
> of 'modern' radio - that is, Satellite or HD radio, the audio
> bandwidth is
> about 15 kHz and the SNR is about 70dB - just like conventional/
> current
> 'analog' FM.
That's because 70 dB SNR is pretty darned good! If you can
get an honest 70 dB in audio, that's as good as the best
audiophile records (e.g., Mobile Fidelity Sound Labs'
half-speed masters) ever got.
--
Randy Yates % "So now it's getting late,
Digital Signal Labs % and those who hesitate
mailto://yates@ieee.org % got no one..."
http://www.digitalsignallabs.com % 'Waterfall', *Face The Music*, ELO
Reply by tomcee●May 7, 20112011-05-07
On May 6, 4:27�pm, Mark <makol...@yahoo.com> wrote:
> On May 6, 11:00�am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
>
> > Mark wrote:
> > > I've always wanted to see how a good threshold extended demod would
> > > work for narrow band voice FM.
>
> > The ultimate threshold extended demodulator makes the efficient input
> > bandwidth equal to the bandwidth of the demodulated signal. Hence the
> > gain is input BW/output BW.
>
> > Vladimir Vassilevsky
> > DSP and Mixed Signal Design Consultanthttp://www.abvolt.com
>
> conventional "communications narrow band voice FM" �uses +/- 5 kHz
> deviation with an RF BW of about 13 kHz and a demodulated BW of about
> 3 kHz so some worth while "extension" might be possible.
>
> It would be interesting to see how it works in practice.
>
> I thought it would be interesting to combine a very narrow band
> threshold extended demod that was as narrow as possible to optimize
> recovered SNR at the expense of some distortion with a conventional
> demod that would automatically switch in whenever �the signal was
> above threshold.
>
> Of course the world has passed analog FM by...
>
> Mark
This is a common misconception - if you study the actual performance
of 'modern' radio - that is, Satellite or HD radio, the audio
bandwidth is
about 15 kHz and the SNR is about 70dB - just like conventional/
current
'analog' FM.
TomC
Reply by David Kirkby●May 7, 20112011-05-07
On May 6, 6:04�pm, Clay <c...@claysturner.com> wrote:
> On May 6, 12:51�pm, Clay <c...@claysturner.com> wrote:
>
>
>
> > On May 5, 7:44�pm, David Kirkby <drkir...@gmail.com> wrote:
>
> > > On May 5, 8:10�pm, Clay <c...@claysturner.com> wrote:
>
> > > > David, I was working with narrowband FM - i.e., the spectral occupancy
> > > > was 30kHz. If you are looking at commercial braodcast FM - you are
> > > > then looking at a much higher spectral occupancy. For example the
> > > > typical broadcast channel is 200kHz wide so you will need to sample at
> > > > least 400kHz. The channel is wider than the deviation which in the US
> > > > is +-75kHz with mono stations being allowed to go an extra 10%. But
> > > > you can see your samping rate needs to be pretty high.
>
> > > I'm looking only at narrow band FM for amateur radio use, where
> > > typically the channel spacing is 12.5 KHz, so onereally needs to keep
> > > the modulation under that. I'm not interested in commercial FM
> > > transmissions. Hence I would have thought that using a 24-bit sound
> > > card able to sample at 192 KHz would have �been suitable.
>
> > > > You can cheat and narrow the channel down with filtering before
> > > > demodulating, but then you lose your quieting and then end up with a
> > > > not so high hi-fi.
>
> > > > Perhaps the DSP that was already in the receiver was was maxed out
> > > > with its other tasks. And that is why they added another one.
>
> > > > IHTH,
> > > > Clay
>
> > > Perhaps that's the reason, but the fact I know the FlexRadio has FM
> > > demodulation issues makes me wonder if demodulating even narrow band
> > > FM is more difficult than I thought it would be.
>
> > > Dave
>
> > Hello Dave,
>
> > When I demodulated narrowband FM, I did (as Vladimir suggests) and
> > made my analytic signal in software. Yes this does burn some cycles,
> > but then you can achieve nearly perfect 90 degree phase difference and
> > matching amplitudes across the band of interest. THe IdQ-QDi approach
> > can actually work pretty well if you do the right approximations.
>
> > Clay- Hide quoted text -
>
> > - Show quoted text -
>
> Dave,
>
> See
>
> http://www.claysturner.com/dsp/instantaneousfrequency.pdf
>
> for one example of a simplified form of the cross correlation
> approach. It uses low order differentiators so it won't try to magnify
> the noise too much.
>
> IHTH,
> Clay
Cheers Clay,
I think I understand the problems a bit better now, and possibly have
some approaches to trial that may work, which are less suseptable to
noise than my initial idea.
I must admit though, I'm beginning to doubt if this will be as good as
a cheap FM demodulator IC.
I'm going to pour over the manual of some of the very expensive (over
$10,000) DSP based transceivers - e.g.
http://www.icomuk.co.uk/categoryRender.asp?categoryID=3785&productID=730&tID=647
and see if that, does the demodulation of FM in hardware or software.
Dave
Reply by David Kirkby●May 7, 20112011-05-07
On May 6, 6:23�am, Randy Yates <ya...@ieee.org> wrote:
> Hi Dave,
Hi Randy
> I've watched, with some amusement, all the responses and I don't
> think anyone has really hit the mark.
Yes, I get that feeling too.
The comment like "Devil is in the details." does not actually add very
much to the discussion.
> Let me take a swing at it.
> FM discrimination using the formula you show above, or by using
> "standard" differentiation of the phase, involves differentiation.
> Differentiation is an inherently noisy process.
Yes, that is true. That certainly sounds a logical explanation of why
the system might fail.
As a matter of interest, today I was looking at the service manual on
an older Kenwood transceiver, the TS-950S. That uses DSP for
demodulating SSB and CW, but not AM or FM. They use a diode detector
for AM demodulation in that old rig. At least the more modern
transceivers I've look at use the DSP for AM.
I would imagine AM demodulation is very easy. In fact, there's a video
on u-tube of where someone used a Stanford Research Devices RF lock-in
amplifier as an AM radio. I used to be surrounded by lock-in
amplifiers and RF components at a previous place of employment. Shame
I never beat him to making a very expensive AM radio.
> One technique which does NOT use differentiation would be a
> least-squares estimator of the "instantaneous" frequency. You
> do this by finding the phase (arctan) of the I/Q samples, then
> fitting the phase values to a line: y = m*x + b. "m" is then
> your frequency.
Yes, that sounds logical.
> I'm pretty sure this is going to perform better, but you have to be
> careful, e.g., you need to carefully evaluate how much of a slice of
> time serves for "instantaneous," and to be well-oversampled so that
> you get quite a few samples over that slice.
Yes.
Thanks Randy. You have given me something to think about.
> --
> Randy Yates � � � � � � � � � � �% "So now it's getting late,
> Digital Signal Labs � � � � � � �% � �and those who hesitate
> mailto://ya...@ieee.org � � � � �% � �got no one..."http://www.digitalsignallabs.com% 'Waterfall', *Face The Music*, ELO
Reply by Vladimir Vassilevsky●May 6, 20112011-05-06
Mark wrote:
> On May 6, 11:00 am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
>
>>Mark wrote:
>>
>>>I've always wanted to see how a good threshold extended demod would
>>>work for narrow band voice FM.
>>
>>The ultimate threshold extended demodulator makes the efficient input
>>bandwidth equal to the bandwidth of the demodulated signal. Hence the
>>gain is input BW/output BW.
>>
> conventional "communications narrow band voice FM" uses +/- 5 kHz
> deviation with an RF BW of about 13 kHz
Actually there are two widely accepted variants of narrow FM: used with
the channel spacing of 12.5kHz and 25kHz correspondingly. The modulation
parameters and the RF bandwidth are somewhat different.
> and a demodulated BW of about
> 3 kHz so some worth while "extension" might be possible.
>
> It would be interesting to see how it works in practice.
>
> I thought it would be interesting to combine a very narrow band
> threshold extended demod that was as narrow as possible to optimize
> recovered SNR at the expense of some distortion with a conventional
> demod that would automatically switch in whenever the signal was
> above threshold.
I thought many times about the optimal FM demodulator for the voiceband
communications. It would be an interesting DSP project, and it could
have practical application. I would be happy to develop such product if
someone sponsors my effort.
> Of course the world has passed analog FM by...
Not really; there are still zillions of analog FM channels arround.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
Reply by Mark●May 6, 20112011-05-06
On May 6, 11:00�am, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
> Mark wrote:
> > I've always wanted to see how a good threshold extended demod would
> > work for narrow band voice FM.
>
> The ultimate threshold extended demodulator makes the efficient input
> bandwidth equal to the bandwidth of the demodulated signal. Hence the
> gain is input BW/output BW.
>
> Vladimir Vassilevsky
> DSP and Mixed Signal Design Consultanthttp://www.abvolt.com
conventional "communications narrow band voice FM" uses +/- 5 kHz
deviation with an RF BW of about 13 kHz and a demodulated BW of about
3 kHz so some worth while "extension" might be possible.
It would be interesting to see how it works in practice.
I thought it would be interesting to combine a very narrow band
threshold extended demod that was as narrow as possible to optimize
recovered SNR at the expense of some distortion with a conventional
demod that would automatically switch in whenever the signal was
above threshold.
Of course the world has passed analog FM by...
Mark
Reply by Clay●May 6, 20112011-05-06
On May 6, 12:51�pm, Clay <c...@claysturner.com> wrote:
> On May 5, 7:44�pm, David Kirkby <drkir...@gmail.com> wrote:
>
>
>
>
>
> > On May 5, 8:10�pm, Clay <c...@claysturner.com> wrote:
>
> > > David, I was working with narrowband FM - i.e., the spectral occupancy
> > > was 30kHz. If you are looking at commercial braodcast FM - you are
> > > then looking at a much higher spectral occupancy. For example the
> > > typical broadcast channel is 200kHz wide so you will need to sample at
> > > least 400kHz. The channel is wider than the deviation which in the US
> > > is +-75kHz with mono stations being allowed to go an extra 10%. But
> > > you can see your samping rate needs to be pretty high.
>
> > I'm looking only at narrow band FM for amateur radio use, where
> > typically the channel spacing is 12.5 KHz, so onereally needs to keep
> > the modulation under that. I'm not interested in commercial FM
> > transmissions. Hence I would have thought that using a 24-bit sound
> > card able to sample at 192 KHz would have �been suitable.
>
> > > You can cheat and narrow the channel down with filtering before
> > > demodulating, but then you lose your quieting and then end up with a
> > > not so high hi-fi.
>
> > > Perhaps the DSP that was already in the receiver was was maxed out
> > > with its other tasks. And that is why they added another one.
>
> > > IHTH,
> > > Clay
>
> > Perhaps that's the reason, but the fact I know the FlexRadio has FM
> > demodulation issues makes me wonder if demodulating even narrow band
> > FM is more difficult than I thought it would be.
>
> > Dave
>
> Hello Dave,
>
> When I demodulated narrowband FM, I did (as Vladimir suggests) and
> made my analytic signal in software. Yes this does burn some cycles,
> but then you can achieve nearly perfect 90 degree phase difference and
> matching amplitudes across the band of interest. THe IdQ-QDi approach
> can actually work pretty well if you do the right approximations.
>
> Clay- Hide quoted text -
>
> - Show quoted text -
Dave,
See
http://www.claysturner.com/dsp/instantaneousfrequency.pdf
for one example of a simplified form of the cross correlation
approach. It uses low order differentiators so it won't try to magnify
the noise too much.
IHTH,
Clay
Reply by Clay●May 6, 20112011-05-06
On May 5, 7:44�pm, David Kirkby <drkir...@gmail.com> wrote:
> On May 5, 8:10�pm, Clay <c...@claysturner.com> wrote:
>
> > David, I was working with narrowband FM - i.e., the spectral occupancy
> > was 30kHz. If you are looking at commercial braodcast FM - you are
> > then looking at a much higher spectral occupancy. For example the
> > typical broadcast channel is 200kHz wide so you will need to sample at
> > least 400kHz. The channel is wider than the deviation which in the US
> > is +-75kHz with mono stations being allowed to go an extra 10%. But
> > you can see your samping rate needs to be pretty high.
>
> I'm looking only at narrow band FM for amateur radio use, where
> typically the channel spacing is 12.5 KHz, so onereally needs to keep
> the modulation under that. I'm not interested in commercial FM
> transmissions. Hence I would have thought that using a 24-bit sound
> card able to sample at 192 KHz would have �been suitable.
>
> > You can cheat and narrow the channel down with filtering before
> > demodulating, but then you lose your quieting and then end up with a
> > not so high hi-fi.
>
> > Perhaps the DSP that was already in the receiver was was maxed out
> > with its other tasks. And that is why they added another one.
>
> > IHTH,
> > Clay
>
> Perhaps that's the reason, but the fact I know the FlexRadio has FM
> demodulation issues makes me wonder if demodulating even narrow band
> FM is more difficult than I thought it would be.
>
> Dave
Hello Dave,
When I demodulated narrowband FM, I did (as Vladimir suggests) and
made my analytic signal in software. Yes this does burn some cycles,
but then you can achieve nearly perfect 90 degree phase difference and
matching amplitudes across the band of interest. THe IdQ-QDi approach
can actually work pretty well if you do the right approximations.
Clay