Reply by glen herrmannsfeldt●June 10, 20052005-06-10
Jerry Avins wrote:
*snip)
> Who, me? I know the difference. The actual carrier swing is 100 kHz, but
> sidebands extend the sum signal width to 150 kHz, The channels are 200
> KHz apart. Generally, the highest modulating frequency is 15 kHz. US and
> UK preemphasis have different time constants. These numbers are modified
> for stereo and SCA. Any broadcast FM detector that the OP builds should
> accommodate stereo and perhaps SCA too.
The distinction I was originally trying to make was between amplitude
and peak-to-peak. As far as I know, FM is normally described as
amplitude of deviation, that is, plus or minus from the carrier
frequency,
http://www.fcc.gov/mb/audio/subcarriers/
but a web search found plenty of web sites giving the
peak-to-peak value.
Television sound, at least in the US (and probably other NTSC countries)
is FM with 25kHz deviation amplitude. Some FM receivers will tune
down to the channel 6 sound frequency, which will be somewhat quieter
than a normal FM station.
http://edocket.access.gpo.gov/cfr_2004/octqtr/47cfr73.681.htm
Broadcast FM has a 75kHz deviation amplitude, or 150kHz p-p.
Unlike AM where the side bands are a simple function of the modulating
signal FM sidebands go to infinity even with a modulating sine wave.
Broadcast FM signals may have a 38kHz DSB/SC stereo subcarrier
and also a 67kHz FM SCA subcarrier, and possibly other subcarriers.
The actual bandwidth used is then a complicated function of the
modulation frequencies and amplitudes.
In addition, it seems that according the the fcc.gov site above,
FM stations with subcarriers are allowed up to 110% modulation
(relative to the +/- 75kHz) total.
-- glen
Reply by Jerry Avins●June 9, 20052005-06-09
Rimmer wrote:
> "glen herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message
> news:8IqdnQDbEueQ8zrfRVn-rg@comcast.com...
>
>>Jerry Avins wrote:
>>
>>
>>>sheepshaggerx@yahoo.co.uk wrote:
>>>
>>>
>>>>My application is in software radio so that the signal will have a
>>>>spectrum (random) anything up to say 15kHz for a normal FM stereo
>>>>radio. I use the sine wave as an example as we normally test systems on
>>>>sine waves and sweep them to do a freq response.
>>
>>>Broadcast FM has a carrier deviation of 150 KHz in a channel 200 KHz
>
> wide.
>
>>
> Thats the modulation depth -not the modulating freq.
The audio has a spectrum of 15 KHz; the FM signal, 150. It's the 150
that needs to be demodulated.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Jerry Avins●June 9, 20052005-06-09
Rimmer wrote:
> "Jerry Avins" <jya@ieee.org> wrote in message
> news:dZWdnfYyaMl_DjrfRVn-vQ@rcn.net...
>
>>glen herrmannsfeldt wrote:
>>
>>>Jerry Avins wrote:
>>>
>>>
>>>>sheepshaggerx@yahoo.co.uk wrote:
>>>>
>>>>
>>>>>My application is in software radio so that the signal will have a
>>>>>spectrum (random) anything up to say 15kHz for a normal FM stereo
>>>>>radio. I use the sine wave as an example as we normally test systems
>
> on
>
>>>>>sine waves and sweep them to do a freq response.
>>>
>>>
>>>>Broadcast FM has a carrier deviation of 150 KHz in a channel 200 KHz
>>>>wide.
>>>
>>>
>>>I thought it was 75kHz, but it is plus or minus 75kHz so the bandwidth
>>>is (approximately) 150kHz.
>>
>>75 is for Television. Sheepshagger is off by a factor of 10. In the US,
>>AM channels are at least 10 KHz wide; some used to be 30.
>>
>>
>
> You are confusing depth of modulation and modulating freq. The mod freq goes
> up to 15kHz and the depth of modulation is 100kHz. The ratio of the two
> gives beta the FM modulation index.
> Sounds to me that this is something overlooked by the digital boys -
> comparison with a simple PLL.They never were much good at closing loops.
Who, me? I know the difference. The actual carrier swing is 100 kHz, but
sidebands extend the sum signal width to 150 kHz, The channels are 200
KHz apart. Generally, the highest modulating frequency is 15 kHz. US and
UK preemphasis have different time constants. These numbers are modified
for stereo and SCA. Any broadcast FM detector that the OP builds should
accommodate stereo and perhaps SCA too.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Rimmer●June 8, 20052005-06-08
"Jerry Avins" <jya@ieee.org> wrote in message
news:dZWdnfYyaMl_DjrfRVn-vQ@rcn.net...
> glen herrmannsfeldt wrote:
> > Jerry Avins wrote:
> >
> >> sheepshaggerx@yahoo.co.uk wrote:
> >>
> >>> My application is in software radio so that the signal will have a
> >>> spectrum (random) anything up to say 15kHz for a normal FM stereo
> >>> radio. I use the sine wave as an example as we normally test systems
on
> >>> sine waves and sweep them to do a freq response.
> >
> >
> >> Broadcast FM has a carrier deviation of 150 KHz in a channel 200 KHz
> >> wide.
> >
> >
> > I thought it was 75kHz, but it is plus or minus 75kHz so the bandwidth
> > is (approximately) 150kHz.
>
> 75 is for Television. Sheepshagger is off by a factor of 10. In the US,
> AM channels are at least 10 KHz wide; some used to be 30.
>
>
You are confusing depth of modulation and modulating freq. The mod freq goes
up to 15kHz and the depth of modulation is 100kHz. The ratio of the two
gives beta the FM modulation index.
Sounds to me that this is something overlooked by the digital boys -
comparison with a simple PLL.They never were much good at closing loops.
Rimmer
Reply by Rimmer●June 8, 20052005-06-08
"glen herrmannsfeldt" <gah@ugcs.caltech.edu> wrote in message
news:8IqdnQDbEueQ8zrfRVn-rg@comcast.com...
> Jerry Avins wrote:
>
> > sheepshaggerx@yahoo.co.uk wrote:
> >
> >> My application is in software radio so that the signal will have a
> >> spectrum (random) anything up to say 15kHz for a normal FM stereo
> >> radio. I use the sine wave as an example as we normally test systems on
> >> sine waves and sweep them to do a freq response.
>
> > Broadcast FM has a carrier deviation of 150 KHz in a channel 200 KHz
wide.
>
>
Thats the modulation depth -not the modulating freq.
Rimmer
Reply by Jerry Avins●June 8, 20052005-06-08
glen herrmannsfeldt wrote:
> Jerry Avins wrote:
>
>> sheepshaggerx@yahoo.co.uk wrote:
>>
>>> My application is in software radio so that the signal will have a
>>> spectrum (random) anything up to say 15kHz for a normal FM stereo
>>> radio. I use the sine wave as an example as we normally test systems on
>>> sine waves and sweep them to do a freq response.
>
>
>> Broadcast FM has a carrier deviation of 150 KHz in a channel 200 KHz
>> wide.
>
>
> I thought it was 75kHz, but it is plus or minus 75kHz so the bandwidth
> is (approximately) 150kHz.
75 is for Television. Sheepshagger is off by a factor of 10. In the US,
AM channels are at least 10 KHz wide; some used to be 30.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by glen herrmannsfeldt●June 8, 20052005-06-08
Jerry Avins wrote:
> sheepshaggerx@yahoo.co.uk wrote:
>
>> My application is in software radio so that the signal will have a
>> spectrum (random) anything up to say 15kHz for a normal FM stereo
>> radio. I use the sine wave as an example as we normally test systems on
>> sine waves and sweep them to do a freq response.
> Broadcast FM has a carrier deviation of 150 KHz in a channel 200 KHz wide.
I thought it was 75kHz, but it is plus or minus 75kHz so the bandwidth
is (approximately) 150kHz.
-- glen
Reply by Jerry Avins●June 8, 20052005-06-08
sheepshaggerx@yahoo.co.uk wrote:
> My application is in software radio so that the signal will have a
> spectrum (random) anything up to say 15kHz for a normal FM stereo
> radio. I use the sine wave as an example as we normally test systems on
> sine waves and sweep them to do a freq response.
Broadcast FM has a carrier deviation of 150 KHz in a channel 200 KHz wide.
>
> I am worried about the digital differentiation bit and noise. I though
> the whole point of a PLL was that we went away from the old ideas of
> differentiation (Foster Seeley).The PLL gave better performance. Well
> here we are in the digital age and we go back to old ideas. Whats the
> point in using digital demodulation if the SNR is poorer than analogue?
If it were worse, it wouldn't be used. Theory be damned. Use whatever
technique works best.
What about the Foster-Seely discriminator or the Seely-Avins ratio
detector involves differentiation? The _difference_ between the diode
outputs is not _differentiation_.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by ●June 8, 20052005-06-08
My application is in software radio so that the signal will have a
spectrum (random) anything up to say 15kHz for a normal FM stereo
radio. I use the sine wave as an example as we normally test systems on
sine waves and sweep them to do a freq response.
I am worried about the digital differentiation bit and noise. I though
the whole point of a PLL was that we went away from the old ideas of
differentiation (Foster Seeley).The PLL gave better performance. Well
here we are in the digital age and we go back to old ideas.Whats the
point in using digital demodulation if the SNR is poorer than analogue?
Thanks
Reply by Thomas Magma●June 8, 20052005-06-08
Hey Gumboots,
It really depends on what the signal looks like that you are trying to demod
and what hardware and processing constraints are. (And what side of the bed
you woke up on that morning) For instance, if you wanted to extract every
possible bit of information out of a captured signal, and you had all the
time in the world to do this, then I would use an I/Q arctan type approach.
But if your processing power/time is limited and your demoding in real time,
then I would use a software PLL. (roughly speaking)
You'll have to be very specific on your application if you want answers that
are somewhat applicable. They both have many advantages and disadvantages.
Thomas
<sheepshaggerx@yahoo.co.uk> wrote in message
news:1118196820.637569.107180@g47g2000cwa.googlegroups.com...
> Which gives best performance, a Phase-locked-loop (say all digital -
> software) or I-Q (ie using arctan and then differentiating)
> demodulation.Reason I am asking is that a PLL is supposed to be the
> best - is it better than just the pure number crunching.It woudl appear
> to me that when you differentiate you introduce noise and the whole
> point of a PLL is that it a servo which tracks rate of change of phase
> and minimises noise.Also why is I-Q prefered rather than software PLL
> in software radio?
>
>
> Thanks
>