> On Mon, 20 Sep 2004 19:06:08 GMT, eric.jacobsen@ieee.org (Eric
> Jacobsen) wrote:
>
>
>>The idea is that if FM modulation is used and one
>>wishes to keep the transmitted spectrum relatively flat (which is a
>>good thing to do to maximize SNR in the receiver), then some sort of
>>pre-emphasis is needed if the input spectrum isn't flat. I think
>>that'll be true for any transmissions system with linear modulation.
>
>
> Don't forget the "triangular noise" spectrum for FM. If we assume
> that the IF noise is flat (a reasonable assumption in the power
> limited case) then the demodulated noise will increase 20dB/decade
> with the baseband frequency.
>
> This makes preemphasis much more important for FM that most other
> communications channels. Not only does the programme material have
> less power at higher frequencies, the noise floor increases too.
>
> Here's my derivation:
> http://groups.google.com/groups?selm=37733912.21210660%40newshost
>
> Regards,
> Allan
There's yet another way to look at FM preemphasis. PM has a flat noise
spectrum but the detector is very insensitive to low frequencies. FM
with high-frequency preemphasis is indistinguishable from PM with
low-frequency pre-emphasis. The signal is FM below the preemphasis
turnover and PM above.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Allan Herriman●September 21, 20042004-09-21
>The idea is that if FM modulation is used and one
>wishes to keep the transmitted spectrum relatively flat (which is a
>good thing to do to maximize SNR in the receiver), then some sort of
>pre-emphasis is needed if the input spectrum isn't flat. I think
>that'll be true for any transmissions system with linear modulation.
Don't forget the "triangular noise" spectrum for FM. If we assume
that the IF noise is flat (a reasonable assumption in the power
limited case) then the demodulated noise will increase 20dB/decade
with the baseband frequency.
This makes preemphasis much more important for FM that most other
communications channels. Not only does the programme material have
less power at higher frequencies, the noise floor increases too.
Here's my derivation:
http://groups.google.com/groups?selm=37733912.21210660%40newshost
Regards,
Allan
Reply by Jerry Avins●September 20, 20042004-09-20
Eric Jacobsen wrote:
> On Mon, 20 Sep 2004 15:56:08 -0400, Jerry Avins <jya@ieee.org> wrote:
>
>
>>Eric Jacobsen wrote:
>>
>>
>>>Regardless, I think my point wasn't dependent on the use of a PLL to
>>>demodulate the signal (although I'm not quite sure how to demodulate
>>>FM without one). ...
>>
>>Is that limited to digital, or are analog techniques included?
>>
>>Jerry
>
>
> Ooh! I'd be _really_ interested in alternative analog techniques!
>
>
> Eric Jacobsen
> Minister of Algorithms, Intel Corp.
> My opinions may not be Intel's opinions.
> http://www.ericjacobsen.org
The most primitive method, suitable only for narrow-band FM, is slope
detection. The signal is tuned to one side of an AM detector's response
curve, and the deviation converts FM to AM. Then there's Armstrong's
twin AM detectors, tuned a little further apart than the peak deviation,
one giving positive output, the other, negative, and their outputs
summed. The next level of sophistication is the Foster-Seely
discriminator. It too uses bucking detectors, but they are driven from a
special winding on the last IF transformer. The vector diagram is
interesting. It provides inherent linearity, so the separate detector
outputs of Armstrong's design, which need careful matching, are
dispensed with. It was the first design I know of suitable for mass
production without tweaking for linearity. All these detectors are
sensitive to AM as well, so two stages of limiting are needed for good
AM rejection.
The next advance was the Seely-Avins ratio detector. (My cousin Jack
Avins.) At first glance, it looks much like a discriminator, but the
output is the ratio of the opposing detectors, not their sum. Its lack
of response to AM made it standard in cheap sets and gave it a
reputation for poor quality. Given the resources demanded by a
discriminator, a ratio detector outperformed one. In good sets, it was
usually used with one limiter both to provide phenomenally good AM
rejection and to provide a stable operating point for the AFC circuit.
See http://www.see.ed.ac.uk/~gjrp/EE3/Comms/Lecture11/sld005.htm and the
few panels following
Jack designed an unusual pentode detector for television sets. A high-Q
tank circuit tuned to the intercarrier sound carrier at 4.5 MHz was
connected to the screen grid and passively driven by it. The grid wad
driven by the high-passed output of the video detector. The tank
responded to the average signal frequency, like a flywheel. Given the
nature of a coupled tank, it oscillated in quadrature with the
unmodulated carrier. I called it a flywheel detector when I saw it.
It was the detector used in a B/W set I bought in 1963, and when there
was a buzz from the vertical bleeding through, I got out the schematic
and studied it. When I thought I had it figure out, I cursed the cheap
design that brought me the buzz, and consulted Jack to see if my
understanding of the "weird" detector was adequate. He told me it was
his design and continued along the lines of, "I called it a quadrature
detector, and I'll have you know that it's in over 300,000 sets so far
and going strong. It's not only at least a dime cheaper than what else
we might have used, but the sound is better and there's only one tweak.
If there's a buzz, it's not from that. Humph!" What could I say but
"Oops!"? He told me where to find the buzz (the sync separator was out
of alignment) and I went home and fixed it.
Later, Jack headed up an IC lab that designed circuits for TV and FM.
That Signetics FM chip That Steve U. cited uses some of the patents that
came from that group. His first significant patent was the VTVM. He had
about 55 by the time he retired.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Phil Martel●September 20, 20042004-09-20
"Eric Jacobsen" <eric.jacobsen@ieee.org> wrote in message
news:414f5bb8.442614203@news.west.cox.net...
> On Mon, 20 Sep 2004 15:56:08 -0400, Jerry Avins <jya@ieee.org> wrote:
>
> >Eric Jacobsen wrote:
> >
> >> Regardless, I think my point wasn't dependent on the use of a PLL to
> >> demodulate the signal (although I'm not quite sure how to demodulate
> >> FM without one). ...
> >
> >Is that limited to digital, or are analog techniques included?
> >
> >Jerry
>
> Ooh! I'd be _really_ interested in alternative analog techniques!
Well, if you have a filter that has the FM signal's frequency in its
transition band, the varying frequency of the FM will produce a varying
amplitude, which can then be rectified to recover the modulation on the FM
signal. The quality is usually not too good, but it does work with narrow
band FM.
Best wishes,
--Phil Martel
>
>
> Eric Jacobsen
> Minister of Algorithms, Intel Corp.
> My opinions may not be Intel's opinions.
> http://www.ericjacobsen.org
Reply by Eric Jacobsen●September 20, 20042004-09-20
On Mon, 20 Sep 2004 15:56:08 -0400, Jerry Avins <jya@ieee.org> wrote:
>Eric Jacobsen wrote:
>
>> Regardless, I think my point wasn't dependent on the use of a PLL to
>> demodulate the signal (although I'm not quite sure how to demodulate
>> FM without one). ...
>
>Is that limited to digital, or are analog techniques included?
>
>Jerry
Ooh! I'd be _really_ interested in alternative analog techniques!
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org
Reply by Jerry Avins●September 20, 20042004-09-20
Eric Jacobsen wrote:
> Regardless, I think my point wasn't dependent on the use of a PLL to
> demodulate the signal (although I'm not quite sure how to demodulate
> FM without one). ...
Is that limited to digital, or are analog techniques included?
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Eric Jacobsen●September 20, 20042004-09-20
Regardless, I think my point wasn't dependent on the use of a PLL to
demodulate the signal (although I'm not quite sure how to demodulate
FM without one). The idea is that if FM modulation is used and one
wishes to keep the transmitted spectrum relatively flat (which is a
good thing to do to maximize SNR in the receiver), then some sort of
pre-emphasis is needed if the input spectrum isn't flat. I think
that'll be true for any transmissions system with linear modulation.
On Mon, 20 Sep 2004 16:34:25 +0800, Steve Underwood <steveu@dis.org>
wrote:
>Jerry Avins wrote:
>
>> Steve Underwood wrote:
>>
>>> Eric Jacobsen wrote:
>>>
>>>> On 16 Sep 2004 12:17:18 -0400, Randy Yates
>>>> <randy.yates@sonyericsson.com> wrote:
>>>>
>>>>
>>>>
>>>>>> The audio signal being recorded tends to have less power at high
>>>>>> frequencies.
>>>>>
>>>>>
>>>>> Why?
>>>>>
>>>>
>>>>
>>>> This thread is hard to keep up with!
>>>>
>>>> Randy, I think this is the same reason that FM radio systems use
>>>> preemphasis/deemphasis. To keep the PLLs operating efficiently and
>>>> keep the SNR from getting really crappy at the high frequencies,
>>>> preemphasis is applied.
>>>>
>>>>
>>> Eh? Had anyone ever built a PLL when the FM standards were being
>>> defined? :-) Certainly nobody envisaged it being practical to use one
>>> in a consumer radio back then.
>>>
>>> Regards,
>>> Steve
>>
>>
>> Oh? There was AFC in Pre-war FM tuners when FM was where TV channel 1
>> would be if there were a channel 1. AFC is frequency lock. Add an
>> integrator, and it becomes a phase lock. We knew how to integrate in
>> 1936, but AFC didn't need it.
>>
>> Jerry
>
>Did you know how to integrate in 1936? I know you outdo most of us for
>seniority, but if you learned integration at, say, 14, that would make
>you...... :-)
>
>Seriously, I don't think anyone used PLLs in consumer equipment until
>Signetics launched the first practical single chip implementations.
>Their FM recovery devices, and a little later their stereo decoders,
>revolutionised the stability of FM receivers. That was a long time after
>the standards were set, though.
>
>Regards,
>Steve
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org
Reply by Jerry Avins●September 20, 20042004-09-20
Steve Underwood wrote:
> Jerry Avins wrote:
>
>> Steve Underwood wrote:
>>
>>> Eric Jacobsen wrote:
>>>
>>>> On 16 Sep 2004 12:17:18 -0400, Randy Yates
>>>> <randy.yates@sonyericsson.com> wrote:
>>>>
>>>>
>>>>
>>>>>> The audio signal being recorded tends to have less power at high
>>>>>> frequencies.
>>>>>
>>>>>
>>>>>
>>>>> Why?
>>>>>
>>>>
>>>>
>>>>
>>>> This thread is hard to keep up with!
>>>>
>>>> Randy, I think this is the same reason that FM radio systems use
>>>> preemphasis/deemphasis. To keep the PLLs operating efficiently and
>>>> keep the SNR from getting really crappy at the high frequencies,
>>>> preemphasis is applied.
>>>>
>>>>
>>> Eh? Had anyone ever built a PLL when the FM standards were being
>>> defined? :-) Certainly nobody envisaged it being practical to use one
>>> in a consumer radio back then.
>>>
>>> Regards,
>>> Steve
>>
>>
>>
>> Oh? There was AFC in Pre-war FM tuners when FM was where TV channel 1
>> would be if there were a channel 1. AFC is frequency lock. Add an
>> integrator, and it becomes a phase lock. We knew how to integrate in
>> 1936, but AFC didn't need it.
>>
>> Jerry
>
>
> Did you know how to integrate in 1936? I know you outdo most of us for
> seniority, but if you learned integration at, say, 14, that would make
> you...... :-)
>
> Seriously, I don't think anyone used PLLs in consumer equipment until
> Signetics launched the first practical single chip implementations.
> Their FM recovery devices, and a little later their stereo decoders,
> revolutionised the stability of FM receivers. That was a long time after
> the standards were set, though.
>
> Regards,
> Steve
Steve,
I could integrate (well, understand integration in principle) at 14, but
I was only four in 1936. By "we", I meant the electronic engineering
community. I hear you claiming that phase-locked loops didn't exist (or
at least weren't in use) until integrated circuits were commercial. Did
I get that right? I remember data separators for magnetic storage drums.
They had PLLs built with discrete components. Later, disks -- remember
those 14" platters? -- had them too. TV synch circuits depend on phase
locking. That's where I got 1936 from, but it could have been earlier.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������
Reply by Steve Underwood●September 20, 20042004-09-20
Jerry Avins wrote:
> Steve Underwood wrote:
>
>> Eric Jacobsen wrote:
>>
>>> On 16 Sep 2004 12:17:18 -0400, Randy Yates
>>> <randy.yates@sonyericsson.com> wrote:
>>>
>>>
>>>
>>>>> The audio signal being recorded tends to have less power at high
>>>>> frequencies.
>>>>
>>>>
>>>> Why?
>>>>
>>>
>>>
>>> This thread is hard to keep up with!
>>>
>>> Randy, I think this is the same reason that FM radio systems use
>>> preemphasis/deemphasis. To keep the PLLs operating efficiently and
>>> keep the SNR from getting really crappy at the high frequencies,
>>> preemphasis is applied.
>>>
>>>
>> Eh? Had anyone ever built a PLL when the FM standards were being
>> defined? :-) Certainly nobody envisaged it being practical to use one
>> in a consumer radio back then.
>>
>> Regards,
>> Steve
>
>
> Oh? There was AFC in Pre-war FM tuners when FM was where TV channel 1
> would be if there were a channel 1. AFC is frequency lock. Add an
> integrator, and it becomes a phase lock. We knew how to integrate in
> 1936, but AFC didn't need it.
>
> Jerry
Did you know how to integrate in 1936? I know you outdo most of us for
seniority, but if you learned integration at, say, 14, that would make
you...... :-)
Seriously, I don't think anyone used PLLs in consumer equipment until
Signetics launched the first practical single chip implementations.
Their FM recovery devices, and a little later their stereo decoders,
revolutionised the stability of FM receivers. That was a long time after
the standards were set, though.
Regards,
Steve
Reply by Jerry Avins●September 19, 20042004-09-19
Steve Underwood wrote:
> Eric Jacobsen wrote:
>
>> On 16 Sep 2004 12:17:18 -0400, Randy Yates
>> <randy.yates@sonyericsson.com> wrote:
>>
>>
>>
>>>> The audio signal being recorded tends to have less power at high
>>>> frequencies.
>>>
>>> Why?
>>>
>>
>> This thread is hard to keep up with!
>>
>> Randy, I think this is the same reason that FM radio systems use
>> preemphasis/deemphasis. To keep the PLLs operating efficiently and
>> keep the SNR from getting really crappy at the high frequencies,
>> preemphasis is applied.
>>
>>
> Eh? Had anyone ever built a PLL when the FM standards were being
> defined? :-) Certainly nobody envisaged it being practical to use one in
> a consumer radio back then.
>
> Regards,
> Steve
Oh? There was AFC in Pre-war FM tuners when FM was where TV channel 1
would be if there were a channel 1. AFC is frequency lock. Add an
integrator, and it becomes a phase lock. We knew how to integrate in
1936, but AFC didn't need it.
Jerry
--
Engineering is the art of making what you want from things you can get.
�����������������������������������������������������������������������