RockBear wrote:

> But what i said is exist in digital audio effects box, just like Digitch
> XMM\DF7, or BOSS GT3 and so on. I know there usually is a pre-emphasis
> circuit or pre-emphasis IIR in the FM system, but what i said is different
> from this pre-emphasis module.
> The pre-emphasis circuit in the digital effects box usually boost the
> 1kHz~15kHz about 3dB~5dB. and it has a attenuation from 15kHz~20kHz,
> whitch to reduce the higher frequency(more than 20kHz) that you had
> mentioned.
> You can see the circuit here:
> http://www.experimentalistsanonymous.com/diy/Schematics/Vibrato%20and%20Pitch%20Shift/Digitech%20XP100.pdf
> In the page 2, the C7 and R41 boost the 1kHz~20kHz about 3dB.

the corner frequency for the treble boost start at 1/(2*pi*(R41 +
R11||R10)*C7) and rolls back level at 1/(2*pi*R41*C7).  now i didn't
analyze the output section but it looks like the de-emphasis (for the
left output) is a function of R32, C23, R27, and C15.  someone else can
work on that one.

anyway, the reason for it is as i stated.  somebody thought that adding
the de-emphasis on the output makes the noise sound quieter.  the
pre-emphasis is needed to counter the effect of the de-emphasis for the
desired signal.

r b-j

robert bristow-johnson wrote:
> Allan Herriman wrote:
> 
>>On 9 Apr 2006 18:44:55 -0700, "robert bristow-johnson"
>><rbj@audioimagination.com> wrote:
>>
>>
>>>in FM and FM stereo transmission, they pre-emphasize the audio going in
>>>in the manner you suggest because the nature of additive noise to the
>>>FM signal will end up in the high frequencies.  to reduce that noise,
>>>they de-emphasize the audio, reducing that trebley noise, but that also
>>>reduces the treble of the audio, so to fix that, they pre-emphasize it
>>>at the transmitter.  there is a standard for that so that all FM
>>>receivers can count on the pre-emphasis being the same for all
>>>transmitters, and vis-versa.
>>
>>"There is a standard"  I only wish that was the case.  Even in FM
>>radio, different pre-emphasis time constants are used depending on
>>your locale.  75us and 50us are the time constants encountered.  75us
>>was chosen at a time before it was realised that FM radio would be
>>used for music.  (It's good for voice.)  50us came later, and is
>>probably a better choice overall.
> 
> 
> wow.  i never heard of the 50us pre-emphasis "standard", but i guess i
> didn't look deep enough:
> 
> http://www.pcs-electronics.com/en/products.php?sub=stereo_encod
> 
> in the U.S., doesn't the FCC set preemphsis spec by regulation?
> 
> http://www.fcc.gov/Bureaus/Mass_Media/Databases/documents_collection/pn740710.pdf
> 
> i can't find a more definitive online doc at the moment, but frankly
> this 50 us pre-emphasis for FM broadcast was unheard of by me unitl
> this moment.
> 
> too bad they didn't pick 50us to begin with.  i would have thunk that
> they had dealt with the psychoacoustic issues and that is what
> indicated the 75us pre-emphasis to begin with.  how much worse than 50
> can it be?  and what do modern stereo receivers use?  what's worse than
> the wrong standard (that is slightly off) is when there are
> inconsistencies in the standard.  kinda like the pin 2 or pin 3 hot
> issue of those XLR connectors.

75 microseconds seems to be highly correlated with 60 Hz power. The rest 
of the world uses 50 microseconds. (At least one receiver had a switch.) 
Note that the preemphasis is a single R-C time constant; minimum phase. 
If the deemphasis is not likewise minimum phase, you won't get the 
original waveform back.

There's another way to look at FM preemphasis. You could just as well 
say that the modulation is phase modulation, flat on the high end, and 
rolled off below the turnover frequency. Some of the whys and wherefores 
actually make more sense that way.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;

>
>Allan Herriman wrote:
>> On 9 Apr 2006 18:44:55 -0700, "robert bristow-johnson"
>> <rbj@audioimagination.com> wrote:
>>
>> >in FM and FM stereo transmission, they pre-emphasize the audio going
in
>> >in the manner you suggest because the nature of additive noise to the
>> >FM signal will end up in the high frequencies.  to reduce that noise,
>> >they de-emphasize the audio, reducing that trebley noise, but that
also
>> >reduces the treble of the audio, so to fix that, they pre-emphasize
it
>> >at the transmitter.  there is a standard for that so that all FM
>> >receivers can count on the pre-emphasis being the same for all
>> >transmitters, and vis-versa.
>>
>> "There is a standard"  I only wish that was the case.  Even in FM
>> radio, different pre-emphasis time constants are used depending on
>> your locale.  75us and 50us are the time constants encountered.  75us
>> was chosen at a time before it was realised that FM radio would be
>> used for music.  (It's good for voice.)  50us came later, and is
>> probably a better choice overall.
>
>wow.  i never heard of the 50us pre-emphasis "standard", but i guess i
>didn't look deep enough:
>
>http://www.pcs-electronics.com/en/products.php?sub=stereo_encod
>
>in the U.S., doesn't the FCC set preemphsis spec by regulation?
>
>http://www.fcc.gov/Bureaus/Mass_Media/Databases/documents_collection/pn740710.pdf
>
>i can't find a more definitive online doc at the moment, but frankly
>this 50 us pre-emphasis for FM broadcast was unheard of by me unitl
>this moment.
>
>too bad they didn't pick 50us to begin with.  i would have thunk that
>they had dealt with the psychoacoustic issues and that is what
>indicated the 75us pre-emphasis to begin with.  how much worse than 50
>can it be?  and what do modern stereo receivers use?  what's worse than
>the wrong standard (that is slightly off) is when there are
>inconsistencies in the standard.  kinda like the pin 2 or pin 3 hot
>issue of those XLR connectors.
>
>r b-j
>
>

Thank you for your answers! 
Buy what i said is exist in digital audio effects box, just like Digitch
XMM\DF7, or BOSS GT3 and so on. I know there usually is a pre-emphasis
circuit or pre-emphasis IIR in the FM system, but what i said is different
from this pre-emphasis module.
The pre-emphasis circuit in the digital effects box usually boost the
1kHz~15kHz about 3dB~5dB. and it has a attenuation from 15kHz~20kHz,
whitch to reduce the higher frequency(more than 20kHz) that you had
mentioned.
You can see the circuit here:
http://www.experimentalistsanonymous.com/diy/Schematics/Vibrato%20and%20Pitch%20Shift/Digitech%20XP100.pdf
In the page 2, the C7 and R41 boost the 1kHz~20kHz about 3dB.

Allan Herriman wrote:
> On 9 Apr 2006 18:44:55 -0700, "robert bristow-johnson"
> <rbj@audioimagination.com> wrote:
>
> >in FM and FM stereo transmission, they pre-emphasize the audio going in
> >in the manner you suggest because the nature of additive noise to the
> >FM signal will end up in the high frequencies.  to reduce that noise,
> >they de-emphasize the audio, reducing that trebley noise, but that also
> >reduces the treble of the audio, so to fix that, they pre-emphasize it
> >at the transmitter.  there is a standard for that so that all FM
> >receivers can count on the pre-emphasis being the same for all
> >transmitters, and vis-versa.
>
> "There is a standard"  I only wish that was the case.  Even in FM
> radio, different pre-emphasis time constants are used depending on
> your locale.  75us and 50us are the time constants encountered.  75us
> was chosen at a time before it was realised that FM radio would be
> used for music.  (It's good for voice.)  50us came later, and is
> probably a better choice overall.

wow.  i never heard of the 50us pre-emphasis "standard", but i guess i
didn't look deep enough:

http://www.pcs-electronics.com/en/products.php?sub=stereo_encod

in the U.S., doesn't the FCC set preemphsis spec by regulation?

http://www.fcc.gov/Bureaus/Mass_Media/Databases/documents_collection/pn740710.pdf

i can't find a more definitive online doc at the moment, but frankly
this 50 us pre-emphasis for FM broadcast was unheard of by me unitl
this moment.

too bad they didn't pick 50us to begin with.  i would have thunk that
they had dealt with the psychoacoustic issues and that is what
indicated the 75us pre-emphasis to begin with.  how much worse than 50
can it be?  and what do modern stereo receivers use?  what's worse than
the wrong standard (that is slightly off) is when there are
inconsistencies in the standard.  kinda like the pin 2 or pin 3 hot
issue of those XLR connectors.

r b-j

On 9 Apr 2006 18:44:55 -0700, "robert bristow-johnson"
<rbj@audioimagination.com> wrote:

>
>RockBear wrote:
>> >Hi,guys!
>> >I have a question about digital audio system.
>> >As we know, a lot of digital audio system has a Pre-Emphasis circuit
>> >before the A/D circuit. The audio signal must pass a treble-boost filter
>> >before they go to the A/D.
>> >And my question is, what's the effect of this Pre-Emphasis circuit? To
>> >reduce the noise? To enhance the A/D's performance? Or other effect?
>> >
>> >Thanks!
>
>ya gotta be careful about thanking people in advance for help.
>
>> Anybody here?
>
>oh, all right. i would expect the opposite.  i would expect a LPF for
>anti-aliasing before the A/D.  in modern audio systems the A/D is a
>sigma-delta type which is grossly oversampled (by 64 or 128 times).
>usually the manufacturer of the A/D recommends a simple passive RC LPF
>before the A/D which, by itself is insufficient to do the anti-aliasing
>job, but the decimation stages of the A/D (where they down sample from
>3 MHz to 48 kHz) does the rest.  so it surprize me that they would do
>that pre-emphasizing as you say.
>
>in FM and FM stereo transmission, they pre-emphasize the audio going in
>in the manner you suggest because the nature of additive noise to the
>FM signal will end up in the high frequencies.  to reduce that noise,
>they de-emphasize the audio, reducing that trebley noise, but that also
>reduces the treble of the audio, so to fix that, they pre-emphasize it
>at the transmitter.  there is a standard for that so that all FM
>receivers can count on the pre-emphasis being the same for all
>transmitters, and vis-versa.

"There is a standard"  I only wish that was the case.  Even in FM
radio, different pre-emphasis time constants are used depending on
your locale.  75us and 50us are the time constants encountered.  75us
was chosen at a time before it was realised that FM radio would be
used for music.  (It's good for voice.)  50us came later, and is
probably a better choice overall.

>i suppose the reason why they may have a pre-emphasis like that in your
>audio system is that someone thought that the quantization noise (along
>with the analog front end noise) was "hissy" or trebley in some way so
>they de-emphasized it in the output and pre-emphasized the audio in the
>input to fix it.  but, frankly, it seems backwards to me.  i am not
>sure that i accept the premise of your question and that is why i
>didn't respond when i first read it.

CD audio has the option for pre- and de-emphasis, but I don't think
I've ever seen a disk that's used it.  The older ('80s) players would
indicate that the pre-emphasis flag had been seen on the disk.  More
modern players don't bother to do that.

Regards,
Allan

RockBear wrote:
> >Hi,guys!
> >I have a question about digital audio system.
> >As we know, a lot of digital audio system has a Pre-Emphasis circuit
> >before the A/D circuit. The audio signal must pass a treble-boost filter
> >before they go to the A/D.
> >And my question is, what's the effect of this Pre-Emphasis circuit? To
> >reduce the noise? To enhance the A/D's performance? Or other effect?
> >
> >Thanks!

ya gotta be careful about thanking people in advance for help.

> Anybody here?

oh, all right. i would expect the opposite.  i would expect a LPF for
anti-aliasing before the A/D.  in modern audio systems the A/D is a
sigma-delta type which is grossly oversampled (by 64 or 128 times).
usually the manufacturer of the A/D recommends a simple passive RC LPF
before the A/D which, by itself is insufficient to do the anti-aliasing
job, but the decimation stages of the A/D (where they down sample from
3 MHz to 48 kHz) does the rest.  so it surprize me that they would do
that pre-emphasizing as you say.

in FM and FM stereo transmission, they pre-emphasize the audio going in
in the manner you suggest because the nature of additive noise to the
FM signal will end up in the high frequencies.  to reduce that noise,
they de-emphasize the audio, reducing that trebley noise, but that also
reduces the treble of the audio, so to fix that, they pre-emphasize it
at the transmitter.  there is a standard for that so that all FM
receivers can count on the pre-emphasis being the same for all
transmitters, and vis-versa.

i suppose the reason why they may have a pre-emphasis like that in your
audio system is that someone thought that the quantization noise (along
with the analog front end noise) was "hissy" or trebley in some way so
they de-emphasized it in the output and pre-emphasized the audio in the
input to fix it.  but, frankly, it seems backwards to me.  i am not
sure that i accept the premise of your question and that is why i
didn't respond when i first read it.

r b-j