# modulation: audio DC offset

Started by December 23, 2009
```*I'm having difficulty posting. This may be a repeat.*

Tim Wescott wrote:

[see previous message if curious]

> Huh.  Good 'technician' course.

Yes. We got into Laplace transforms, vector analysis (for EM fields and
waves) and more. A blind engineer named Lieljestrand taught us how to
solve AC circuits with vector diagrams. He kept string in his pocket for
drawing circles on the blackboard.

> I'm still going to stand by the notion that the reality may be
described as a high-level 1-quadrant multiplier with built-in offset,
but while that works well for modeling it's not necessarily how you want
to think about it when you're debugging an AM set.

There's more than one way to describe almost anything, but that
description is accurate and apt. One of the multipliers is a constant
(if you can call RF constant). So gain proportional to plate voltage
accomplished the multiplication. If there's enough drive always to drive
the output to the rail, that's all that's needed.

> Incidentally, I've often wondered if you couldn't get decent SSB
performance with an AM modulator that goes all the way down to 0,
coupled with an exciter that shifts phase at the appropriate spots.
Needing a DC-
> coupled audio amp to the RF deck plate is probably the least of your
practical difficulties, though.

I think you're describing a complicated way to make a balanced
modulator. If so, there are still two sidebands.

Jerry
--
Engineering is the art of making what you want from things you can get.
&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;&macr;
```
```Another possible repost:

>
>
> Tim Wescott wrote:
>
>
>> Incidentally, I've often wondered if you couldn't get decent SSB
>> performance with an AM modulator that goes all the way down to 0,
>> coupled with an exciter that shifts phase at the appropriate spots.
>> Needing a DC-coupled audio amp to the RF deck plate is probably the
>> least of your practical difficulties, though.
>
> This is what was referred as synthetic method of SSB generation. The
> problem with it is the accuracy of phase and amplitude modulators.

I knew it (from RCA Institutes and ARRL handbook) as the phasing method.

Did you know that, early on, vestigial sideband TV was accomplished by a
filter in the antenna feed? The first filters I designed apart from tone
controls were done with waveguide stubs and a Smith chart.

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;

```
```hello, and thanks everyone for your replies.
I feel like I'm on the same page with everyone.

The basic equation you provided validates my concept of the
offset and shows me where to place the offset.

if I use (a) for audio and (c) for carrier,
2cos(c) + 2cos(a+c) + 2cos(a-c)
implies that a is the carrier on the RHS, but the identity:
cos(x)=cos(-x) =>
2cos(a-c) = 2cos(c-a)
fixes thngs.

Back to physical representation.

If the tube plate of the modulator is connected to top tank
circuit, and the tank bottom is in series with the modulator
( transformer ), which then connects to Eb:

IOW,
If I see a plate circuit, there is the ep, et, em and Eb
Eb is the static plate supply
em is the modulation
et is the plate RF tank
ep is the plate voltage itself.

Then the plate supply ( static )
Eb = et + em + ep

The voltages add.  I see no multiplication here.

I have been also taught that non-linear effects make the
modulation.  Unless it's evident in this example, I will
ignore it.

However, the only addition two things I can think of that might
in some way contribute to the explanation are:
1. there is some "power" in the calculation that helps explain
the multiplication ( V x I  is a multiplication ).
2. the fact the additions above are hiding the "X" effect ( select 1 )
static E's
dynamic e's
??

Not to stray too far from the technical aspects of
how modulation happens and it's translation to DSP,
I would like to know, since I am simulating from a
learning experience ( using C#, not Matlab ), what ( in general )
would be the ratio ( range) of digital modulations often used
with "computers".  For example, would someone use a 2 ghz pentium
and a 65536 sized array to simulate a 100 hz signal onto a
5 Mhz ( 50 mhz, 500 Mhz ) carrier ??  Is there a rule of thumb ?
( would it matter if it was some digital modulation or linear ?)

The thread has been quite enlightening.

Walt......

```
```waltech wrote:
> hello, and thanks everyone for your replies.
> I feel like I'm on the same page with everyone.
>
> The basic equation you provided validates my concept of the
> offset and shows me where to place the offset.
>
> if I use (a) for audio and (c) for carrier,
> 2cos(c) + 2cos(a+c) + 2cos(a-c)
> implies that a is the carrier on the RHS, but the identity:
> cos(x)=cos(-x) =>
> 2cos(a-c) = 2cos(c-a)
> fixes thngs.
>
> Back to physical representation.
>
> If the tube plate of the modulator is connected to top tank
> circuit, and the tank bottom is in series with the modulator
> ( transformer ), which then connects to Eb:
>
> IOW,
> If I see a plate circuit, there is the ep, et, em and Eb
> Eb is the static plate supply
> em is the modulation
> et is the plate RF tank
> ep is the plate voltage itself.
>
> Then the plate supply ( static )
> Eb = et + em + ep
>
> The voltages add.  I see no multiplication here.

The modulated stage operated Class-C with a tank Q of about 3. That's
damped enough to allow cycle-to-cycle changes of amplitude, but live
enough to swing the plate up to the power supply when the tube id cut
off. Therefore, the peak RF amplitude follows the modulation voltage
coming out of the transformer (if there is one. See "Heising modulator")

> I have been also taught that non-linear effects make the
> modulation.  Unless it's evident in this example, I will
> ignore it.

Multiplication is a non-linear operation. One can also modulate with diodes.

> However, the only addition two things I can think of that might
> in some way contribute to the explanation are:
> 1. there is some "power" in the calculation that helps explain
> the multiplication ( V x I  is a multiplication ).
> 2. the fact the additions above are hiding the "X" effect ( select 1 )
> static E's
> dynamic e's
> ??

Don't stretch it.

> Not to stray too far from the technical aspects of
> how modulation happens and it's translation to DSP,
> I would like to know, since I am simulating from a
> learning experience ( using C#, not Matlab ), what ( in general )
> would be the ratio ( range) of digital modulations often used
> with "computers".  For example, would someone use a 2 ghz pentium
> and a 65536 sized array to simulate a 100 hz signal onto a
> 5 Mhz ( 50 mhz, 500 Mhz ) carrier ??  Is there a rule of thumb ?
> ( would it matter if it was some digital modulation or linear ?)

AM is rarely used with digital modulation. It is primarily an analog medium.

> The thread has been quite enlightening.

You're welcome.

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;
```
```On Thu, 24 Dec 2009 10:07:23 -0500, Jerry Avins wrote:

> *I'm having difficulty posting. This may be a repeat.*
>
> Tim Wescott wrote:
>
>    [see previous message if curious]
>
>  > Huh.  Good 'technician' course.
>
> Yes. We got into Laplace transforms, vector analysis (for EM fields and
> waves) and more. A blind engineer named Lieljestrand taught us how to
> solve AC circuits with vector diagrams. He kept string in his pocket for
> drawing circles on the blackboard.
>
>  > I'm still going to stand by the notion that the reality may be
> described as a high-level 1-quadrant multiplier with built-in offset,
> but while that works well for modeling it's not necessarily how you want
> to think about it when you're debugging an AM set.
>
> There's more than one way to describe almost anything, but that
> description is accurate and apt. One of the multipliers is a constant
> (if you can call RF constant). So gain proportional to plate voltage
> accomplished the multiplication. If there's enough drive always to drive
> the output to the rail, that's all that's needed.
>
>  > Incidentally, I've often wondered if you couldn't get decent SSB
> performance with an AM modulator that goes all the way down to 0,
> coupled with an exciter that shifts phase at the appropriate spots.
> Needing a DC-
>  > coupled audio amp to the RF deck plate is probably the least of your
> practical difficulties, though.
>
> I think you're describing a complicated way to make a balanced
> modulator. If so, there are still two sidebands.

Only if the exciter phase is limited to 0 and 180 degrees.  If you take
the low-level SSB signal and separately run it through a limiter and find
it's envelope, then use the limited signal to drive the RF input to the
final and the envelope to drive the power to the final, it would work in
theory.

--
www.wescottdesign.com
```
```> AM is rarely used with digital modulation. It is primarily an analog medium.
>
>
Um, what abut QAM?   :-)

Mark
```
```
> One of the multipliers is a constant
> (if you can call RF constant). So gain proportional to plate voltage
> accomplished the multiplication.

I kinda like the way the above sounds.
I want to think on it a bit more.

>
>> AM is rarely used with digital modulation. It is primarily an analog
medium.
>>
>>
>Um, what abut QAM?   :-)
>
>Mark

as a radio transmission.  QAM will do ..

thanks for mentioning it.

By the way, I ran across a term that suggested low level
modulation using phase and then at the power amp, converting to AM.
The term was ampliphase.  Didn't know if it was related to the
discussion on SSB  but it sounded a bit like it.

Walt......

```
```Mark wrote:
>> AM is rarely used with digital modulation. It is primarily an analog medium.
>>
>>
> Um, what abut QAM?   :-)

:-) indeed! You don't make QAM with a plate modulator, nor do you decode
it with a peak detector.

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;
```
```On Thu, 24 Dec 2009 14:43:58 -0500, Jerry Avins <jya@ieee.org> wrote:
>
>AM is rarely used with digital modulation. It is primarily an analog medium.

This maybe true for carrier based (and/or wireless) systems but for
base-band systems almost all modulation is AM. Best examples are 1GbE
(PAM5) and 10GbE (over 100m copper) which is 16 PAM.
--
Muzaffer Kal

DSPIA INC.
ASIC/FPGA Design Services

http://www.dspia.com
```
```Muzaffer Kal wrote:
> On Thu, 24 Dec 2009 14:43:58 -0500, Jerry Avins <jya@ieee.org> wrote:
>> AM is rarely used with digital modulation. It is primarily an analog medium.
>
> This maybe true for carrier based (and/or wireless) systems but for
> base-band systems almost all modulation is AM. Best examples are 1GbE
> (PAM5) and 10GbE (over 100m copper) which is 16 PAM.

I should clarify. By AM, I meant the system in which a carrier is varied
in amplitude to create an envelope that follows the modulating signal,
and can as a result be demodulated with an envelope detector. Variations
that require taking phase into account lie outside the scope of my
statement.

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;
```