AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency

[Bob Myers]

NTSC TV video uses "vestigial sideband", but I don't know why - maybe
they need more power down at the V. sync freq. or something.

Nope - VSB was chosen because there simply wasn't enough room for
anything else. At the time, broadcast television was the most bandwidth-
hungry form of communications there was, at least in widespread use, and
was allocated (originally) 13 channels of 6 MHz each - a whopping 78 MHz
of spectrum allocated to a single service! (Remember, this was at a time
when
the only things living much above the VHF frequencies were such things as
radar.) You need more than 4 MHz of each channel just for the luminance
information, and that is at baseband - putting the video out as full DSB
would've meant at leadt 10 MHz-wide TV channels. But keeping the
carrier (and as a side effect, some of the lower sideband, which really
doesn't do much except force some oddities in the receiver's response
curve) kept the receiver design fairly simple.

Bob M.

 

[Hein ten Horn]

Of course you heard beats. What you didn't hear is the sum of the
frequencies. I've had the same setup on my bench for several months.
It's also one of the experiments the students do in the first year
physics labs. Someone had made the claim a while back that what we
hear is the 'average' of the two frequencies. Didn't make any sense
so I did the experiment. The results are as I have explained.

We hear the average of two frequencies if both frequencies
are indistinguishably close, say with a difference of some few
hertz. For example, the combination of a 220 Hz signal and
a 224 Hz signal with the same amplitude will be perceived as
a 4 Hz beat of a 222 Hz tone.

gr, Hein

 

[Tommy Tootles]

Tommy Tootles wrote:

Uh, John...respectfully, I have to wonder just who is on drugs.

The original poster *ASKED* about *DSB* vs. AM

YOU *ANSWERED* about *SSB*. Here is the correct answer...

There are two broad types of DSB (double sideband) transmission:

DSB-RC and DSB-SC, meaning BOTH sidebands are transmitted, but with
either a (R)educed (C)arrier or a (S)urpressed (C)arrier. AM sends
both sidebands and full carrier.

Hope that answers the OP's question>

Yeah, you just discovered that for all intents and purposes double
sideband is am, and suppressed carrier is just like suppressed
carrier am?

Oh well, better late than never ...

JS

What *I* discovered is -not- the point. And for all intents and
purposes, "AM" and DSB are two distinct (but certainly related) things.
Different hardware to create (balanced modulator for DSB vs high level
plate modulation for 'classic' AM), more power required for AM and
finally, back in the day, the FCC had -different- emission designators
for AM vs DSB. Now, if they were the same, why would you think the FCC
gave them -different- emission designators?

What IS the point is:

1) The original poster asked a question about "x".

2) You gave a half-assed answer to "y".

And then, you had the bare faced gall to accuse the original poster of
being on drugs!

Look at the good news--even though you gave a partially wrong answer to
a question that wasn't even asked, you at least resolved the issue of
which of the two of you is on drugs... ;-)

 

[Roy Lewallen]

What is the difference between AM and DSB?

The two actually describe different properties, so a signal can be be
AM, DSB, neither, or both.

And here we run into some trouble between technical correctness and
common usage.

DSB stands for Double SideBand. Although I suppose an FM signal could be
called DSB because it has two *sets* of sidebands, and a narrowband FM
signal has only one significant pair like an AM signal, in my experience
the term DSB virtually always refers to a signal generated by amplitude
modulation.

AM is Amplitude Modulation. Straightforward amplitude modulation such as
done for AM broadcasting produces a carrier and two sidebands, or DSB
with carrier. Either the carrier or one sideband, or both, can be
suppressed. If you suppress the carrier (or don't generate it in the
first place), you get DSB with suppressed carrier, or DSB-SC. If you
suppress one sideband, you get SSB. Usually, but not always, the carrier
is also suppressed along with the one sideband, resulting in SSB-SC.
NTSC television transmission is VSB -- AM with a carrier and "vestigial"
or partially suppressed sideband and a full second sideband. Partial
suppression of the carrier is also done for some broadcast purposes.

So a commercial AM broadcast station broadcasts a signal that's both AM
and DSB. A typical amateur or military SSB transmission is AM but not
DSB. A QPSK signal is neither. And, as I mentioned, some signals like FM
could be considered DSB but not AM (although this isn't common usage).

In common amateur parlance, however,

"AM" usually means AM with two sidebands and carrier.
"DSB" usually means AM with two sidebands and suppressed carrier
"SSB" usually means AM with a single sideband and suppressed carrier

Roy Lewallen, W7EL

 

[Ron Baker, Pluralitas!]

I don't understand what effect you're referring to here.

When I was tuned to the 3rd harmonic sometimes
I would hear it and sometimes not.
It would come and go rather abruptly. It didn't seem
to be gradual fading.

I'm a physicist/engineer, and have been for a long time. I have always

The you understand Fourier transforms and convolution.

maintained that if the only way one can understand physical phenomena is
by solving the differential equations that describe them, then one does
not understand the phenomena at all. If you can express a thing in
words, such that a person with little mathematical ability can
understand what's going on, *then* you have a good grasp of it.

I too am a fan of the intuitive approach.
But I find that theory is often irreplacable.

 

[isw]

On 7/6/07 9:36 AM, in article
[email protected], "isw" <[email protected]>
wrote:

On 7/5/07 10:27 PM, in article [email protected],


On 7/5/07 12:00 AM, in article
[email protected],


On 7/4/07 8:42 PM, in article
[email protected],
"Ron


On 7/4/07 10:16 AM, in article
[email protected],


On 7/4/07 7:52 AM, in article
[email protected],
"Ron

<snip>


cos(a) * cos(b) = 0.5 * (cos[a+b] + cos[a-b])

Basically: multiplying two sine waves is
the same as adding the (half amplitude)
sum and difference frequencies.

No, they aren't the same at all, they only appear to be the
same
before
they are examined. The two sidebands will not have the correct
phase
relationship.

What do you mean? What is the "correct"
relationship?


One could, temporarily, mistake the added combination for a full
carrier
with independent sidebands, however.




(For sines it is
sin(a) * sin(b) = 0.5 * (cos[a-b]-cos[a+b])
= 0.5 * (sin[a-b+90degrees] -
sin[a+b+90degrees])
= 0.5 * (sin[a-b+90degrees] +
sin[a+b-90degrees])
)

--
rb





When AM is correctly accomplished (a single voiceband signal is
modulated

The questions I posed were not about AM. The
subject could have been viewed as DSB but that
wasn't the specific intent either.

What was the subject of your question?

Copying from my original post:

Suppose you have a 1 MHz sine wave whose amplitude
is multiplied by a 0.1 MHz sine wave.
What would it look like on an oscilloscope?
What would it look like on a spectrum analyzer?

Then suppose you have a 1.1 MHz sine wave added
to a 0.9 MHz sine wave.
What would that look like on an oscilloscope?
What would that look like on a spectrum analyzer?




So the first (1) is an AM question and the second (2) is a non-AM
question......

What is the difference between AM and DSB?




AM is a process. DSB (double sideband), with carrier, is it's most
simple
result. DSB without carrier (suppressed carrier dsb) requires using, at
least, a balanced mixer as the AM multiplier.

And requires, for proper reception, that a carrier be recreated at the
receiver which has not only the amplitude of the original,

There is no need at all to match the carrier amplitude of the original
signal. You can use an excessively high carrier injection amplitude with
no
detrimental affect, but if the injected carrier is too little, the
demodulated signal will be over modulated and sound distorted.

but also its exact phase.

Exact, not required. The closer the better, however.

Well, OK, the phase must at least bear a constant relationship to the
one that created the signal. If you inject a carrier that has a
quadrature relationship to the one that created the DSB signal, the
output will be PM (phase modulation). In between zero and 90 degrees,
the output is a combination of the two. If the injected carrier is not
at precisely the proper frequency, the phase will roll around and the
output will be unintelligible.

Not unintelligible.... Donald Duckish.

I think you are confusing *single* sideband, for which that is correct,
and *double* sideband (which we were discussing), for which it is not
true.

On a more practical side, however, most receiver filters for ssb will
essentially remove one sideband if there are two, and can attenuate a
carrier so the local product detector can do it's job resulting in improved
receiving conditions. But this is more advanced than the Ops questions.

Doing it that way will work, but it's not "fair", because you are not
actually demodulating a DSB signal (which was the subject of the
discussion).

Isaac

 

[isw]

The two actually describe different properties, so a signal can be be
AM, DSB, neither, or both.

And here we run into some trouble between technical correctness and
common usage.

DSB stands for Double SideBand. Although I suppose an FM signal could be
called DSB because it has two *sets* of sidebands

Um, actually, it has a lot more than that. A carrier FM modulated by a
single sine wave has an infinite number of sidebands. If the modulating
signal is more complex, then things get really complicated.

Isaac

 

[Ron Baker, Pluralitas!]

Yes, and in fact, that multiplication referred to above creates a
DSB-suppressed-carrier signal. To get "real" AM, you need to add back
the carrier *at the proper phase*.

So does the multiplication in the first example really make
it amplitude modulation?

 

[Ron Baker, Pluralitas!]

What *I* discovered is -not- the point. And for all intents and purposes,
"AM" and DSB are two distinct (but certainly related) things. Different
hardware to create (balanced modulator for DSB vs high level plate
modulation for 'classic' AM), more power required for AM and finally, back
in the day, the FCC had -different- emission designators for AM vs DSB.
Now, if they were the same, why would you think the FCC gave
them -different- emission designators?

You make good, relevant points there.

What IS the point is:

1) The original poster asked a question about "x".

2) You gave a half-assed answer to "y".

And then, you had the bare faced gall to accuse the original poster of
being on drugs!

Look at the good news--even though you gave a partially wrong answer to a
question that wasn't even asked, you at least resolved the issue of which
of the two of you is on drugs... ;-)

I would agree with the above also, but don't
wish to be provocative.

 

[John Smith I]

[chit]

Hmmm, why be a half-assed-idiot when you can be a full fledged one? I
see your point ...

Point is, DSB IS AM, you can receive it on any am receiver, get a life,
get off drugs and certainly get off the news groups, you are ill suited
to be here ...

JS

 

[Ron Baker, Pluralitas!]

A well-designed filter running into a bolometer would be. You can make
the filter narrow enough to respond to only one frequency component at

Any real spectrum analyzer has a lower limit
to its resolution bandwidth, does it not?
The resolution bandwidth of the human ear is non-zero
and not really adjustable, is it not?

the time, and a bolometer just turns the signal power into heat; nothing
nonlinear there...

Really?
You said you are a physicist/engineer.
What does "linear" mean?

 

[Don Bowey]

On 7/6/07 9:36 AM, in article
[email protected], "isw" <[email protected]>
wrote:

On 7/5/07 10:27 PM, in article [email protected],


On 7/5/07 12:00 AM, in article
[email protected],


On 7/4/07 8:42 PM, in article
[email protected],
"Ron


On 7/4/07 10:16 AM, in article
[email protected],


On 7/4/07 7:52 AM, in article
[email protected],
"Ron

<snip>


cos(a) * cos(b) = 0.5 * (cos[a+b] + cos[a-b])

Basically: multiplying two sine waves is
the same as adding the (half amplitude)
sum and difference frequencies.

No, they aren't the same at all, they only appear to be the
same
before
they are examined. The two sidebands will not have the correct
phase
relationship.

What do you mean? What is the "correct"
relationship?


One could, temporarily, mistake the added combination for a full
carrier
with independent sidebands, however.




(For sines it is
sin(a) * sin(b) = 0.5 * (cos[a-b]-cos[a+b])
= 0.5 * (sin[a-b+90degrees] -
sin[a+b+90degrees])
= 0.5 * (sin[a-b+90degrees] +
sin[a+b-90degrees])
)

--
rb





When AM is correctly accomplished (a single voiceband signal is
modulated

The questions I posed were not about AM. The
subject could have been viewed as DSB but that
wasn't the specific intent either.

What was the subject of your question?

Copying from my original post:

Suppose you have a 1 MHz sine wave whose amplitude
is multiplied by a 0.1 MHz sine wave.
What would it look like on an oscilloscope?
What would it look like on a spectrum analyzer?

Then suppose you have a 1.1 MHz sine wave added
to a 0.9 MHz sine wave.
What would that look like on an oscilloscope?
What would that look like on a spectrum analyzer?




So the first (1) is an AM question and the second (2) is a non-AM
question......

What is the difference between AM and DSB?




AM is a process. DSB (double sideband), with carrier, is it's most
simple
result. DSB without carrier (suppressed carrier dsb) requires using, at
least, a balanced mixer as the AM multiplier.

And requires, for proper reception, that a carrier be recreated at the
receiver which has not only the amplitude of the original,

There is no need at all to match the carrier amplitude of the original
signal. You can use an excessively high carrier injection amplitude with
no
detrimental affect, but if the injected carrier is too little, the
demodulated signal will be over modulated and sound distorted.

but also its exact phase.

Exact, not required. The closer the better, however.

Well, OK, the phase must at least bear a constant relationship to the
one that created the signal. If you inject a carrier that has a
quadrature relationship to the one that created the DSB signal, the
output will be PM (phase modulation). In between zero and 90 degrees,
the output is a combination of the two. If the injected carrier is not
at precisely the proper frequency, the phase will roll around and the
output will be unintelligible.

Not unintelligible.... Donald Duckish.

I think you are confusing *single* sideband, for which that is correct,
and *double* sideband (which we were discussing), for which it is not
true.

What do you propose the term be for the output of a slightly de-tuned
demodulator of a DSB sans carrier, signal?

Doing it that way will work, but it's not "fair", because you are not
actually demodulating a DSB signal (which was the subject of the
discussion).

I don't believe the OP stated whether the DSB signal was with or without
carrier. If without carrier, demodulation is certainly called for. If with
carrier, it hardly merits discussion.

 

[Don Bowey]

What *I* discovered is -not- the point. And for all intents and
purposes, "AM" and DSB are two distinct (but certainly related) things
Different hardware to create (balanced modulator for DSB vs high level
plate modulation for 'classic' AM), more power required for AM and
finally, back in the day, the FCC had -different- emission designators
for AM vs DSB. Now, if they were the same, why would you think the FCC
gave them -different- emission designators?

You are confusing FCC use codes and technical processes. Do you believe the
FCC Designator of "J" for ssbsc says HOW to do it. Not for an instant.

 

[Roy Lewallen]

Um, actually, it has a lot more than that. A carrier FM modulated by a
single sine wave has an infinite number of sidebands. If the modulating
signal is more complex, then things get really complicated.

Sometimes it's difficult to communicate. A "set" can consist of more
than one. In the case of FM, each set includes an infinite number,
although only a limited number contain a significant amount of energy.
The remainder can be ignored without any substantial degradation of
received signal quality. This is true regardless of the complexity of
the modulating signal.

Roy Lewallen, W7EL

 

[Tommy Tootles]

Point is, DSB IS AM, you can receive it on any am receiver,

Well, another of your half-assed answers. You can receive *DSB-RC* on
any AM receiver because the carrier, although reduced, allows reception
via a simple envelope detector. On the other hand, DSB-SC requires a
product detector, a coherent detector or a Costas Loop, detectors NOT
available on "any" AM receiver. So, yet another "half-an-answer" on your
part.

get off drugs and certainly get off the news groups, you are ill suited
to be here ...

A person asks about "a", *you* give them an answer to "b", then accuse
-them- of being on drugs and say -they- are ill-suited to be here. May I
suggest that you look in the mirror if you are concerned about
suitability...

Your thought processes and (lack of) logic seem quite odd. Odd enough to
question who the drug user might be.

 

[Tommy Tootles]

You are confusing FCC use codes and technical processes. Do you believe the
FCC Designator of "J" for ssbsc says HOW to do it. Not for an instant.

Don,

I believe you are misinterpreting or misunderstanding what I wrote--and
my apologies if I wasn't clear enough in my statement above. Let me
clarify...

I was NOT confusing FCC use codes and technical processes. I made two
separate, *stand-alone* statements:

Statement 1 (re: technical processes)--low level balanced modulator
vs.high level plate modulation. A true statement.

Statement 2 (re: FCC emission designators)-- that the FCC had different
emission designators for AM, DSB-SC and DSB-RC *BACK IN THE DAY*. A true
statement.

By "back in the day", I was referring to the late 50s and early 60s when
sideband (of all varieties) was just coming in to usage in the ham radio
world. Everything was so new that SSB hadn't yet emerged as the mode of
choice. Some rigs back then were capable of both SSB and the two flavors
of DSB. The FCC (AT THAT TIME--"back in the day") had designators for
all of the above. The FCC emission designators have changed at least
once (and maybe more) since those days.

In any event, they were meant to be two stand-alone statements, with
-no- implication intended that the designator tells how to do it. So,
either you need to read more carefully, I need to write more carefully
or all of the above...

 

[isw]

--bunch of stuff trimmed off--

What do you propose the term be for the output of a slightly de-tuned
demodulator of a DSB sans carrier, signal?

I'm not sure it has a name. The output is constantly swishing around
between AM and PM, at a rate determined by the frequency error of the
reinjected carrier. Most detectors will have a problem with it.

Isaac

 

[isw]

Any real spectrum analyzer has a lower limit
to its resolution bandwidth, does it not?
The resolution bandwidth of the human ear is non-zero
and not really adjustable, is it not?


Really?
You said you are a physicist/engineer.
What does "linear" mean?

Let's not get too far off the subject here. We were discussing whether
the "tuning beat" that you use to tune a musical instrument involved a
nonlinear process (ie. "modulation"). I said that it does not, and that
it could be detected by instrumentation which was proveably linear (i.e.
not "perfectly" linear, because that's not required, but certainly
linear enough to discount the requirement for "modulation").

That's all.

Isaac

 

[isw]

So does the multiplication in the first example really make
it amplitude modulation?

Yes, because the output signal varies in amplitude with modulation. For
suppressed carrier SSB or DSB, the output is zero when there's no
modulating signal, while for "traditional AM", the output is 50% for no
modulation.

Compare to FM or PM, where the output is constant regardless of the
modulation level. True, FM has a lot of sidebands that vary in
amplitude, but if you add them all together, the output is constant.

Run an SSB, DSB, or AM rig into a dummy load and it'll get hotter with
modulation, while with FM the temperature won't change.

--

But recall that if you take that DSB signal you got by multiplication,
and reinject the carrier in quadrature, you no longer have amplitude
modulation.

Isaac

 

[Ron Baker, Pluralitas!]

Let's not get too far off the subject here. We were discussing whether
the "tuning beat" that you use to tune a musical instrument involved a
nonlinear process (ie. "modulation").

Then linearity is at the core of the matter.
What does "linear" (or "nonlinear") mean to you?

I said that it does not, and that
it could be detected by instrumentation which was proveably linear (i.e.
not "perfectly" linear, because that's not required, but certainly
linear enough to discount the requirement for "modulation").

No nonlinearity is necessary in order to hear
a beat?
Where does the beat come from?