Reply by glen herrmannsfeldt●July 28, 20142014-07-28
Rick Lyons <R.Lyons@_bogus_ieee.org> wrote:
(snip, someone wrote)
>>Well, it certainly would be if only we could figure out how to get those
>>damned imaginary numbers to actually make an antenna do something.
>>Usually, by the time you translate things to voltages on a wire you
>>_really_ want to have everything real.
(snip)
>> (a), it isn't, (b), if you're thinking AM, it is no longer a
>> very common modulation method,
> Whoa. Wait. I'm listenin' to AM radio right now in my
> office!
I most often listen to AM for sports and traffic reports.
There is some music there, but FM stations have more.
Maybe, though, when far away from big cities, AM stations also
reach out farther.
>>and (c), AM was conceived and executed because it's an
>>EASY modulation method with 1920's technology.
> Standard broadcast AM radio has some important shortcomings.
> Its RF bandwidth is twice that of the information signal's
> bandwidth, AM broadcasts are quite susceptable to
> RF noise between the transmitter and the AM receiver, and
> (if I recall correctly) only one quarter of the transmitter's
> power goes toward radiating a single sideband of the
> transmitted RF.
OK, but there are two sidebands, so half the power is in the
sidebands, assuming the usual demodulator.
Reminds me that for vistigial sideband (analog TV), since
one sideband has reduced bandwidth, and both sidebands go into
the demodulator, the receiver has to correct for the frequency
response of the sum. Higher frequencies have half the amplitude
of the lower ones.
> Given all that, broadcast AM radio has one overwhelming
> quality. The AM receivers were simple and affordable!
But now FM radios, using modern analog ICs, are pretty simple
and affordable, too. And if not, go to a thrift store and buy
a used one.
Seems to me the main advantage now is that the lower AM frequencies
can reach farther, especially at night.
We now have HD radio, with digital subcarriers on top of the usual
AM and FM stations, though I don't know anyone with a receiver
for them.
-- glen
Reply by Rick Lyons●July 28, 20142014-07-28
On Sat, 26 Jul 2014 18:03:39 -0500, Tim Wescott
<tim@seemywebsite.really> wrote:
>On Sat, 26 Jul 2014 01:44:47 -0500, SBR123 wrote:
>
>> Hello All,
>>
>> The classical modulation scheme is defined as,
>>
>> x(n)*cos(w_0*n) <-> 0.5*[X(w+w_0) + X(w-w_0)] - eq(1)
>
>That is double sideband modulation, which is neither classical nor very
>good.
>
>Perhaps you are thinking of amplitude modulation, which would be
>
>(1 + x(n)) * cos(w_0 * n)
>
>assuming that x(n) is restricted to the interval (-1, +1)?
>
>>
>> Similarly, text books also define frequency shifting as,
>>
>> x(n)*exp(j*w_0*n) <-> X(w-w_0) - eq(2)
>>
>>
>> It appears to me that scheme in eq(2) is much cleaner way of modulating
>> a signal than scheme in eq(1).
>
>Well, it certainly would be if only we could figure out how to get those
>damned imaginary numbers to actually make an antenna do something.
>Usually, by the time you translate things to voltages on a wire you
>_really_ want to have everything real.
Hi Tim,
Cute. But actually, you've touched upon a
critically important topic here. Now and then I
teach a DSP class and one part of my class covers
the basics of quadrature (I/Q) processing with
signal samples having both real and imaginary parts.
For example, an analog complex down-conversion followed
by two analog lowpass filters driving two A/D converters.
I show some sort of block diagram on the screen and then
show some algebraic equations that describe the mathematical
relationships of various signals in the block diagram.
The equations always contain the 'j-operator' signifying
a complex signal somewhere in the block diagram.
I then go through my standard spiel suggesting that the
students do NOT think of the 'j' symbol as a number, but
rather think of it as a "operation" performed on a
number to generate a new number.
Invariably, a thoughtful student asks, "How do we implement
the j-operator in hardware?" Tim, ...that question is not
at all silly! And it's a question whose answer is not
given enough attention in the standard college
DSP textbooks.
>> Second scheme requires complex
>> multiplication in time while first scheme requires some additional
>> manipulation.
>>
>> So, I am wondering why eq(1) is normally defined as a standard
>> modulation method.
>
>(a), it isn't, (b), if you're thinking AM, it is no longer a very common
>modulation method,
Whoa. Wait. I'm listenin' to AM radio right now in my
office!
>and (c), AM was conceived and executed because it's an
>EASY modulation method with 1920's technology.
Standard broadcast AM radio has some important shortcomings.
Its RF bandwidth is twice that of the information signal's
bandwidth, AM broadcasts are quite susceptable to
RF noise between the transmitter and the AM receiver, and
(if I recall correctly) only one quarter of the transmitter's
power goes toward radiating a single sideband of the
transmitted RF.
Given all that, broadcast AM radio has one overwhelming
quality. The AM receivers were simple and affordable!
[-Rick-]
Reply by Rick Lyons●July 28, 20142014-07-28
On Sat, 26 Jul 2014 22:24:44 -0500, "SBR123" <100967@dsprelated>
wrote:
>Thank you very much, Mr. Dvsarvate & Mr. Wescott,
>
>I think I get an idea but still not yet fluent with modulation,
>transmission etc.
>When I get there, I will come back on this question
>
>_____________________________
>Posted through www.DSPRelated.com
Reply by SBR123●July 27, 20142014-07-27
Thank you very much, Mr. Dvsarvate & Mr. Wescott,
I think I get an idea but still not yet fluent with modulation,
transmission etc.
When I get there, I will come back on this question
_____________________________
Posted through www.DSPRelated.com
Reply by Tim Wescott●July 26, 20142014-07-26
On Sat, 26 Jul 2014 01:44:47 -0500, SBR123 wrote:
> Hello All,
>
> The classical modulation scheme is defined as,
>
> x(n)*cos(w_0*n) <-> 0.5*[X(w+w_0) + X(w-w_0)] - eq(1)
That is double sideband modulation, which is neither classical nor very
good.
Perhaps you are thinking of amplitude modulation, which would be
(1 + x(n)) * cos(w_0 * n)
assuming that x(n) is restricted to the interval (-1, +1)?
>
> Similarly, text books also define frequency shifting as,
>
> x(n)*exp(j*w_0*n) <-> X(w-w_0) - eq(2)
>
>
> It appears to me that scheme in eq(2) is much cleaner way of modulating
> a signal than scheme in eq(1).
Well, it certainly would be if only we could figure out how to get those
damned imaginary numbers to actually make an antenna do something.
Usually, by the time you translate things to voltages on a wire you
_really_ want to have everything real.
> Second scheme requires complex
> multiplication in time while first scheme requires some additional
> manipulation.
>
> So, I am wondering why eq(1) is normally defined as a standard
> modulation method.
(a), it isn't, (b), if you're thinking AM, it is no longer a very common
modulation method, and (c), AM was conceived and executed because it's an
EASY modulation method with 1920's technology.
You can make a decent AM transmitter with three to five vacuum tubes (MOPA
RF section using two tubes, plus a two-tube audio preamp/final amp stage
in class A, or a three-tube audio preamp/final amp stage in class B or
AB). You can make a workable AM receiver with just one tube, or with a
diode and a really good set of earphones -- a table-top set in the 1920's
that would be considered "nice" may have had just three or four tubes,
and with two tubes in the receiver you could hear shortwave AM from
around the world.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Reply by dvsarwate●July 26, 20142014-07-26
In your eq.(1), the result of the modulation is
a real-valued signal while eq.(2) gives a
complex-valued signal. The identity
cos(theta) = 0.5*[ e^{j theta} + e^{-j theta} ]
relates the two notions.
Reply by SBR123●July 26, 20142014-07-26
Hello All,
The classical modulation scheme is defined as,
x(n)*cos(w_0*n) <-> 0.5*[X(w+w_0) + X(w-w_0)] - eq(1)
Similarly, text books also define frequency shifting as,
x(n)*exp(j*w_0*n) <-> X(w-w_0) - eq(2)
It appears to me that scheme in eq(2) is much cleaner way of modulating a
signal than scheme in eq(1). Second scheme requires complex multiplication
in time while first scheme requires some additional manipulation.
So, I am wondering why eq(1) is normally defined as a standard modulation
method.
_____________________________
Posted through www.DSPRelated.com