"Roy Lewallen" wrote in message
...
The average value of a signal is its DC value. To receive and reproduce
a signal with a non-zero DC component means that your antenna, as well
as your receiver, has to have response to DC. To generate such a signal
would require a static electric and/or magnetic field, which can't
propagate. So it's not possible for a signal you're receiving to have a
non-zero average value.

sure it is. and while the 'static' field itself doesn't propagate the
leading edge of a step from 0 to some 'static' value can propagate and if
you measure as it passes you will see the received signal go from 0 to the
static value and then stay there.

On Sun, 6 Feb 2005 15:59:36 -0000, "Dave" wrote:

leading edge of a step from 0 to some 'static' value

Hi Dave,

....negates the usage of "static" - clearly.

A step pulse is not "static" and in fact contains an infinite range of
frequencies all of which are NOT DC. This is called the genii out of
the bottle and no one here can (but no doubt will try to) put it back.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
On Sun, 6 Feb 2005 15:59:36 -0000, "Dave" wrote:

leading edge of a step from 0 to some 'static' value

...negates the usage of "static" - clearly.

A step pulse is not "static" and in fact contains an infinite range of
frequencies all of which are NOT DC. This is called the genii out of
the bottle and no one here can (but no doubt will try to) put it back.
______________

You mis-read. There is a DC component _required_ to convey the steady
voltage values preceding and following the step pulse transition. He's not
saying that the step pulse transition itself is comprised of "DC."

RF

On Sun, 6 Feb 2005 12:12:40 -0600, "Richard Fry"
wrote:
"Richard Clark" wrote in message
On Sun, 6 Feb 2005 15:59:36 -0000, "Dave" wrote:
leading edge of a step from 0 to some 'static' value
...negates the usage of "static" - clearly.
You mis-read.

Hi OM,

Your failure to reconcile the meaning of static with a non-static
description is as much as I need read. This class of discussion is
revealed in the hallmark re-definition of terms to suit poor
arguments.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sun, 6 Feb 2005 12:12:40 -0600, "Richard Fry"
wrote:
"Richard Clark" wrote in message
On Sun, 6 Feb 2005 15:59:36 -0000, "Dave" wrote:
leading edge of a step from 0 to some 'static' value
...negates the usage of "static" - clearly.
You mis-read.

Hi OM,

Your failure to reconcile the meaning of static with a non-static
description is as much as I need read. This class of discussion is
revealed in the hallmark re-definition of terms to suit poor
arguments.

73's
Richard Clark, KB7QHC
________________

Your rhetoric is evasive and blustery -- no doubt so you won't have to admit
publicly to an incomplete understanding of what you are responding to.

RF

Surely as a former broadcast engineer you're acquainted with a circuit
called a "DC restorer". This is a circuit which is always present in a
TV receiver. In its simplest form, it's just a diode clamping circuit,
although I've made very good ones with an FET switch and hold capacitor.
What it does is to set the sync pulse tip to a fixed DC value, which
then causes the rest of the TV waveform to be at a fixed DC value. This
is how the DC information is "transmitted". The actual TV waveform is AC
coupled, as it must be, and its DC values are established in the
receiver by the DC restorer.

No DC value is transmitted.

You should be able to find an explanation of this in any basic text on
the principles of television transmission and reception.

Roy Lewallen, W7EL

Richard Fry wrote:
______________

You mis-read. There is a DC component _required_ to convey the steady
voltage values preceding and following the step pulse transition. He's
not saying that the step pulse transition itself is comprised of "DC."

RF

"Roy Lewallen" wrote
Surely as a former broadcast engineer you're acquainted with a circuit
called a "DC restorer". This is a circuit which is always present in a TV
receiver. In its simplest form, it's just a diode clamping circuit,
although I've made very good ones with an FET switch and hold capacitor.
What it does is to set the sync pulse tip to a fixed DC value, which then
causes the rest of the TV waveform to be at a fixed DC value. This is how
the DC information is "transmitted". The actual TV waveform is AC coupled,
as it must be, and its DC values are established in the receiver by the DC
restorer....
_________________________

I must respectfully disagree with that, Sir. The baseband video signal is *
DC coupled * through analog TV transmitters . The peak power of a
transmitted TV RF waveform is a fixed value, at the power corresponding to
the licensed ERP of the station, occurring at the peak of sync pulses, and
independent of program video. Transmitted _average_ power is a function of
the video envelope.

Video is transmitted with negative polarity; 75% modulation when video is
black, and 12-1/2% modulation when it is white. The video waveform AS
TRANSMITTED can contain a steady state (DC) value throughout the video field
for any amplitude at or between those values (actually the color subcarrier
can exceed these for some conditions).

The circuits in a TV receiver cannot pass the DC component, thus the need
for a DC restorer following the video demodulator. But the point remains
that the transmitted TV waveform can, and often does contain a DC component,
and that this DC component is required for accurate reproduction of the
original video on the TV display.

RF

PS: I still AM a broadcast engineer.

"Roy Lewallen" wrote about broadcast television waveforms:
... No DC value is transmitted....
and
...The actual TV waveform is AC coupled, as it must be, and its
DC values are established in the receiver by the DC restorer...
___________________________

Please think about this. Standard TV color bars are produced by mixing the
outputs of three video pulse generators. The pulses are square waves, with
amplitude limits of 0 and 0.7VDC (~0.25µs transitions). These three video
waveforms are shown in Figure 7-1 in the paper hyperlinked at the bottom of
this post.

The DC present during the "on" time of these video pulses is
indistinguishable from the DC supplied by a continuous 0.7VDC source, over
the same, steady-state time interval. The DC and near-DC values of this
video must be available to the TV display device in order to accurately show
the color bar signal (and any other video waveform).

Low frequency content is lost when video is AC-coupled through a TV
transmitter (or any other circuit). A TV set DC restorer sets and
maintains the DC axis on which the pulse rides, but that does not correct
the distortion of the pulse waveform resulting from loss of its low
frequency content near zero hertz (DC).

Also recall that even the shortest pulse* that can pass undistorted through
the ~4MHz video bandwidth of the US broadcast TV standard contains most of
its energy at, and near zero hertz, e.g., DC.

*a sin² pulse with 0.25µs transitions

So DC _is_ , and must be conveyed by analog broadcast (and other) television
systems, because television wouldn't work well otherwise. It isn't DC as
you may think of it coming out of a battery, but it is DC, nevertheless, and
identical to battery DC of the same amplitude, when compared over equal,
steady-state intervals.

http://www.tek.com/Measurement/cgi-b...Set=television

RF

"Richard Clark" wrote in message
...
On Sun, 6 Feb 2005 15:59:36 -0000, "Dave" wrote:

leading edge of a step from 0 to some 'static' value

Hi Dave,

...negates the usage of "static" - clearly.

A step pulse is not "static" and in fact contains an infinite range of
frequencies all of which are NOT DC. This is called the genii out of
the bottle and no one here can (but no doubt will try to) put it back.

73's
Richard Clark, KB7QHC

you will of course note that i quoted the term 'static' to denote that it
was indeed not static in the infinite sense of mathematics, but in the real
sense of it being a constant value over some measured time period.

the other obvious thing you are missing is that DC is a frequency... and it
does come out in the Fourier transform as a frequency of zero with some
finite magnitude for any waveform who's average value is not zero. it does
not even require that the waveform stay at some value forever, a short pulse
going from 0v to 1v and back to 0v will have a DC component in its spectrum
both mathematically and on any meter that can properly respond to it.

On Sun, 6 Feb 2005 19:35:55 -0000, "Dave" wrote:

you will of course note that i quoted the term 'static' to denote that it
was indeed not static in the infinite sense of mathematics, but in the real
sense of it being a constant value over some measured time period.

Hi Dave,

This is fine as an elaboration that grows beyond the horizon of the
question. But it really serves no purpose but to embroider a glaring
lack of facts.

the other obvious thing you are missing is that DC is a frequency... and it
does come out in the Fourier transform as a frequency of zero with some
finite magnitude for any waveform who's average value is not zero. it does
not even require that the waveform stay at some value forever, a short pulse
going from 0v to 1v and back to 0v will have a DC component in its spectrum
both mathematically and on any meter that can properly respond to it.

After having done a quite rigorous contract with HP deeply involved in
both the strict math and the mechanics of Fourier, I am well versed to
know how not all Fourier Transforms offer equal outcomes - especially
with, or without D.C. as a product.

Pure Fourier mandates a waveform constant throughout all time (from
its inception to its end) and is strictly one of those "thought
experiments" when it comes to transients - obviously. This has been,
and is the first issue that comes with its classic requirements
clashing with utility.

Practical Fourier mandates that you remove all D.C. components through
what is called "windowing" your data. You can choose not to and enjoy
all the artifacts of spurious behavior (like frequency folding,
frequency blurring, and false baselines) and translate those results
into blighted proofs too. With transients, practical Fourier
techniques mandate the presumption that they are periodic through all
time by literally forcing the data set (through extensive reworking)
to fit this requirement (which returns us to classic expectation).
Such reworking of original data demands that you make a choice: you
either discard amplitude accuracy for the sake of frequency accuracy,
or discard frequency accuracy for the sake of amplitude accuracy. The
irony of this choice is that you have already discard D.C. and its
appearance within the product of results is imbued with error. Need I
ask if you know how much error? Ignorance to what accuracy your
transforms bring reduce arguments to platitudes.

Simple, day to day, practical applications of Fourier discard these
issues as trivialities immediately and offer no pretense of "accuracy"
because you couldn't appreciate it anyway (unless the designer of an
IIR or FIR was so incompetent as to be that obvious).

However, this goes beyond the pale to justify a glaring presumption
"average signal magnitude not zero" with the trappings of Fourier.
For one, absolutely no one knows what is meant by "impulsive
wideband signal (500 - 2.5 GHz)." This is so extremely vague as to be
supportable by a multitude of waveforms that have wildly different
Fourier results. For instance, there is the classic impulse of
physics, lightning, which is notoriously low in frequency content over
the interval cited. Clearly such impulsions suffered are not from
nature. But this begs forcing the nomenclature to fit the problem.
There are other impulses like a gated sine wave (which obviously
renders a zero average), or a swept sine wave (same obvious average),
or a ramp wave, or a saw tooth wave, or a triangle wave, or chirp
wave. The list of impulsive wideband signals is exhausting, but
hardly accommodating to the same universal expectation.

If our correspondent, Galilea, cannot come to terms with responding to
technical issues when seeking help, the growth of forced speculation
creates its own illusory discussion that is best limited to idle
conversation overheard in a ticket line waiting for a movie box office
to open.

73's
Richard Clark, KB7QHC