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

Richard Fry wrote:

Before calling this reality absurd, consider that a television station
transmits a video signal in/on an RF channel. The demodulated video
waveform in the TV receiver will be identical to the baseband video
signal applied to the TV tx -- including its DC components (subject to
any distortions along the transmission path).

If it wasn't ~ identical, a TV set could never "fade to black" when the
original image did, and low-luminance colors such as blue, red, brown
etc would be impossible to reproduce with their original chromaticity.

RF (ex-RCA Field Engineer, and installer of
hundreds of TV color studio and film cameras)

You perhaps never installed a receiver. Without its DC restorer circuit,
the problems you mention do indeed exist. In a black-and-white set, a
large white area will cause everything else to go black, to maintain a
zero average. But the DC information is transmitted as a level
difference between the sync tip and the "porches". At the receiver (as I
mentioned in another post), this voltage *difference* is turned into a
DC value. No DC is transmitted.

Surely a little bit of thought will establish why direct transmission of
DC isn't possible beyond the range of a static field.

Roy Lewallen, W7EL

"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

But the DC information is transmitted as a level difference
between the sync tip and the "porches". At the receiver (as I mentioned in
another post), this voltage *difference* is turned into a DC value. No DC
is transmitted.
_________________

As I wrote in my slightly earlier post to you on this, the actual video
waveform is DC coupled through an analog TV transmitter. Please refer to
that post for more on this topic.

RF

"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:

"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.

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.

Perhaps the measured DC value is not the result of EM waves
but the result of mechanical depositing of charge on the
antenna. I assume that wind/snow static is DC - enough DC
to cause arcing across a coax connector.
--
73, Cecil http://www.qsl.net/w5dxp


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Richard,

Sorry about the triple. I was so caught up in the excitement (constant
throughout all time) of your message I just couldn't stop.

Quote:

"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)"


I can ignore the name-dropping, but I cannot ignore the incorrect
statement about "Pure Fourier". There is no mandate of constancy even
for the purest Fourier transform. The function needs only to be
moderately well-behaved, including single valued and integrable.

Perhaps you are confusing Fourier series analysis with Fourier transform
analysis?

(By the way, for any of the lurkers out there, I did not say a word
about "DC". That is a trivial matter subject only to one's choice of
wording. There is no apparent disagreement on the reality, only the
description.)


73,
Gene
W4SZ


Richard Clark wrote:
On Sun, 06 Feb 2005 21:58:05 GMT, Gene Fuller
wrote:


Utter nonsense.


Hi Gene,

You had to post that three times?

Oh well, seems the season for deeply insightful criticism that appears
to be following the value of the dollar in the world market. Was that
your 1½¢ worth?

Barring any discussion of conflicting experience, I would say it was
worth every penny. You can put it into a privatized SS account and
recoup massive earnings - if the dollar would just tread water. Don't
invest in cotton or rice futures though.... [Feb. 5 - President Bush
will seek deep cuts in farm and commodity programs in his new budget]

73's
Richard Clark, KB7QHC

On Mon, 07 Feb 2005 14:34:38 GMT, Gene Fuller
wrote:

I can ignore the name-dropping, but I cannot ignore the incorrect
statement about "Pure Fourier". There is no mandate of constancy even
for the purest Fourier transform. The function needs only to be
moderately well-behaved, including single valued and integrable.

Hi Gene,

In this case you are seriously wrong. There are no IFs ANDs or BUTs.
The loop hole of well-behaved is not enough with it being far too
inspecific.

The ONLY case where the Fourier Series resolves a correct
transformation is if you limit your data set (or for an Integration,
you define your limits) over an interval of n · 2 · PI for a periodic
function where n is an integer from 1..m. Further, you are resolution
limited if you fail to observe Nyquist's laws and under sample, or
fail to frequency limit your real data. This also segues into
Shannon's laws where you can observe the S+N/N in the transform
(discussed below). These concerns are EXTERNAL to the simple act of
transforming data, but are necessary correlatives that MUST be taken
into account.

If you fail even in this simple regard for periodicity (say looking at
only 359 degrees of the periodic function), the result is quite
dramatically different in the Fourier output. Even the casual
observer can immediately see the difference between the correct and
incorrect results, there is nothing ambiguous about it at all.

Perhaps you are confusing Fourier series analysis with Fourier transform
analysis?

No, I have done both, and I will drop the name again, at HP with their
work on Fourier Analysis equipment where I tested their FFT algorithms
(call them what you may, the basic underlying requirements do not
change). I was working with 24 Mathematicians AND Engineers - there
was nothing sloppy about the quality of up-front preparation. This
was a project 5 years in the making. They even wrote their own Pascal
compiler for 1000000 lines of code. I have also done IIRs and FIRs,
Wavelets, and a host of other frequency/time series decimation
analysis.

ALL Fourier techniques have requirements that go beyond the Fourier
math. These requirements (if you have any interest in accuracy)
cannot be ignored. If you have no interest in accuracy, you still
have to perform some of them, which is to say there are trade offs as
I mentioned previously. Ignoring them all simply reduces real data
into transformed garbage.

I have written FFT software that has resolved pure sine waves into a
transformation to a single bin with a statistical noise floor and ALL
spurious response down 200dB. To give an example of what 1° of
decimation error will do, it will inject 120dB of noise into the
product and spurs that are barely 10 to 20 dB down from the principle
bin (which also exhibits about 3dB error).

Much of what is available through college texts and on the web are
seriously under powered in their scope. College is not very
interested in scope, simply introduction. That is, unless you find
yourself in a undergrad (more probably grad school with the additional
considerations taken into account) engineering course dedicated to
modern implementations (practical Fourier) now largely focused on DSP
(which had its genesis in the IIR and FIR earlier implementations).

73's
Richard Clark, KB7QHC

Cecil Moore wrote:

Tam/WB2TT wrote:

"Galilea" wrote

When analysing wideband impulse signals from a wideband antenna I have
realised that the average signal magnitude is not zero. I have
thought this
is because the reactance of the antenna at different frequencies
varies and
since it is a wideband antenna there can be energy measured since it
is only
for an extremely small period of time of 2-4 us. However, I am not
sure and
would greatly appreciate the views of this newsgroup.

Are you saying the DC value is not 0 ?


He seems to be saying the DC value is not 0 for a sampling
time period of 2-4 uS which would of course be true for a
sine wave if, e.g., an odd number of positive cycles were
sampled along with an even number of negative cycles. Or
if the frequency was lower than 250 kHz. Plus all the
signals and noise are superposed.

Chances are fair that something is doing some rectifying somewhere.

ac6xg

Jim Kelley wrote:
Chances are fair that something is doing some rectifying somewhere.

It later occurred to me that wind/snow noise is
carried by static DC charged particles.
--
73, Cecil http://www.qsl.net/w5dxp


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