Why is video inverted for transmission?

[JosephKK]

glen herrmannsfeldt [email protected] posted to
sci.electronics.design:

Gary Tait wrote:
(snip)



I remember in the days of UHF getting more popular, and using
a converter box that output on VHF channel 3. This can be done
with the appropriate mixer and LO, without converting the input
to baseband. It does involve tuned circuits, but does not
convert all (70) UHF channels to be tuned on a (12 channel)
VHF tuner.

I believe there were/are block converters from cable channels
to UHF which do convert all at once. Most cable boxes don't
do that, though. Early (analog) ones did the down conversion
similar to the UHF conversion described above. Most now likely
go to baseband and then remodulate for those without video inputs.

The distinction between 'tuner' and 'converter' is fuzzy.
I don't believe that there is a convenient way to block convert
the ATSC input to NTSC output. One could build a box with
multiple tuners, decoders, and modulators but I doubt that
would be for the consumer market.

While i have not see one, there is nothing that prevents it
technologically. I have seen receivers that can receive the entire
AM band (in stereo as broadcast) at the same time. I have seen
receivers that receive over half of the FM band (in stereo)
simultaneously. Only ADC, DAC, and compute power available preclude
block conversion. Dig around bit on software defined radios and you
can find the done devices that i have found.

 

[ChairmanOfTheBored]

Gary Tait wrote:
(snip)



I remember in the days of UHF getting more popular, and using
a converter box that output on VHF channel 3. This can be done
with the appropriate mixer and LO, without converting the input
to baseband. It does involve tuned circuits, but does not
convert all (70) UHF channels to be tuned on a (12 channel)
VHF tuner.

I believe there were/are block converters from cable channels
to UHF which do convert all at once. Most cable boxes don't
do that, though. Early (analog) ones did the down conversion
similar to the UHF conversion described above. Most now likely
go to baseband and then remodulate for those without video inputs.

The distinction between 'tuner' and 'converter' is fuzzy.
I don't believe that there is a convenient way to block convert
the ATSC input to NTSC output. One could build a box with
multiple tuners, decoders, and modulators but I doubt that
would be for the consumer market.

The box needs an ATSC tuner, and the logic to convert the
result to an NTSC analog signal. Most likely with both video
and RF outputs. The output of the ATSC tuner is the digital
signal, not suitable for an analog TV, so the box needs
both a tuner and converter.

Which is why the item I describe IS a converter!

Mainly... because it HAS ONE IN IT!

 

[IAmTheSlime]

It's easy to see even fairly dim stars against a dark nighttime sky.
Seeing black specks against a bright daytime sky is much harder. The
contrast ratio may be the same, but size matters.

Jerry

The "farmer" that discovered Pluto after years of intensive slide
examination knows a thing or two about contrast.

The plates were all developed so the examiner got black "stars" or
"objects" on a white "space" background.

There is a reason for this. It has to do with the human eye, and how
the brain processes visual information.

 

[[email protected]]

The "farmer" that discovered Pluto after years of intensive slide
examination knows a thing or two about contrast.

The plates were all developed so the examiner got black "stars" or
"objects" on a white "space" background.

There is a reason for this. It has to do with the human eye, and how
the brain processes visual information.

I would have thought that the primary reason for using a white
background was to avoid introducing extra noise from the photographic
processing required to convert back to a positive image.

 

[glen herrmannsfeldt]

JosephKK wrote:

(snip)

While i have not see one, there is nothing that prevents it
technologically. I have seen receivers that can receive the entire
AM band (in stereo as broadcast) at the same time. I have seen
receivers that receive over half of the FM band (in stereo)
simultaneously. Only ADC, DAC, and compute power available preclude
block conversion. Dig around bit on software defined radios and you
can find the done devices that i have found.

You can block convert a band to another, shifted, band with a
single mixer. I suppose processors are fast enough now to
demodulate a downshifted group of FM band signals, maybe
AM without downconversion.

I think I said before that a block ATSC to NTSC converter might
be useful in a commercial setting, such as a hotel. I would
seem to have a very small advantage for home use, more than I
would expect for the cost difference.

-- glen

 

[Jerry Avins]

Which is why the item I describe IS a converter!

Mainly... because it HAS ONE IN IT!

You have a prick, therefore you are a prick? Maybe so.

Jerry

 

[Gary Tait]

While i have not see one, there is nothing that prevents it
technologically.

For a $40 box, it won't be a blik digital to analog convertor.

 

[Gary Tait]

I most certainly does exactly that. It is meant for HDTV, and puts out
a wide from factor picture. It ALSO has a built in converter, which
allows the user to set it up for use with an older 4:3 analog TV.

It does so flawlessly, and yes, that does qualify it 100% as a
"converter".

No, they are ATSC digital tuners. It is a matter of course they
"decode" the digital signal downconvert HD (sub) channels for display on
an SD set. ATSC/HD tuners have been doing that for years.

You're an idiot, and are NOT familiar with the device I refer to.

Well what devices are you familiar with then?

The ones I know of are ATSC digitial STB tuners.
They tune one 8VSB carrier, demux a subchannel out of that, decompress
an MPEG2/AC3 program stream, output out component or HDMI, as well as
downconvert to SD out S-video/composite, and if need be directly tune SD
subchannels and feed out the SD outputs, or upconvert to HD output.

Bullshit side topic of the basic argument in this sub-thread.

The fact remains that the tuner I mentioned IS an HD OTA tuner that
does 100% all channel conversion to a letterboxes 4:3 output for wide
HDTV Digital Content, and standard 4:3 for "digital TV" channels.

Do I really need to hook the fucker up, and take photo screen shots of
the set-up menus? Sheesh.

Such tuners do exist, and have for years, and will for years to come.

It is just that, at least as far as I know, no such tuners eligible for
the digital TV voucher are on shelves yet. I could be wrong on that.

 

[Gary Tait]

Gary Tait wrote:
(snip)



I remember in the days of UHF getting more popular, and using
a converter box that output on VHF channel 3. This can be done
with the appropriate mixer and LO, without converting the input
to baseband. It does involve tuned circuits, but does not
convert all (70) UHF channels to be tuned on a (12 channel)
VHF tuner.

Yes, that is a traditional cable convertor, which wen out of fasion when
integrated descramblers came to be. Even the tuner block in an analog
TV is a sort of frequency convertor, it just converts the incoming RF
channel to an IF, which the circuits detect the video baseband from.

I believe there were/are block converters from cable channels
to UHF which do convert all at once. Most cable boxes don't
do that, though. Early (analog) ones did the down conversion
similar to the UHF conversion described above. Most now likely
go to baseband and then remodulate for those without video inputs.

Yes, there were block upconvertors, which converted a block of cable
channels to UHF frequencies.

Modern digital cable and satellite boxes (and late era analog cable
tuner/descramblers) output composite video/stereo audio, not baseband.
Baseband is the signal that comes off the primary video detector, and
has the audio subcarrier (s) on it still. Those signals are filtered off
and the signal clamped to make composite video. Digital boxes just make
the composite or other video signal from the data from the video
deocder.

The distinction between 'tuner' and 'converter' is fuzzy.

A convertor just converts frequency. A tuner tunes a signal an outputs
"line" video and audio.

I don't believe that there is a convenient way to block convert
the ATSC input to NTSC output. One could build a box with
multiple tuners, decoders, and modulators but I doubt that
would be for the consumer market.

Not for a $40 box.

The box needs an ATSC tuner, and the logic to convert the
result to an NTSC analog signal. Most likely with both video
and RF outputs. The output of the ATSC tuner is the digital
signal, not suitable for an analog TV, so the box needs
both a tuner and converter.

All ATSC tuner boxes do that now. They tune the ATSC hannel and output a
viewiable SD or HD video signal. Some may lack RF out though.

It is just they have to make one that meets the requirements for a
voucher (which has certain stipulations an STB manufacturer could easily
meet, and some boxes could likely technically meet today).

 

[John Monro]

I would have thought that the primary reason for using a white
background was to avoid introducing extra noise from the photographic
processing required to convert back to a positive image.

Astronomical plates were normally developed as negatives because
negative processing is simpler and quicker than positive 'reversal'
processing. Ease of finding unexpected objects would not be a
consideration because the plates are rarely used for this purpose, most
imaging being done on known objects.

Also, I think the alternative 'reversal' developing process would
introduce additional error into the relationship between the
astronomical object's actual brightness and the optical density of its
image as recorded on the plate. Precise recording of object brightness
is a common requirement of astronomers.


Regards,
John

 

[IAmTheSlime]

You have a prick, therefore you are a prick? Maybe so.

Jerry

Trust me, dumbshit... everyone in this thread is a prick.

 

[ChairmanOfTheBored]

Astronomical plates were normally developed as negatives because
negative processing is simpler and quicker than positive 'reversal'
processing. Ease of finding unexpected objects would not be a
consideration because the plates are rarely used for this purpose, most
imaging being done on known objects.

Bullshit. That is EXACTLY what the plates the man who discovered Pluto
was using them for! He was hired because no scientist wanted to perform
the painstaking and laborious task required.

Also, I think the alternative 'reversal' developing process would
introduce additional error into the relationship between the
astronomical object's actual brightness and the optical density of its
image as recorded on the plate.

Brightness wasn't a factor. The plates were specifically used to FIND
previously undiscovered moving objects. Contrast ratio was the important
factor.

Precise recording of object brightness
is a common requirement of astronomers.

The task being done being the dependent factor. In THIS case,
brightness was NOT a factor, was NOT gauged, and the ONLY thing being
sought was a new, previously undiscovered object.

The discovery of Pluto was the result:

http://en.wikipedia.org/wiki/Pluto#Discovery

The plates shown were reversed in the wiki page, but Mr. Tombaugh
examined negatives.

 

[John Monro]

Bullshit. That is EXACTLY what the plates the man who discovered Pluto
was using them for! He was hired because no scientist wanted to perform
the painstaking and laborious task required.

Think carefully about this and you may learn something.
Did the person undertake a new photographic survey or did he use the
ovservatory's archival material?
If the latter, do you think the original purpose of the astronomers who
produced the material may have had something to do with whether
negatives or positives were produced?

 

[ChairmanOfTheBored]

Think carefully about this and you may learn something.
Did the person undertake a new photographic survey or did he use the
ovservatory's archival material?

IDIOT! HE created the plates on a daily basis! HE made the entire
archive!

WAKE UP!

If the latter, do you think the original purpose of the astronomers who
produced the material may have had something to do with whether
negatives or positives were produced?

HE WAS THE ONLY RESEARCHER, you dingledorf!