On Sat, 14 May 2022 at 15:48:39, Roderick Stewart
<rjfs@escapetime.myzen.co.uk> wrote (my responses usually FOLLOW):
[]

In the decoder, RGB signals are derived by matrixing the luminance
signal with the colour difference signals, which have also been
derived from the gamma corrected RGB signals in the encoder. The
"un-gamma" process is done to individual colours by the CRT itself.

[]
I. e. the phosphor.

Do modern LC displays, and other technologies - Plasma? OLED? - have a
similar gamma curve to CRT phosphors? If not, presumably modern sets
have an un-phosphor-gamma followed by gamma correction for whatever they
_are_ using (assuming it's needed). Do you think we'll ever see a
different (or no) curve start to used, with archive material doomed to
be viewed slightly wrongly, apart from by true enthusiasts? Or has this
already happened? (I imagine it could be quite subtle so not thought
worth bothering with by many.)
--
J. P. Gilliver. UMRA: 1960/<1985 MB++G()AL-IS-Ch++(p)Ar@T+H+Sh0!:`)DNAf

Thay have a saying for it: /Geiz ist geil/, which roughly translates as, "It's sexy to be stingly". - Joe Fattorini, RT insert 2016/9/10-16

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"J. P. Gilliver (John)" <G6JPG@255soft.uk> wrote in message news:R$cU4NThT8fiFwr0@a.a...

On Sat, 14 May 2022 at 15:48:39, Roderick Stewart <rjfs@escapetime.myzen.co.uk> wrote (my responses usually FOLLOW):
[]

In the decoder, RGB signals are derived by matrixing the luminance
signal with the colour difference signals, which have also been
derived from the gamma corrected RGB signals in the encoder. The
"un-gamma" process is done to individual colours by the CRT itself.

[]
I. e. the phosphor.

Do modern LC displays, and other technologies - Plasma? OLED? - have a similar gamma curve to CRT phosphors? If not, presumably modern sets have
an un-phosphor-gamma followed by gamma correction for whatever they _are_ using (assuming it's needed). Do you think we'll ever see a different (or
no) curve start to used, with archive material doomed to be viewed
slightly wrongly, apart from by true enthusiasts? Or has this already happened? (I imagine it could be quite subtle so not thought worth
bothering with by many.)

I imagine that gamma is "hardcoded" into the design of any camera, signal processing, broadcasting, reception, display chain so that it cannot be
changed without rendering all current and past TVs "obsolete" in the sense
that images without gamma (or with a very different value) would look
"wrong".

I believe UK and US use different values of gamma which is why some US made-on-video TV programmes look vaguely wrong if they are shown on UK TV.
It's either gamma or black level/pedestal which is different. I noticed it before I first read about it, and thought the pictures looked a bit like the "colour plates" in books from the 1930s/40s/50s - low contrast, excessive saturation, too much shadow detail and too little highlight detail. It seems
to have been an analogue-only thing.


What was the purpose of gamma? Was it to make the signal more immune to
noise (a bit like LPs apply HF emphasis on the record and the record player
has inverse de-emphasis)? Or was it because CRTs had an inbuilt gamma
because of the phosphor and transfer characteristics of a CRT, and it was easier/cheaper to correct for this at the broadcast end rather than
requiring the correction circuitry in every TV?

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On Sat, 14 May 2022 18:12:22 +0100, NY wrote:

was it because CRTs had an inbuilt gamma because of the phosphor and
transfer characteristics of a CRT, and it was easier/cheaper to correct
for this at the broadcast end rather than requiring the correction
circuitry in every TV?

Yes! :)

--
TOJ.

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On Sat, 14 May 2022 18:12:22 +0100, NY <me@privacy.invalid> wrote:

What was the purpose of gamma? Was it to make the signal more immune to
noise (a bit like LPs apply HF emphasis on the record and the record player has inverse de-emphasis)? Or was it because CRTs had an inbuilt gamma
because of the phosphor and transfer characteristics of a CRT, and it was easier/cheaper to correct for this at the broadcast end rather than
requiring the correction circuitry in every TV?

The latter.

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"Paul Ratcliffe" <abuse@orac12.clara34.co56.uk78> wrote in message news:slrnt80143.2jg.abuse@news.pr.network...

On Sat, 14 May 2022 18:12:22 +0100, NY <me@privacy.invalid> wrote:

What was the purpose of gamma? Was it to make the signal more immune to
noise (a bit like LPs apply HF emphasis on the record and the record
player
has inverse de-emphasis)? Or was it because CRTs had an inbuilt gamma
because of the phosphor and transfer characteristics of a CRT, and it was
easier/cheaper to correct for this at the broadcast end rather than
requiring the correction circuitry in every TV?

The latter.

So if (in an alternative universe!) TV had been developed when LCD screens
were the normal display technology, TV would have been linear rather than parabolic output = input ^ gamma and life would have been a great deal
simpler ;-) Assuming that LCD screens have linear light output for linear input pixel value and/or voltage.

As a matter of interest, is digital TV linear (ie do pixels with values 10,
50 and 100 have brightnesses in the ratio 1:5:10) and is the
gamma-correction for the SCART-output of a set-top-box done locally, or is gamma at the prescribed value still applied between the camera and the
encoded pixel values in the video stream? Is what JP Gilliver said true -
that an LCD TV applies inverse gamma conversion, followed by any non-linear parameter that might apply to an LCD screen?

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On Sat, 14 May 2022 21:29:02 +0100, "NY" <me@privacy.invalid> wrote:

As a matter of interest, is digital TV linear (ie do pixels with values 10, >50 and 100 have brightnesses in the ratio 1:5:10) and is the
gamma-correction for the SCART-output of a set-top-box done locally, or is >gamma at the prescribed value still applied between the camera and the >encoded pixel values in the video stream? Is what JP Gilliver said true - >that an LCD TV applies inverse gamma conversion, followed by any non-linear >parameter that might apply to an LCD screen?

When I got my first digital stills camera (a Nikon Coolpix) way back
in year 2000, I wondered about this too, and as I was working with
broadcast television cameras at the time, I was able to photograph the
various test charts we used for those, and measure the numerical
outputs using the histogram in graphics software on the computer.

We had the BBC No 57 greyscale chart, and various other charts that
included greyscales. I found the greyscale in the Macbeth chart to
give particularly clear results.

I also had a laptop with a video capture card that enabled me to save
still images of the same charts as JPG files like the ones from the
Nikon camera, and make the same measurements.

Since then, I've done the same kinds of tests with various other
digital stills cameras. And the answer... is that digital stills
cameras and broadcast television cameras generate signals with exactly
the same gamma characteristics as far as I could measure them, so they
must all have the same kind of gamma correction circuitry, or its
digital equivalent. It's only what you would expect if you think about
it, because we can view images originating from both television and
stills cameras on the same screens, side by side if you like, and they
all look similar. Broadcast cameras usually have a means of bypassing
or switching off the gamma correction circuitry (sometimes used as
part of lineup) and the effect makes most of the image, notably face
tones, grotesquely dark and oversaturated, and everything looking very contrasty, so it would be immediately obvious if this were missing.

Also, electronic images of all sorts that we used to view on CRTs can
now be viewed on modern flat screen displays without adjusting them,
so it seems that gamma correction is one thing that has been
maintained as a universally recognised standard.

Rod.

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