How Analogue Colour Televisions Works: the Coding and Decoding Process
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- čas přidán 30. 11. 2022
- Pye History Trust presents the third part in its video series: “How Analogue Television Works”.
Part 1 ( • How monochrome analogu... ) explained monochrome (black & white) television broadcast and receiver technology.
Part 2 ( • How analogue colour te... ) explained how the eye perceives colour; how colour cameras work and how the television receiver displays the original picture.
In this Part 3: a simplified explanation of how the colour signal is encoded onto the video signal by the camera and how the television receiver decodes this signal.
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Audience
- Students: physics, optics
- Electronics engineers
- Broadcast enthusiasts
Contents:
0 -1 min: Introduction to the Pye Story at Cambridge Museum of Technology
1 - 2 mins: introduction to colour television
2 - 3 mins Luminance
3 - 12 mins: National Television System Committee (NTSC) system, developed in the USA
13 -20 mins Phase Alternating Line (PAL) system, developed in Europe
Published under Creative Commons Licence:
Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)
www.creativecommons.org/licenses/by-nc-nd/4.0
The Pye Story (www.pye-story.org) permanent exhibition at Cambridge Museum of Technology (www.museumoftechnology.com) displays examples of analogue television broadcast and receiver equipment produced by Pye during the 20th century.
Video produced by R N Bates and R Ellis for The Pye History Trust, 2022.
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#electronics
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#television - Věda a technologie
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Really excellent - thank you for the clarity and simplicity of the explanation.
Thank you for a fine explanation of this complex process. Well done!
NTSC originally proposed the phase alternating process. RCA called it PLA. This was dropped due to the already massive cost of a color TV receiver in 1953 dollars. By eliminating the additional delay line, a very expensive component back then, they cut some of the final cost from the product.
I observed a PAL receiver at the NAB convention, in the early 1990s, that was suffering severe phase distortion and looking closely at the screen I observed that it suffered the same color shift issues as NTSC. However, with the phase alternation, one line was green, the next purple. Stepping back a short distance, they eye blended the two lines and the picture was acceptable.
At one of my jobs, throughout my long career in video product engineering, I had the opportunity to connect a three tube camera to a video monitor with direct RGB (525/30i). The result was extremely astounding. This was how I learned about how "bad" chroma encoding schemes could be.
A further detail of the NTSC I&Q signals is they were not equal bandwidth. The I signal, which was reds, yellow and orange image areas, was 1.5mhz. The Q signal which was the blues and violets, was 0.6mhz. This was done to give more detail to the brighter nature and flesh tone colors. The problem with this was a more costly receiver decoder design. Two different bandwidth filters were needed and because they were different frequency passbands, they had different delay. So yet another delay line was required in the I signal to keep it registered with the Q signal. Only a few early color receivers, like the RCA CT100 used the true I&Q system. This is why the color reproduction on a restored RCA CT100 is so good. But all USA TV manufactures including RCA as well as the Japanese soon went with the equal bandwidth R-Y, B-Y system to reduce consumer receiver costs. Most broadcast NTSC encoders continued to use the I&Q system but the advantage was lost in the later receivers that used equal bandwidth, about 500khz, R-Y, B-Y.
I've actually implemented PAL encoding and decoding in a shader for Media Player Classic back when I was on Windows (the program is Windows-only, despite being open-source, unfortunately, and can't be ported to anything else as it's designed entirely around Windows' APIs).
The simple decoder should produce alternating light and dark versions of a colour, not alternating hues. There's a subtraction step that will produce alternating hues if left out. Implementing something equivalent to the delay-line decoder via a second shader (the framebuffer inbetween shader passes giving storage equivalent to the delay-line) required leaving out that step, but the averaging in the second shader removed the alternating hues.
Excellent video! I'm surprised about how similar PAL and NTSC were. I was stationed in the US Army and was dismayed by how much better their PAL broadcasts were than NTSC (until I learned how recently they got Color TV compared to the US). Specifically, blacks were blacker. I think reds were a deeper red. And PAL seemed to lack a lot of artifacting NTSC had, like if there's white text on a black background, the white text creates a band, or ghost image, to the right where it should be black. Colors didn't bleed as much, but this could be due to greater bandwidth. The 50hz flicker was a problem for me, though.
It's a problem even if you grow up with it. Having a 50Hz TV in your peripheral vision is incredibly distracting. The 60Hz default for 640*480 and above on computers isn't much better, so I am really glad CRTs are dead. I was constantly going into the settings in Windows on other people's computers and putting up the refresh rate (usually to 85Hz) because I found the thing unusable at 60Hz.
Simple PAL was not the first version of PAL but a way for cheap manufacturers to avoid paying royalties for the PAL patents to Tekefunken. Most noticable being the first line of Trinitrons from Sony. The simple PAL receiver has a hue button like in NTSC. But no other set or brand made simple PAL receivers ever again. And PAL “D” does not exist, it is called by the CCIR notation for B/G, I, D/K, M or N which denotes line count/frame rate, channel bandwidth, sound offset, and channel spacing.
at 14:28, the PAL subcarrier should be specified as MHz, instead of kHz, of course.
Apologies. Looks like our proof reading failed.
We used to refer to NTSC as 'Never Twice the Same Color' :)
If you spelled it "analogue", you probably had PAL. If you spelled it "analog", probably NTSC. If you only answered back, if someone spoke to you in proper French, with no accent, then definitely SECAM !
I thought PAL was 'Prey and Learn' not 'Peace at Last'.
There were more interpretations:
Perfection at last
Pay another licence (because PAL wasnt free, it was developed and licenced by Telefunken)
Pay and look
I and Q are _not_ the same as R-Y and B-Y. They are 33° rotations from R-Y and B-Y.
People Always Last.... given BBC colour was 20 years late.
Totally, though it was mot their fault. The BBC and ITV wanted to implement NTSC-A in the late 50’s but were blocked by the UK government who set up the Pilkington Committee who were clearly told to say wait for the German system to please the Common Market. There was a lot of dissent in the UK industry over this as NTSC-A would have allowed a renaissance in artistic efforts, boost the UK industry, allow war impoverished Brits to continue to use their existing tv’s and not require a new tv for BBC2, then colour, then any tv. It would also have saved millions in saved transmitter replacements and additions. Sad sad story.
@@richjames2540Not just BBC. Philips experimented with NTSC on 625 lines in 1964 on their private transmitter as they preferred a fast to market and NTSC was ready and as far as I know royalty free by then. But for some reason in politics it meant to wait for PAL. In the flat Netherlands NTSC phase problems were minimal, but Germany had mountains so was more problematic. Philips did not like it since it meant a delay and also additional licensing fees to Telefunken for the PAL patent. In the end it was a wise choice to have gone for PAL anyway.
Yes, it was Politics and the Common Market that held up UK Colour TV. I am not sure it was worth the wait in the UK. It was very soon after the start of PAL in Europe that Solid State replaced valves and the phase error problem in NTSC went away. The big loss in the UK was having a non compatible system meaning, new TV's in the home and new Transmitters with many more relay stations and poorer coverage in rural areas which took about 30 years to be resolved. The majority of this coming from the abandonment of VHF rather than adopting NTSC-A and then adding an NTSC-I later. Philips would have benefitted enormously if the NTSC system had been adopted in late 50's. Still all history now. PAL, PAL Plus, D-MAC all gone as is NTSC and SECAM. We could have had a world standard for HD TV but as you know whilst 1080i and 720p were adopted as formats the three major HD territorial transmission standards are incompatible and UK and Europe are still stuck with the lower frame rate, though Flat panels running at 50p or 25i do not present as many issues as CRT sets did with migraine inducing flicker. Now all computers seem to use 60p the world could move from 29.997 and 25 frames per second to 30 or 60 without any real issues, I suspect politicians got in the way of that too,
NTSC has been jokingly --- and deservingly --- called "Never Twice the Same Colour"
because the chroma phase tends to vary and the user adjustment provides no absolute reference.
The Magnavox console i grew up with had both colour and tint buttons on the remote
and the knobs on the set were motorised.
Later NTSC sets had automatic tint and color control,which resulted in less of an issue.
@@900Yugo But still inaccurate hues unless proper adjustments were made
by someone who knew how to do them right.
I have had a few TVs in which the "automatic" functions
were so screwy I couldn't stand to use them.
@@spacemissingMany of the cheaper TV sets were not so good at color reproduction,I am from Taiwan and we used NTSC for color encoding. PAL does have a better contrast image. SECAM was worse than PAL or NTSC though.
The problem was mostly caused by differential phase and gain errors in the high power transmitters and to some extent. tube based receiver circuits. This problem was largely fixed by the mid 1970s with improved electronics.
@@900Yugo Indeed, a signal was inserted into the vertical interval to help with automatic color and hue/tint (NTSC land) for TV's enabled with this feature. VIRS, or Vertical Interval Reference Signal, for Chroma reference, it contained many other test signals to check for signal quality back at the studio. RCA first implemented it with their TV's as they had come up with a method. I remember our Toshiba 19" TV had an "Auto color" button on the front panel, probably using this helper signal.
Later on they modified the helper signals to include a GCR, or Ghost Canceller Reference. This was added by Philips, and I believe they had a chip manufactures could add to their TV's for this feature that did everything, auto color, ghost cancelation, probably even included closed captioning when that became a requirement for all TVs. Cable companies could use a rack mounted piece of gear that would lock into this VIRS/GCR reference and clean up any off air signal before they passed it onto subscribers. It worked surprisingly well, I tested a unit once with this feature.
Like others said, by the mid 70's TV's had become solid state and were pretty stable (versus the tube era) and with the reference signals implemented in the 80's along with integrated circuit chips now common, NTSC had much more stable color reproduction. Plus if you were on cable, there was virtually no phase errors, so long as the cable company had good amplifiers. I don't remember anyone messing about with the hue/tint adjustment by the mid 80s to early 90's much anymore. I do remember if you had over the air reception, if an airplane flew right overhead you would get multipath ghosting and the color would shift around. I guess if you were someone who lived in the flight path of a major airport that would be a pain..