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Video Confusion Abounds
by Paul McGoldrick
I have received a number of e-mails this week from readers who think I have been unfair in my criticisms of TI’s TVP515* digital video encoders (the company calls them decoders, and perhaps we’d all be better off just to call them composite video ADCs). Most of the messages missed the central issue that I was raising: there was no direct criticism of the parts themselves. Indeed the data sheet leaves too many questions over performance than it answers. How do you measure the quoted differential gain and differential phase in digital component signals, for example?
What I was criticizing was the misuse of video terminology. Not understanding the difference between baseband composite video standards and transmission standards is, to me, totally unforgivable for an engineer working in the video space. I have a little more forgiveness for the inability to recognize the difference between dc restoration and a clamp – but only with some difficulty…
Having been involved with video since, basically, the very idea of PAL was being raised among my peers in the BBC (I had a boss whose career track ended when he supported NTSC for the UK – which we were working with in Studio H in Lime Grove); having taught video technology for some years at college level; having been involved with the very definition of video standards, the testing of them, and the award of Emmys for them – I feel I have more than the simple right of spouting off about the misuse of terminology.
As a little test of where the industry is in its understanding these days I looked through just the first page of results from a Google search for “analog video.”
The first to come up was from Wikipedia, that totally reliable source of information (major sarcasm, this is the first time EN-Genius has allowed links to Wikipedia). You have to look at the page yourself: even Wikipedia thinks it is a joke!
Next up was an application note from Maxim Integrated Products, where there are a number of gems but the section I like most (least?) is: “The third and fourth lines (of a Table) are the two forms of component video, color difference (Y'PbPr/Y'UV/Y'IQ) and Luma-Chroma (Y'-C). Sometimes, there is confusion about the terms used. Some texts use the terms Luminance and Chrominance, which are from Color Science. Here we'll use Luma and Chroma where the Luma term is written with a prime (Y') to indicate the non-linear video form.”
Where to start? Gamma predistortion (“the non-linear video form”) usually referred to, incorrectly, as gamma correction, is a predistortion to correct for the non-linearity of the display system which for many years was, of course, the cathode-ray tube. Broadcasters wanted to relieve the receiver manufacturers of as many build costs as possible. Predistorting the video with the CRT offering the opposite characteristic was one way to help. This has become a really complicated topic in these days of newer display technologies.
YUV: U and V are the weighted color-difference signals for PAL encoding. I have no idea what YIQ is supposed to be.
“There are two forms of component video.” Yes, there are, and Y-C is not one of them. Apart from luminance and color-difference signals, the other component video standard is RGB. (Which we spent a long time in the video T&M industry trying to change to GBR because the same connector is used for G as for Y, but the computer industry won out.)
Luma is modulated luminance. Chroma is modulated chrominance. The terms are not interchangeable. And why all the capital letters?
More? On to a NASA paper by John McGowan now apparently only available by citation, and here we go, again, that dreaded misuse of UV.
And how about this interesting encyclopedia entry further down the Google results page, as a definition of analog video: “The original video recording method that stores continuous waves of red, green and blue intensities. In analog video, the number of rows is fixed. There are no real columns, and the maximum detail is determined by the frequency response of the analog system.”
If anybody works out what those three sentences are saying perhaps you’ll let me know. Meanwhile I will continue to defend the terminology of video systems.
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