Digital video signal

The digitization of the video signal developed in the 1970s, culminating in the publication in 1982 of Opinion 601 of the International Radio Consultative Committee (CCIR), which in 1995 became Recommendation ITU-R BT.601-5 on the 4:2:2 format. It's legitimate to ask why the video signal was digitized, given the remarkable quality of the analog signal.

Digital technology offers a number of advantages. For recording, for example, noise increases with each generation in analog, which is not the case in digital. Transmission quality remains constant. In both cases, there is no degradation. What's more, digital technology enables numerous special effects and color corrections, which previously had to be made at source. For example, since inlays have gone digital, the "blue" has disappeared from the weatherman's hair.

Digital technology was first used in isolated applications, such as the TBC (Time Base Corrector) for VTRs, and was then introduced into analog chains (e.g. Thomson's digital truqueur mixer, marketed with analog inputs and outputs), making it possible to benefit from some of the advantages of digital technology without having to change equipment entirely. The principle of digital islands in an analog chain involved a large number of analog-to-digital and digital-to-analog conversions, with the attendant problems. To overcome this, the processing chain had to be entirely digital, which was achieved by the 4:2:2 format for production, while broadcasting was still predominantly analog with NTSC, PAL and SECAM systems.

With three analog coding systems for broadcasting, identified by two different standards (625 lines at 25 frames per second with the PAL and SECAM systems, and 525 lines at 30 frames per second with the NTSC system - 29.97 to be exact), there was a great risk of finding ourselves in the same situation for digital. We'll see how this was avoided.

The first step in this standardization process was to define the nature of the signal to be digitized. Color video signals can be in "composite" form, such as NTSC, PAL or SECAM, or in "component" form, i.e. one luminance signal and two color difference signals. The latter, much more flexible solution was chosen. From then on, it was necessary to define the quality ratio to be established between luminance and color component information.

This article first describes the characteristics of video signal digitization, followed by the specific features of the recommendations.