Autor: Redaktion

Video Signals Overview

The RGB signal offers full colour resolution with a ratio of the sampling frequencies of 4: 4: 4.

RGB (4: 4: 4) and YCrCb (4: 2: 2)

The ratio 4: 4: 4 describes the relationship of the sampling frequencies used in each of the three colour signals Red, Green and Blue. The 4: 4: 4 sampling is used mainly for Mastering (high-quality) and production (eg camera, film scanning, colour correction).

In contrast codec colour signals like YCrCb are usually sampled in 4: 2: 2. In this case, the ratio describes the relationship of the sampling frequencies of the luminance signal Y and the chrominance signals CR and CB. In the case of 4: 2: 2 sampling, the chrominance signals (only every second pixel is sampled) have only half the colour resolution (bandwidth) of the luminance signal.

This roughly corresponds to the behavior and interpretation of colour signals by the human eye, in which the brightness signals are about twice that of colour signals. The 4: 2: 2 signal is used in television production over the 4: 4: 4 signal, as it requires less bandwidth (data transfer rate), and is therefore more economic for broadcasting.

More subsampling structures

If additional subsampling ( for example, 4: 2: 0 or 3: 1: 1) is applied, then the required bandwidth (data transfer rate) is further reduced. However, multiple processing steps lead to both a reduction in colour resolution and /or luminance, and a potential quality reduction.

Sampling structure 4: 2: 0

Here the chrominance signals CR and CB are each calculated from two adjacent rows. This calculation is performed on each second pixel. Coding MPEG2 with this sampling structure no longer corresponds to the technical delivery requirements for HD. Many HDSLR and HDSLM cameras work with codecs that only scan in 4: 2: 0. Pay attention to this, because - despite the often excellent sensors of this class of camera - almost all broadcasters in Europe will reject 4:2:0 deliveries, or only play them out in very exceptional circumstances.

Sampling structure 3: 1: 1

The sampling structure 3: 1: 1 was used primarily in Sony HDCAM in the late 1990s. This sampling structure, as well as the special form of 3: 1.5: 1.5 (DVCProHD), may not be accepted for programme delivery to broadcasters in Europe.

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