As mentioned in section “The Path to the Video”, the Contribution links is all the processing undergone by the video signal from its capture at the source until it is sent to the broadcasting head-end, where it is broadcasted to the final user.
Therefore, the aim is to allow several Encoding-Decoding processes (Multi-Generation) with the least loss of quality possible.
The two types of deterioration suffered by the signal during the whole Multi-Generation process are:
1.- Loss of quality due to the encoding process itself, which will essentially depend on the bandwidth available, as well as on the features of the transport network.
2.- Additional loss in the processing of the signal, for example as a result of Chrominance conversion.
o The video capture is done in 4:4:4 (RGB) format.
o The format used in Broadcast production environments is 4:2:2 (the horizontal resolution of the colour information is reduced by half against the luminance). This is the most common format within the Broadcast world.
o The format used in the final broadcasting to the user is always 4:2:0 (the colour information is reduced by half again, in this case its vertical resolution).
The contribution links are usually made in 4:2:2 format, which allows for maintaining the picture quality in the different Multi-Generation steps.
Since they are the last encoding process, distribution links are made in 4:2:0 format, which allows for the required bandwidth reduction for encoding, taking advantage of the fact that the human eye is less sensitive to chrominance than it is to luminance.
Sapec’s solutions for these Contribution links are based on the maintenance of quality at very high levels (depending on the bandwidth required for the broadcasting), so that the signal arrives as similar to the original as possible to the last point in the chain. This way, although in this last process the signal is very compressed for its broadcasting through reduced bandwidth channels, the final quality is much higher than when the signal gradually deteriorates along the different steps in the chain.
The table on the right shows an example of a Contribution link with THREE Encoding-Decoding processes (Multi-Generation).
The chart shows the quality levels (measured in PSNR) for different encoding Bit Rates for each of the 3 steps, comparing the use of H.264 encoders with professional profiles for contribution, known as Hi 4:2:2 and with High Profile distribution profiles in 4:2:0. Sapec offers Encoding and Decoding devices with 4:2:2 profiles both for MPEG-2 and H.264 encoding.