When we talk about Ultra High Definition or UHD, for short, we only tend to think about the increase in the definition that this implies. UHD is not just more pixels; instead, it is better pixels. In this series of articles, TM Broadcast will delve into what UHD really means and the most important aspects of this new technology.
Let’s start with the obvious: resolution and transport of the baseband signal.
When digital TV began to take root and SDI (Serial Digital Interface) was born, the advance was clear. Analogue video was left behind, either due to components, with the high complexity of carrying out any production due to it having to transport each component of colour separately, or for composite video, with the limitations of quality and its low reliability as it was very susceptible to interference. Two standards emerged at the time:
- 94, retro-compatible with NTSC standard.
- 579i50, retro-compatible with PAL standard.
At the time, retro-compatibility was necessary when setting up a new standard. The definition was the same, and the type of scanning remained in the interlacing due to the technical limitations of the monitors and cameras. Its bandwidth of only 177 Mbps was sufficient, however, to carry both video and audio. Another critical aspect of the rapid deployment of SDI was embedded audio, with much less interference and higher reliability than analogue signals.
Only a 4:4 aspect ratio was available initially, but 16:9 anamorphic was immediately implanted. With identical definition but a rectangular pixel instead of square, it allowed transporting panoramic images.
When all this was in place, and everyone understood each other to perfection came a leap in definition -the so-called HD- and with it a little chaos that persists even today. Just as UHD is not just a leap in definition, HD was not so either.
It went from 177 Mbps to 1.5 Gbps. It almost multiplied the bandwidth by ten and, instead of only two signal transport standards, the progressive appeared, allowing the next variety of signals:
So, we can at least forget about mixing aspect ratios, because there is only 16:9. However, the introduction of progressive scan mode, more “cinematic” and better for sport and high-speed scenes, brought sufficient complexity into the equation.
A bandwidth of 1.5 Gbps, on the other hand, did not allow transporting all the desired definition in progressive mode, because the amount of information per frame is twice as much as in interlacing. This led to limiting the definition to 720 pixels and creating a war in the consumer market between HD Ready, 720 and Full HD, 1080. We still find professional colleagues making this distinction, which was just a ploy to sell more home TVs.
Another option was to limit the number of frames per second to transmit, which led to two new signals in all this swarm of signals:
We had to double the bandwidth yet again, up to 3 Gbps, to make it possible to transport the desired Full HD definition, 1920×1080 pixels, with a progressive scan without losing frames per second, but reaching the desired ones as well:
These are signals that, in reality, hardly anyone uses. On the one hand, it requires a 3G video baseband production environment, which is not widespread. The file in question also takes up twice as much storage, and the bandwidth for transmitting it is also doubled. In economic terms, this is a cost overrun that the end customer does not value as a leap in quality between progressive signals or 720 and progressive Full HD. Producers and engineers like this very much, but no financier is willing to pay for it.
An SDI-HD transport system called Dual Link was timidly created, which allowed, using two 1.5 G signals, transporting a complete 3G signal combining both. But there is a problem. If 3G is not used that much, this other means of transport is virtually anecdotal. Our older readers will recall the older technology in which more than one cable was required to carry the signal. There is something else you will remember even more, but let us keep the intrigue for the younger ones.
This brings us, in short, to the following amalgamation of standards:
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