The concept of interlaced video is closely related to CRT displays, which is why it is a subject that does not appear when talking about contemporary equipment, because although the first LCD displays supported resolutions in interlaced mode. , they didn’t take a long time to throw them away as there was enough bandwidth in the digital video signals to avoid having to use this video mode.
What is interlaced video?
For a long time the video standards par excellence were NTSC and PAL, but the latter will be ignored because the advances which have enabled home computing, although it hurts us, did not occur on the European continent. Thus, in order to take advantage of the ability of televisions to create a real-time information terminal, the first mini-computer terminals of the time began to be created, consisting of a modified television set without a radio frequency receiver.
So, at the end of the 60s and the beginning of the 70s, the first screens for PC started to emerge. However, it was quickly realized that there was a bottleneck and that the video signal that could be sent was limited in order to have access to high refresh rates. The answer to this was interlaced video, which involves having the video system draw the odd and even lines on separate frames. This way, although the vertical resolution is reduced with each frame, the number of frames per second is doubled and all without increasing the bandwidth of the video signal.
How a CRT screen works
Think of a CRT monitor as a kind of printer, when a sheet is printed, we can see how the head moves from left to right to print a line, when it ends, the paper goes up and the head goes back to the top. left. In a CRT screen, it’s similar, the electron beam travels through the screen from left to right and top to bottom. When the frame has finished drawing, it returns to the initial position to generate the next one.
Esto lo hace tan deprisa que nuestra vista percibe todas esas líneas en pantalla como una sola, pero si pudiéramos observar nuestro entorno a una alta frecuencia no veríamos una imagen, sino solamente la linea de escaneo por la que acaba de pasar el haz de each moment. It therefore takes advantage of a limitation of our visual system and our brain to create the illusion of generating images very quickly.
NTSC and PAL standards
What defines a standard is a series of parameters and specifications that all manufacturers who adhere to it must adhere to and in the case of NTSC and PAL, manufacturers have had to adapt their screens to a series of times that were The following :
- The refresh rate, 60 Hz for NTSC in interlaced mode and 30 Hz in progressive mode. In PAL 50 and 25 Hz respectively.
- The number of scan lines, 525 in progressive mode and 262.5 in interlaced mode for NTSC. In PAL, they were 625 and 312.5. In interlaced mode, the last half line was not displayed.
- The time it took for the electron beam to cross a line was the same in both modes, but the times are different for each of the two standards. In the NTSC standard, a total row is 63.3 microseconds, but the first 10.9 microseconds are for repositioning. In the case of the European standard, the times are respectively 64 microseconds and 12 microseconds.
Graphics hardware that has to handle CRTs must take these times into account and usually has a series of built-in binary counters responsible for counting the times to know when to send and when not to send the video signal. This is a completely different operation from LCD screens.
Interlaced video on LCD and OLED screens
It is necessary to start from an important point, in an LCD screen there is no electron beam that generates the image, but it is created by illuminating the hundreds of thousands and currently the tens of millions of points that make up the screen. Thus, the periods during which the electron beam is repositioned to draw the next scan line or for the next frame do not seem to exist.
However, due to the way the television signal was transmitted, and since there had to be a period of transition from one type of television to another, LCD controllers were designed to display the lines of scanning one by one as it happened in the CRTs. . For this, the delay time corresponding to the repositioning intervals of the electron beam is added.
On the contrary, today’s video signals are fast enough that you don’t have to rely on tricks like video interlacing. Therefore, they allow high frequency images to be displayed without sacrificing horizontal resolution.
The problem of deinterlacing
If you connect an old console or VHS or DVD player with analog video signal outputting in interlaced mode to an LCD or OLED display, you may see a huge drop in picture quality from what it was seen in. a cathode ray tube. And no, we’re just talking about color representation and contrast level, but a problem called deinterlacing.
This is not a standard process, but a series of algorithms based on reconstruction of the interlaced to progressive image, which often end up generating video signals with enormous jitter, so as not to say that with a much lower image quality, as well as a series of artifacts that obscure the overall experience.
Today there are intermediate devices designed to solve this problem, as well as the creation of algorithms through deep learning that allow us to correctly reconstruct the old interlaced video signal on an LCD screen. On the PC side, the interlaced mode has disappeared with the removal of VGA and DVI in graphics cards, which is normal since today CRT monitors are no longer marketed and we do not have video signals limited by the bandwidth.
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