Signetics 2650 & 2636 programming/Analogue TV
These consoles were manufactured in the days of analogue TV, when the picture was drawn on a cathode-ray tube by an electron beam scanning back and forth across the screen. The programmer doesn't need to know all the details of how these TVs worked, but should be familiar with the basic principle of operation, some of the terminology, and the speed at which the beam crosses the screen. The purpose of this section is to provide an overview and suggest some further reading.
Raster scan
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The electron beam draws one horizontal scan line at a time, starting at the top left of the screen. By the time it reaches the bottom right of the screen a whole picture has been displayed. A long-persistence phosphor coating on the screen retains the image long enough that the human eye does not perceive any flicker.
While the beam is moving left to right the screen is being lit up with the three primary colours, red, green and blue. At the end of each scanline it is moved back, right to left. This movement is faster than in the other direction, and no colours are displayed; this is referred to variously as the horizontal retrace, horizontal flyback or horizontal blanking. In a similar manner, at the end of each frame the position of the beam has to be moved back to the top left of the screen without displaying any colours; this is referred to variously as the vertical retrace, vertical flyback, vertical reset or vertical blanking. The PVI documentation refers to it simply as VRST
PAL and SECAM
[edit | edit source]Colour television was broadcast in three different standards in various parts of the world, NTSC, PAL, or SECAM. As far as is known, the 2636 PVI was never used in any consoles for the North American NTSC system. Most were PAL and those made in France were presumably SECAM. Fortunately these two standards vary only in the way the colour is encoded in the broadcast signal, and both use the same number of scan lines with the same timings. This means that most of the electronics, and more importantly the firmware, will be the same. The only things that need to change are the video encoder and possibly the modulator. PAL and SECAM both fall under a standard known as either 625 lines or 576i. The number 576 comes about because 49 of the 625 lines are not visible during the vertical blanking period.
Interlaced TV pictures
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The concept of interlaced video also needs to be addressed here as it explains the difference between the 625 lines of a standard tv signal and the much smaller number of lines we can specify with an eight-bit register. In an interlaced tv picture every other line is output first, then when the beam sweeps down across the screen again it displays the lines in between. This method was adopted to help reduce flicker. Each vertical pass is known as a field, and it takes two fields to make a frame. In this way a lot more detail can be displayed in the picture.
Non-interlaced console picture
[edit | edit source]These video game consoles generate a non-interlaced signal. Each vertical pass of the screen is a complete frame. Each scanline takes 64μs and each frame has 312 scanlines. Each frame takes a total of 20ms, equivalent to 50Hz. Forty three scanlines are blanked during the vertical retrace, leaving 269 potentially visible.
Further reading
[edit | edit source]Wikipedia has numerous articles related to this subject should the reader what to dig deeper into this subject.