Flicker fixer
A Flickerfixer (engl. About Entflackerer ) is an additional module for Amiga Computers. It improves the image quality and monitor connectivity of Amiga Computer? By a flicker-free display screen modes, the interlaced (Engl. Interlace achieved using).
As an alternative to RGB or video monitors and TV sets, VGA- compatible display devices can be connected to a flicker fixer , provided that the flicker fixer supplies deflection frequencies that are within the permissible frequency range of the monitor used. Many flicker fixers have a built-in "scandoubler" for this purpose.
Internal and external variants
Internal devices must be installed as hardware extensions or as a plug-in card for the video slot (available depending on the Amiga model) and offer a better picture quality than external variants. External devices have the advantage that they only have to be plugged into the RGB port that every Amiga model provides, but internally carry out a new (lossy) A / D conversion of the video signal. An example of this is the "Hedley monitor" A2024 offered by Commodore itself, which converts a 4-bit HiRes image into a 2-bit monochrome image (4 gray levels) with doubled resolution or optionally a black and white image with four times the resolution can convert.
A special feature of the A3000 (T) is a flicker fixer in the form of an amber chip. For older Amigas ( A500 & A2000 ) there was e.g. B. Flickerfixer of the "MultiVision" series from 3-state and products from other companies, for the A2000 from Commodore itself the plug-in card A2320 with the amber chip mentioned. Add-on solutions or separate devices exist in connection with graphics cards .
There are also devices with an S-Video output and the actually unnecessary feature of being able to deflicker all AGA modes in addition to the usual PAL or NTSC (CompServ: AGA Flickerfixer Scandoubler II). There is also a slimmed-down version of the "Flickermagic" baptized Flickerfixer (DCE: Scanmagic), which only provides the Scandoubler function.
functionality
The two main functions of a flicker fixer can be described as follows:
The horizontal deflection frequency is doubled (scandoubler function)
In the classic case (Amiga with OCS chipset) the frequency of the PAL mode is doubled from approx. 15.6 kHz to approx. 31.2 kHz. This function alone enables a standard VGA monitor to be operated on the Amiga, because a VGA monitor with the Amiga-typical 15.6 kHz would not yet work. However, the vertical frequency, 50 Hz for PAL, must also be supported by the monitor. CRT monitors rarely cause problems, but many TFT monitors only work from around 60 Hz.
The result is an effect that halves the height of the black lines between the video image or, subjectively, even eliminates it: A video image is composed of pixels on a digital level , these then appear nicely square on the monitor, and the pixel lines are less streaky. In order to build a bridge to modern graphics cards that can use the so-called doublescan mode (especially in lower image resolutions ), the effect described is doubled again (this mode could be seen as the opposite of interlaced mode).
The fields are buffered and output at the same time ( deinterlace function)
The problem is the following: In order to double the vertical resolution (e.g. from 256 to 512 lines), the Amiga has to use the interlaced mode. Only 2 times 25 half images are output per second (instead of 1 full image 50 times per second), each of which is shifted alternately at normal height and then vertically by a half line . The vertical resolution is doubled, but at the cost of the vertical frequency being effectively halved to 25 Hz. An Amiga in interlaced mode flickers a lot. In televisions without 100 Hertz technology (i.e. with an equally low vertical frequency), flickering is reduced by phosphors that remain luminescent for longer and other technical analog connections. In the case of a CRT screen, the higher the vertical frequency, the more stable the picture, because the electron beam builds up the picture faster. There are studies that have found a particularly pleasant vertical frequency for CRTs, which is between 100 and 130 Hz. In the case of TFT screens , this value no longer has any direct analog effects, but represents a factor in the clock frequency with which the internal video hardware of the TFT screen can work.
To explain the terms: A field is a frame that is halved in vertical resolution, of which there are two types: The first field contains all even lines and the second field all odd lines of the full image. If you now play them back one after the other by interlacing, you get the complete resolution of the full image again. The sluggishness of the human eye is used because the fields are reproduced in quick succession. A half line at the beginning of one field is created by the video hardware and is located between two normal (non-interlaced) lines.
Now the flicker fixer (deinterlacer) comes into play: The flicker fixer "grabs" 2 fields and outputs them together. As a result, the "half-line flickering" is buffered and a flicker-free display is obtained.
Of course, this - "Weave deinterlacing" (English. Interweave baptized - process a big disadvantage): It works only with still images well - moving objects called Ghost Effects occur (movement or comb artifacts called), that an afterimage of previous position of the moving image or object remains visible. The afterimage arises from the fact that the fields were originally generated one fiftieth of a second with a time lag and are now reproduced as a pair at the same time. (Notes: Still images that are created from motion sequences by television companies often only show one field. Each line of the field is displayed twice (drop field) to avoid the problem with ghost effects . VCRs only show the still image half vertical resolution. With progressive video sources, only full images are used, so a deinterlacer is no longer necessary.)
The Scanmagic mentioned in the introduction completely dispenses with the deinterlace function, which has the advantage that moving images with interlace can be displayed on VGA-compatible monitors without ghost effects (but the flicker is there again). There are now programmable flicker fixers with ASIC technology (Indivision ECS / AGA) on the hardware level , which can be fed dynamically and are programmable, and can therefore be used for a second screen (stackable) or to expand the color space.
history
At the time of the Amiga 500 , the acquisition of a flicker fixer in connection with a VGA-compatible monitor was a relatively expensive, but also the only way to get a higher quality computer image.
In the Amiga sector there were some special developments in monitors in order to fully utilize the possible frequency range of the AGA chipset that was recently developed . With the introduction of the ECS chipset, higher deflection frequencies were possible than with PAL and NTSC, which made the purchase of a multi-frequency monitor attractive. Normally, however, these monitors do not support the low vertical deflection frequency for PAL and NTSC and thus a large range of Amiga software that is mainly based on PAL. Therefore, the use of a flicker fixer or a scandoubler is still common practice today to display this software on the monitor (these are mostly games and demos - not programs that adhere to the AmigaOS guidelines for graphics programming). Many of today's Amiga users benefit from the fast and high-resolution graphics of "real" graphics cards and the possibilities and diversity of the native Amiga chipset. A graphics card, the PicassoIV from Village Tronic with an integrated, programmable Scandoubler / Flickerfixer, reads in the native Amiga graphics and outputs them in relatively freely configurable frequency ranges. This means that old software can even be displayed on most TFT monitors. It should be noted that the vertical frequency is composed of whole divisors of the original vertical frequency, e.g. B. 75, 100 or even 150 Hz for PAL, so that the timings coincide and a smooth display is guaranteed.
The development of Amiga flicker fixers has stopped at the level of weave deinterlacing ( see above ). Modern graphics cards and televisions use more sophisticated methods to achieve a flicker-free and clean display of (moving) video images. In the case of digitally processed video images - for example on 100 Hertz televisions - the previous (older) field is included in the calculation, which results in a display that is much better than that of Weave. Some TV manufacturers use very special processes to get the most out of the outdated television standard. When playing interlaced video files on the computer, e.g. B. the VLC media player can be used. This supports various algorithms for deinterlacing.