AMI-RGB2VGAULTIMATE

is a "revised-revised" RGB to VGA adapter for the Amiga A500-A600-A2000 and A1200-A4000, with unique improvements:

• Fully buffered Composite HSync and VSync signals
• Selectable CSync signal for those lucky guys who own a Sony PVM monitor (or feed a GBS-C converter, etc)
• Impedance matched inputs and outputs
• Fully high quality buffered video signals
• Fully filtered spurious input frequencies: no more vertical bars on LCD screens !
• Four user selectable filter settings: precisely match your display resolution for perfect quality !
• Smaller than the original 390682-01/03 VGA dongle by commodore

User can freely select any of the four low-pass filter profiles, to either match resolutions for maximum clarity and clearness; available profiles are:

• SD -> 9.5Mhz
• ED -> 18Mhz
• HD -> 36Mhz
• FHD -> 72Mhz

THE ISSUE

When connecting an Amiga computer (or in general any computer/console designed to be used with an analogue RGB monitor) to a modern LCD screen with a VGA interface, one can observe the typical 'jail bars', annoying horizontal stripes covering the entire screen.

These two screen below show a typical issue:

The VGA input , unlike the scart one, is not bandwidth limited in any way; thus it picks up any spurious noise on the Amiga RGB signals.

More in detail, the basic problem is the VGA monitor: the Amiga video output was designed with CRTs in mind (which was the correct solution at the time); the video bandwidth of those CRTs was something limited to ~10MHz.

Video jailbars artifact are, in the frequency domain, frequency components >28MHz (they come from the fundamental Amiga 28Mhz clock and its higher harmonics) .

Modern VGA/LCD displays, support today video bandwidths of 130MHz and more, so they are able to display these frequency components which take the form of annoying jail bars.

THE SOLUTION

This below is the result of the video filtering and impedance-matching of the adapter:

A practical and effective solution to the issue is to limit the video bandwidth from the source with a proper video amplifier chip with lowpass filter.

The cut-off frequency of the low-pass filter must be correctly calculated in order to eliminate unwanted frequencies, without however compromising the quality and definition of the video information to be displayed.

In the case of the Amiga series computers, it is basically a matter of calibrating the filters for two major cases: OCS/ECS and AGA.

In the first case, filtering calibrated at 9Mhz or 18Mhz is appropriate, while in the second case it is appropriate to increase the bandwidth, filtering around 30Mhz.

The design described here combines in one device the ability to select any of four appropriate cut-off frequencies on the fly, using a commercial chip from Texas Instruments, which performs the function of video amplifier and selectable filter.

The inputs and outputs of the circuit are also calibrated to the correct impedance of the video signals, and the horizontal and vertical synchronisation signals are buffered for maximum possible stability.

Finally, an option has been included to replace the horizontal synchronisation signal with the composite synchronisation signal, so that Sony PVM-type displays or GBS-C-type converters can be connected.

A few other passive components and the necessary VGA (to monitor) and DB-23 (to Amiga) connectors complete the design.

The latter connector, rather rare nowadays, can be purchased from various Chinese suppliers, or made from a common DB-25 connector by simply removing the two outermost pins.

THE CIRCUIT FULL SCHEMATICS

The circuit is quite simple: the Amiga's RGB video components go into the input of the chip integrated video amplifier and programmable filter, through appropriate impedance matching resistors.

A three-way switch allows the filtering to be bypassed completely, rather than selecting any of the four available low-pass filters, calibrated to the appropriate frequencies relative to the Amiga's video modes.

An additional switch allows the selection of horizontal synchronism rather than composite synchronism signal (HSync vs. CSync), so that the adapter can be used to connect screens such as Sony PVM monitors, or converters such as GBS-C or similar.

Similar to the inputs, the video outputs are also correctly impedance matched.

Each video component (R, G, B) is processed according to the following principle scheme:

The switchable frequency lowpass filter show the following figures:

BUILDER's FILES

All builder's files (full schematics and PCB in Altium format, gerber files, BOM, pick&place file) are free to download here (or on the Github site)