[EN] SavoNARA: VBXE2VGA/SVIDEO

[pancio]

Several people have asked me about the possibility of converting RGB to HDMI. There are several solutions available based on the RaspberryPi Pico but they require much greater skills - the subject is quite difficult (for me) and more expensive... Maybe one day I will try to analyze the AN0-AN3 signal from GTIA and generate an image via HDMI completely digitally... someday!

In the meantime, I was thinking about how to send an image from VBXE to HDMI using the cheapest and simplest method (which does not mean the worst!)
VBXE generates an analog RGB signal - the problem is that it does not generate H/V sync signals and this significantly limits the number of available devices that can display RGB images - it basically comes down to old TV receivers with a SCART connector.

The temptation to display an image from VBXE (using the capabilities of the VBXE core!) on computer monitors and monitors equipped with an SVIDEO input was so great that I decided to develop a simple solution. I also wanted the image using all the possibilities of VBXE to be also available on the DIN-5 output in Atari.

Savonara: VBXE2VGA/SVIDEO is the next project created as part of the activities of PTODT (Polskie Towarzystwo Ochrony Dziedzictwa Technicznego, which means the Polish Society for the Protection of Technical Heritage), founded for the protection of technical heritage, especially in the field of informatics and computers. I encourage you to visit the association's website and support our activities:

Polskie Towarzystwo Ochrony Dziedzictwa Technicznego/


License Information
The project was made as Free Hardware under the Creative Commons BY-NC-SA 4.0 license. This means that you can make it yourself, for private use. Commercial use is possible after obtaining the authors' consent.

Schematic
The device structure is shown in the diagram below:

vbxe2vga_v1.0.2.png (185.73 KiB) Viewed 1457 times

The main element here is the AD724 or AD725 chip, which is an integrated decoder of RGB signals on SVIDEO and CVBS.

For correct operation, the AD724/5 requires additional synchronization signals H and V generated by the LM1881 chip - these signals are also needed to create a full VGA. Additionally, the RGB signals are amplified and improved using a video amplifier THS7316DR.

It is also worth mentioning the PAL x4 frequency multiplier implemented on the ICS501 system - it allows for better processing of RGB signals and, consequently, obtaining a nicer SVIDEO image.

The NTSC/PAL operating mode selection is connected by creating a jumper JP2 (shorted for PAL) and the frequency conversion by jumper JP1 (open for 4x PAL). If we do not plan to use the quadruple PAL frequency, we do not need to install the ICS501 integrated circuit

PCB layout
The PCB has dimensions 28.5mm x 49 mm and can be directly connected to VBXE RGB SLOT.:

vbxe2vga_v1.0.2-pcb-front.png (153.47 KiB) Viewed 1457 times

vbxe2vga_v1.0.2-pcb-rear.png (116.75 KiB) Viewed 1457 times

Both output sockets are equipped with a power line that can be used for an additional HDMI converter - especially suitable for the Bitfunx HDMI scanner/grabber that uses Y and C SVIDEO signals. Of course, VGA can also be connected to the HDMI converter, but the image obtained from the VBXE enhanced by sync signals can be used with monitors that have a VGA DSUB-15 input and its quality is very good - especially RGB are amplified and corrected by THS7316DR.


Assembly
It is best to start assembling the components with passive components. The board is relatively large, so there should be no problems. I used mostly solder pads for 0805 components, but it is much more convenient to solder 0603 components on them. In the next stage, we assemble integrated circuits and in the last stage, sockets/pin strips.

One notice here regarding variant v1.0.x... there is no need to have big capacitors (220uF) on the output on SVIDEO and CVBS for PAL - it shall be replaced by short circuit by using wire - I leave these capacitor's footprints because I have no idea that these shall be use for NTSC or not. Probably not but I have no one NTSC TV at home for test.

t.b.c.