Retro Video Adaptor (RVA) in development
Current Status: Work in progress
Date: Jan 2013
License: Creative Commons Attribution 3.0 Unported License
This project was originally intended to be a scan-doubler for the Amiga computer series, when initially conceived back in 2005. The idea was to use an Averlogic AL250 scan-doubler with a suitable Analogue Front Enf (AFE) to sample the RGB video.
As time passed and newer monitors and TVs, with better capabilities emerged on the market, a re-think was required. Modern TVs support RGB SCART, YPbPr component video and a variety of PC digital formats (DVI/HDMI) as well as the older VGA input.
To add flexibility to the design it was decided to make the design modular. The base-board handles sampling of the analogue video source and provides an expansion connector for a plug in module. In addition it has an analogue video encoder device to support a wide variety of formats. This new approach was the main reason to target the design at the retro gaming market as it could be used with a wide variety of 80's and 90's computers and games consoles.
The basic scheme is shown here:
The Video decoder device handles a wide range of SDTV video formats and is surprisingly tolerant of ill-defined video sources (240p/288p). It can handle composite (CVBS), Y/C (S-Video), YPbPr and RGB video inputs, all under software control. The video is sampled and crucially re-timed to provide a more stable sync source to a downstream device and the end-users TV display.
The video encoder supports the same, wide range of video types as the decoder. This also allows format conversions to be achieved digitally, e.g Y/C to YPbPr or RGB to YPbPr or RGB to Y/C. In theory you could convert composite to RGB or YPbPr but why would you want to?
The AL250 scan doubler device accepts the digital video from the decoder, in YUV 4:2:2 format, with no loss of colour information and converts it back to RGB and outputs it at 31 KHz or greater screen refresh rate.
To facilitate future expansion and experimentation, an expansion header has been added.
The HDMI output is of interest but it is dependent on the video decoder device providing a stable LLC clock from the recovered video information.
The Averlogic AL260 is an improved video scandoubler with improved de-interlacing and can provide timebase correction. This warrants evaluation.
Current project status (27th April 2012)
Schematics complete pending final tweaks for the expansion board(s). PCB will take 20-30 hours to track.
Components for two prototypes are currently in stock.
PCBs will be 4 layer, expensive but required for noise immunity and controlled impedance.
Updated 14 May 2018