The Domesday Duplicator is intended to allow high-quality back-ups of the analogue information contained on the BBC Domesday laserdiscs by bypassing most of the 30-year-old electronics in the Philips VP415 player. Direct RF sampling also allows all information on the laserdiscs to be duplicated (unlike conventional RGB sampling of the video output). Since the BBC Domesday AIV laserdiscs are a combination of video, pictures, sound and data (as well as numerous VBI streams), direct RF sampling is the preferred method of preservation.
The Domesday Duplicator project is completely open-source and open-hardware. For details of how to obtain the source code and hardware files please see the software overview page.
The hardware is a USB3 based 10-bit analogue to digital converter designed to allow the backup of BBC Domesday AIV laserdiscs as well as generic PAL and NTSC laserdiscs through the direct sampling of the RF data from the optical head (laser) of a laserdisc player.
The hardware/software solution is designed to act as a sampling front-end to the ld-decode (software decode of laserdiscs) project https://github.com/happycube/ld-decode and replaces the generic TV capture card to provide high-frequency sampling with 4 times the sample resolution. Increasing the sample resolution allows better capture of disc overall however, the primary advantage is that the Domesday Duplicator provides better performance for weaker RF signals especially at the start of a laserdisc (where the RF output has lower amplitude) and when the disc is degraded due to age and surface damage.
Domesday86 would like to thank Chad Page (the author of the ld-decode project) – without his tireless work producing ld-decode and his assistance in modifying the library to support the Domesday Duplicator project, this preservation method would not have been possible.
The following image shows a comparison between RGB capture from a laserdisc player using standard video capture hardware and the output from the Domesday Duplicator RF capture after processing with ld-decode:
The following block-diagram shows the 4 high-level components of the Domesday Duplicator:
These components are described in more detail in the following sections:
The reference laserdisc player for the Domesday Duplicator project is the Pioneer LD-V4300D. Information about this player is available from the following section:
The following diagram shows a typical laserdisc RF signal sampled at 32 MSPS (million samples per second):
An overview of the laserdisc decoding process
The Domesday Duplicator is a high-speed DAQ for capturing the RF output from a laserdisc player. In order to use the RF capture it is necessary ‘decode’ the captured RF sample into video, audio and data. Understanding what’s involved in the decoding process requires a basic knowledge of the design of a laserdisc player. For more information please see the following guide:
Q: Why not just copy the laserdisc digitally like a DVD/CD-ROM?
A: Laserdiscs are analogue, so it’s not possible to simply copy the video files from the disc as you would with digital media.
Q: Don’t the BBC already have a high-quality copy of the Domesday contents?
A: Yes and no. The BBC have a digital betacam version of the Community and National Domesday laserdiscs which includes both the video and audio. However, there is a lot more information on an AIV laserdisc (ADFS data, VBI data, etc.) and, more importantly, there are a number of other AIV discs not covered by this backup. Finally, the digital betacam copy is not accessible to the public; so there’s no way to get a copy.
Q: Why not just use a video capture card on the RGB/SCART/Composite output of the VP415?
A: The video and audio produced by the VP415 has to pass through many analogue stages before it can be viewed as a PAL signal. As the data is analogue each processing stage adds noise and distortion to the signal. By sampling the RF directly from the player’s laser you avoid all of the lossy stages. Furthermore, the RF sampling can be performed using any PAL compatible laserdisc player – VP415 players are quite rare, this technique allows any player to be used to read the AIV disc contents.
Q: I don’t think the picture’s that much better, what’s the big deal?
A: RF sampling preserves the contents of the laserdisc far better than ‘ordinary’ video capture. It helps if you think of the RF sampling as ‘scanning’ the laserdisc surface with a laser; the RF sample is effectively a scan of the disc’s surface. Once scanned the methods for decoding the disc’s contents can be improved over time and more types of information can be extracted from the image. Right now the decoding is primarily for the video and audio content but soon it will be possible to extract VBI and data from the image too. Furthermore, the techniques used to recover content such as video can also be improved leading to better output from the image (without the need to resample the disc). RF sampling preserves the contents of the disc in a way that RGB capture cannot.
Q: Why didn’t you just use an off-the-shelf SDR board?
A: Although there are SDR boards available that can perform RF capture they have RF front-ends designed for radio and therefore would need significant modification for use as a laserdisc sampler. SDR boards are also quite expensive in comparison to the ‘custom’ part of the Domesday Duplicator. A 10-bit single-ended ADC allows the design to be a very cost-effective solution. Domesday Duplicator is also designed (from the beginning) to be a complete open-source, open-hardware solution, so a custom design allows better control over the licensing used for the project.
Q: How big are the RF images?
A: The duplicator outputs the 10-bit samples as 16-bit signed data. The data is created at around 62 Mbytes per second. So a CAV disc capture is around 130 Gbytes per side (260 Gbytes per disc). CLV discs contain more video (up to 60 minutes per side) and require around 220 Gbytes per side.
Q: This project is just the most awesome thing I’ve ever seen! How can I help?
A: Hit the donate button on the page (above-right) and help us fund the necessary equipment needed to make Domesday86 a reality – every little helps! All of the projects around Domesday86 are open-hardware and open-software, so donations are the only means of funding for the project.