Current status of ld-decode / vhs-decode (true "backup" of RF signals)

ld-decode is an open-source effort to provide a "software defined LaserDisc player". The project aims to take raw RF captures of LaserDiscs (produced by the Domesday Duplicator hardware and software) and decode the RF back into usable component parts such as video, analogue audio and digital data and audio too.

The decoding process (like a real LaserDisc player) is a multi-stage process. The raw RF must be demodulated (from the original FM signal) and filtered into video, audio and EFM data. This data is then framed and passed through a digital time-base correction (TBC) process which attempts to remove errors caused by the mechanical nature of a LaserDisc player during capture.

The resulting TBC output is then run through a chroma-decoder (comb-filter in NTSC speak) which recovers the original color as raw RGB. The raw RGB can then be processed into PNG frames or video files such as mp4 or avi for viewing using open tools such as ffmpeg.

Please see the Installation guide for details of how to download and install ld-decode and the basic usage guide for instructions on how to use ld-decode.

An overview of how a LaserDisc player functions (which can help you to understand the component parts of ld-decode) is available from this link.

[^ from ld-decode Github Wiki]

For this test I captured with cxadc, which is a modified driver for capture cards based on the Conexant cx2388x video decoder chip. The driver puts it into some raw mode that makes it output the raw signal (at 28.6 MHz sampling frequency, with 8bit resolution.) The DdD which is used for the laserdisc captures now is 40MHZ 10-bit so it may give a nicer signal, but I don't know as I haven't got one yet.

I used a standard BNC test probe (probes that came with my analog discovery 2 usb scope), using a BNC to RCA adapter to connect it to the capture card. The probe was connected the rf test point on a SONY SLV-SE60N VHS deck, a mid-range 4 head Hi-Fi deck. So nothing fancy. (Had to use the s-video connector on the capture card for whatever reason, so used a S-Video-> RCA dongle from a graphics card I had lying around in reverse to connect.)

Had to adjust the IRE zero-point in the decoder to pick up the syncs correctly. I just eyeballed it from a signal plot, not sure how to calculate it properly, and also disable some audio stuff in the code to avoid it crashing.
Note that the video on the tape is recorded from TV, I think that's the cause of the ghosting you can spot. I made a color bar test tape on a SVHS deck that I will try to capture later.

[Attachment 49930 - Click to enlarge]

Since people are talking about video tape again, here's a status update and a decoded image (PAL) from a VHS tape from the current VHS. I just did some work to approximate the right de-emphasis today which seems to have helped a bit.

I haven't done any colour decoding as of yet. The color carrier is in the captured data, but it has to be phase shifted based on which video track it's on (VHS does this to reduce crosstalk), and heterodyned(or whatever the right term is) up to the right frequency.

The comb filter/color decoder in ld-decode assumes a composite signal at the moment, but for a color-under format like VHS one would ideally want to avoid mixing the color signal in and decode the color separately, so that's also something that needs to be implemented.

VHS (and other videotape vcrs) typically have some extra frequency and phase correction of the colour signal to compensate for the uneven tape speed as well, but I'll leave that for later.

The TBC may need a bit of tweaking too, it gets confused by the head switching point at the moment.

Current project is at https://github.com/oyvindln/ld-decode

I've so far tried to keep it compatible with upstream, so all the VHS stuff are in separate files.

Feel free to contribute, I can upload the raw .lds captures if needed.