Quantcast Decoding VHS RF signal in software? - digitalFAQ Forum
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02-08-2019, 06:01 PM
hodgey hodgey is offline
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I thought some of you may be interested in this.

As have been mentioned before on these forums, the people behind the domesday86 project have been working on decoding the rf signal off laserdiscs for a while now. I've restarted work on using the software to do the same with VHS (and maybe other tape formats eventually). It's still very much early stage work in progress, though I got some interesting results already.

frame_pal_source_1.jpg

https://github.com/happycube/ld-decode/issues/16


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02-08-2019, 06:20 PM
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I don't know what value this has. Explain.

Offhand, I have to wonder if access to the "full" signal might allow software-based timebase correction. Other than that (far-fetched?) idea, I'm not sure why it would be done.

For LD, isn't it essentially allowed LD ripping? (True ripping, not "ripping" misused to mean a capture.)

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02-09-2019, 08:31 PM
hodgey hodgey is offline
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Software-based timebase correction is indeed one advantage of this, as the decoder will has access to the same signal as a VCR or capture device would. The ld-decode software already does correct the horizontal sync to a degree, but as you can see it's a bit inaccurate on the VHS capture decode as of now. You can also do other stuff like dropout compensation (which is partly implemented for the laserdiscs) as well.

Doing it in software means it doesn't have the limitations of a decoder chip that has to do all this with very limited memory space within a very short time, and it can be tweaked to suit each individual capture in post. Also it means that you are not as reliant on needing the top tier SVHS decks with all the processing bells and whistles, as you are bypassing the player processing entirely.

As for the reasoning behind the project for capturing laserdiscs, you can see the FAQ on their website. I suppose you could use the term ripping, The Domesday discs, and many other laserdiscs utilized some of the unique features of the format to do more than just straight video playback. There is also the fact that Laserdiscs don't store luma and chroma separately like VHS (and other tape formats) so having excellent comb filtering is very crucial, which further complicate capturing the "traditional" way.
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02-10-2019, 01:36 PM
latreche34 latreche34 is offline
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Actually the VHS one is more important than laser disc, very few laser discs didn't make it to DVD or Blu-ray so not much interest there when you can just get the HD version of the movie. But for home videos this sounds like a good ideas with great potential for better quality. Especially if the process involves one digitization only vs 3 with the traditional workflow assuming the line and full frame TBC's are used.

Last edited by latreche34; 02-10-2019 at 01:57 PM.
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02-10-2019, 04:32 PM
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This looks like software defined radio in a FPGA chip, driven completely by a firmware upload to the FPGA on boot up. Though I suppose we're getting to the point where CPUs have enough cores and speed to do it by pinning a task off to one part of one core while the rest of the operating system runs on the rest of the CPU.

The trick then becomes one of a proper signal conditioner and an Analog to Digital converter.. which can also be defined in silicon by software.. or as a stock add-on in many system on chip with bundled "peripherals" like the CPU in a cell phone. A CCD camera chip is after all a very simple ADC with many many converters in row and column arrays. Its not like the old days when you needed an analog circuit tied to a photodiode to generate a single "bit" of data.. now you have acres of bits and a flood of digital data to work with in software.
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02-14-2019, 03:03 PM
captainvic captainvic is offline
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Quote:
Originally Posted by latreche34 View Post
Actually the VHS one is more important than laser disc, very few laser discs didn't make it to DVD or Blu-ray so not much interest there when you can just get the HD version of the movie. But for home videos this sounds like a good ideas with great potential for better quality. Especially if the process involves one digitization only vs 3 with the traditional workflow assuming the line and full frame TBC's are used.
Yes, this process could be very important for VHS sources, but to say "very few laser discs didn't make it to DVD or Blu-ray" is not wholly accurate. Approximately 1/3 of my laserdisc collection has yet to be released on DVD or BluRay, probably because most of my laserdiscs are not big-name Hollywood movies.

This process could be important for a variety of analog sources. Thanks for sharing! I find the Domesday Duplicator project to be fascinating.
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03-09-2020, 04:49 PM
Infrid Infrid is offline
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Quote:
I don't know what value this has. Explain.

Offhand, I have to wonder if access to the "full" signal might allow software-based timebase correction. Other than that (far-fetched?) idea, I'm not sure why it would be done.
Don't underestimate the importance of such project, the ability to sample the waveform, save it and process (or reprocess) later is such a huge advancement that can revolutionise how we preserve analogue footage.

A decent VRC will be required, but you can avoid the purchase of an expensive TBC, outdated capture card and complicated driver/capture software setup.

With an high sampling frequency and bit depth you can be more precise on how the colour is filtered and decoded. Not to mention the software time base correction, to me it's a godsend.

The ability to re-process the sampled footage when a new version of the decoder is out, it's simply amazing. A new denoise algorithm is out? just replay the file! A more precise TBC? replay!

If I can sample all the signals from my VHS collection, I won't worry too much about rotting and degradation of the media.

I captured lots of VHS using my limited equipment, dropped frames, A/V sync problems, unstable image , etc etc. I could solve those problems with an investment of 1-2k, but I don't want to spent that money on the equipment.

Projects like this can cut down the cost of digitalising your VHS collection, I am looking forward to this. I really do.
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03-09-2020, 09:13 PM
sevarre sevarre is offline
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Yes I am also following this closely. The documentation on the LD decode project is nice and gives a nice tutorial on building your own DAQ system (think it uses a 40 Mega Samples / second ADC chip). I am checking out some old used NI products that are marketed as "digitizers" but which are basically a DAQ with a couple GS/s and see if I can use one of those instead of building the the thing on the LD Decode project (although that looks like a fun/reasonably easy task given the documentation).

I don't know anything about the RF signal that comes off the tape head, though. Have to research that. I know all about the NTSC composite video signal, but I am guessing the signal coming off the head amp is not simply just that signal. I believe this project is particularly useful for laserdisc because they can sometimes contain auxiliary information that is not simply audiovisual (like how teletext is contained in the vertical blanking interval, maybe?).

The question I have for VHS is whether or not software can extract the audiovisual information from the raw RF head amp signal as well as the electronics in the AG1980P can.
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03-09-2020, 09:48 PM
NJRoadfan NJRoadfan is offline
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This PDF gives an overview of how the signal is stored on tape: https://www.av-iq.com/avcat/images/d...20-%204611.pdf
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03-10-2020, 12:57 AM
SFtheGreat SFtheGreat is offline
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Quote:
Originally Posted by sevarre View Post
I don't know anything about the RF signal that comes off the tape head, though. Have to research that. I know all about the NTSC composite video signal, but I am guessing the signal coming off the head amp is not simply just that signal. I believe this project is particularly useful for laserdisc because they can sometimes contain auxiliary information that is not simply audiovisual (like how teletext is contained in the vertical blanking interval, maybe?).
NYSC VHS also had closed captions, for years I wondered how to play my canadian Star Trek on a PAL VCR and see the text.

Also, the ability to tap into the directs signal off the tape before the electronics interpret it according to being either PAL or NTSC would mean that any system will be able to read any system, as long as cotrol track and tracking are fine, simply grab the RF before it is converted to CVBS and audio signal.

And regarding audio, the Hi-Fi stereo needs to be interpreted from the wave then.
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