FParams_PAL_VHS['video_bpf_low'] = 4500000 #3350000 originalBesides the ability to live preview the results I had to wait for the frames to decode but I think it's good now.RFParams_PAL_VHS['video_bpf_high'] = 7000000 #520000 original
I looked at teletext patterns, I messed with the values until the dropouts were gone. These parameters vary depending on the condition of the tape.
Other values I'll be looking at will be TBC sensitivity, video levels and the color filters.
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Using the same settings I get them on "overused" recording of multiburst pattern. I think middle frequencies aren't filtered enough but I may be wrong.
Without FFT display it's hard to guess the original correct recording frequency.
What do you need the composite output for? Man those samples look pretty stable with only two minor problems, There is white and black short horizontal lines that appear in the video section only I assume those were in the original video and there are some long horizontal white noise lines, it will be nice to compare to normal capture and post them on youtube.
Man that Canal+ scrambled video reminded me of the 90's and my Aston satellite receiver, those were the fun days if you know what I mean (emulator cards !!!).
I see that you posted normal video captures, so those short noise spikes are original.
Last edited by dellsam34; 2nd Dec 2019 at 11:52.
Personally I don't like to spam with links and videos any topics but there's regular capture of this momemt for easy comparison
I failed a bit with the tracking setting - forgot to set it to automatic and didn't have any monitor to see the picture.
I saw the regular video samples and updated my post above, So any progress on the chrominance ?
Dropout compensation isn't properly implemented for VHS yet, so any dropouts become quite visible long noisy white lines like in e.g sample 3. The short white (or in a few cases black) spikes I think are mostly a result of the demodulation (transforming the FM signal frequencies into video levels, or voltage in the analog domain) process picking up the wrong frequency (though there could be some short dropouts in there too.) The luma video signal on the tape is FM-modulated, i.e high frequency wave = white, low frequency wave = black. It makes sense for there to be spikes/noise on a re-used tape as there may be more residual noise and wear from previous recordings that hasn't been entirely eliminated by the erase head, and a worn tape won't store the magnetick tracks as well as a new one. It's sort of the opposite of the "bearding" described in the link where spikes are generated in the modulation stage due to the modulator being overdriven (though I've seen VCRs creating black spikes on playback too).
The filtering and demodulation of the RF signal in vhs-decode needs more work to avoid picking up the wrong frequency. Adjusting the band pass filter currently used to filter waves of too high and low frequency has some impact, but may need something more complex. Demodulation seemed to be done with a somewhat different process in VCRs judging by the books I've read compared to how ld-decode implemented it, so I don't know if I need to change more than just the filtering of the RF signal before demodulating.
As for chroma, it's decoded to a degree now, but it still lacks the frequency and phase correction functionality found in VCRs to compensate for the unstable nature of the signal coming off the tape. Granted, we do TBC it, which makes up for it a little. There's also a bug that results in the chroma not being centered right which needs to be tracked down.
Sorry for complicated wall of text. Been thinking of making a blog post or something detailing the VHS decoding process in a bit more readable manner.
https://en.wikipedia.org/wiki/Differentiator - burst frequency are superimposed over DC bias - check CCIR18 signal for example in https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_application/application_notes/7bm25/7BM25_0E.pdf - drop of bias level passed trough capacitors may be visible as pattern around burst.
I'm going to try and capture from my 1994 RCA VCR this week to post a sample.
Last edited by Titan_91; 8th Dec 2019 at 20:04.
Well, it died. The tracking circuit no longer works at all and the RF and composite output of the machine are crap. Guess I'll re-install the mod in my original VCR since the signal level is about on par with the Spider-Man sample and will be looking at Goodwill for an older/better one.
I want to get going with this right away, but I don't want to have to wait for a Domesday Duplicator to be built because nobody seems to be offering to do do. What other devices are up to the task for this?
Here's the Conexant CX2388 based card I bought.
I searched eBay for "Blackbird capture card" after doing some research. You want one with a CX2388 based broadcast decoder. This has the 28.6MSPS high speed ADC needed for FM signal sampling from a LaserDisc player or VCR. Check the item photos to verify the chip is on the card. Once you have one you will likely need a second PC with a PCI slot to run Linux on.
I got to thinking about how to cleanly use a VGA splitter/amp for capturing. I have a generic brand DVD/VCR combo that plays tapes pretty well. The DVD player tray sensor went bad so I disconnected it. The VCR side still works fine. So I might try to remove the DVD drive and use that space to put my VGA splitter and its power supply. I can wire the supply in to the mains feed for the player and tap the RF as close as possible from the main board. I can then install an RCA jack and just run a cable from the machine to my PC. Using a VGA splitter is detailed in the Google document.
Edit: I pulled the DVD drive. I think I found the RF test point along with the head switching test point. After checking the service manual this connects to pin 79 of the main video IC and is pre-amplified like on the Phillips VCR. I plan on stacking the splitter on top of its power brick to allow the cables to clear. I can then use one of the pre drilled holes in the back of the case for the output.
Would I also need to ground the splitter? Any suggestions to add to this idea?
Last edited by Titan_91; 12th Dec 2019 at 13:03.