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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 25 Aug 2020, 22:58 
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Titan91,

I was waiting for someone more knowledgeable to respond, and they can correct me if i am wrong.

While the spec says the highest frequency is 9.31Mhz (or whatever it is), happycube once told me you needed about ~12.5Mhz of bandwidth, minimum, for the captures. I don't know why that is the case, but that is what i recall (iirc).

Nyquist-Shannon states the minimum, but that is the minimum with the most quantization noise/error. You can't go with the 10 bit 14.4MSPS mode, because it isn't high enough. period. So that leaves you the 2 8 bit modes. The higher msps will get you less quantization noise/error, so i'd go for that.

Just make sure your disk is fast enough to save it.
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 27 Aug 2020, 19:19 
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That makes sense and lines up with what I read. Meanwhile I snagged another Pioneer CLD-V720 and a new old stock Sony demo disc from 1990. The disc was never opened! Both were pretty cheap eBay finds.

The disc itself I'm going to capture at 35.8MSPS/8 bit. I had no issues capturing VHS at this rate. Playing it back it appears to be in great condition with no dropouts that I can see.

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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 28 Aug 2020, 01:06 
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So I went ahead and installed the RF mod. I used part of an old VGA cable spliced to an arcade machine bill acceptor harness. This took me only an hour. The harness is well below the disc transport. Luckily I didn't have to remove the tray, just eject it. The karaoke Aux audio jacks are really handy to make this a clean 100% reversable mod. Wish I had heat shrink tubing though.

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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 28 Aug 2020, 01:53 
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titan91 wrote:
So I went ahead and installed the RF mod. I used part of an old VGA cable spliced to an arcade machine bill acceptor harness. This took me only an hour. The harness is well below the disc transport. Luckily I didn't have to remove the tray, just eject it. The karaoke Aux audio jacks are really handy to make this a clean 100% reversable mod. Wish I had heat shrink tubing though.

for the best signal to noise ratio, it would probably be better to use the cable shield as the ground, rather than the green wire. If you check out the domesday site, coax is recommended (i used an old piece of a wifi antenna cable). Additional, while convenient, the RCA connectors won't provide as good of S/N as BNC. It may not seem like it would matter, but the RF signal is very weak, and easily interfered with.

I speak from experience, i tried to use SMA jacks and cables because i thought they would be "just as good" as BNC and they were easier to mount. When i switched to BNC, S/N improved.
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 29 Aug 2020, 03:05 
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I don't intend on sticking with that type of cable. I made it to get my feet wet and do some initial testing, which is promising so far. I'll see how well the initial samples decode before installing coax and properly connecting the ground.
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 01 Sep 2020, 02:21 
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The signal level is pretty low tapping directly in to my player (as expected). So I added a Radio Shack distribution amp with a not-so-great RCA cable (temporary) and got about 6db of gain. Here's a before spectrum graph in Audacity with the CXADC gain set to 28:

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Now with the distribution amp and gain set to 31:

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I also ran an spectrum plot. Gain of 28 with no distribution amp with the FM envelope peaking at -60db.

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Amp added at 28:

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Amp with max gain at 31:

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I think that's a pretty good result so far for what is mostly just a high quality s-video cable going between the amp and my PC. The amp helped the two audio carriers the most. I am getting a lower harmonic of the main video signal however. Will test again after installing a coax tap.
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 09 Sep 2020, 02:37 
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After replacing my previous cable with coax and properly grounding the shield braid, I probably got 1db of additional gain. I've soldered directly to the test point this time as well. The spectrum and decodes are probably slightly better looking. What else can I do that wasn't already mentioned to get more RF gain out of the player with my Conexant card? Maybe the RF test point isn't the best location to tap? Currently using just a Radio Shack composite video distribution amp with an unregulated power brick to minimize noise from that end. This works well for my vhs-decode captures. Maybe replace this with a low noise HF amplifier? I know the card is capable of capturing better SNR, as I've done so with a stronger signal from my VCR.

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Last edited by titan91 on 10 Sep 2020, 12:49, edited 1 time in total.
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 09 Sep 2020, 03:56 
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4 or so years ago, i did the TV card capture method, and i remember getting results that were better than what the player could display via composite out (although it was a PAL player). I did not even have to use an amplifier in between.

I was using:
1. a CLD 1050
2. a panel mount BNC port installed on the back of the machine
3. no resistor with the coax soldered to the RF test point and panel mount BNC
4. an S-Video to BNC Y adapter for the input into the card.

the 1050 may have had higher output??? If you read back far enough in this thread, i believe happycube said that after he started using the aforementioned BNC Y adapter, he no longer needed the amplifier either. As mentioned before, using the RCA jack is also poor quality:
Quote:
^ from wikipedia.

those are the only things i can say, but that is just based on my experiences. $20 on amazon for the parts (and a few extra BNC panel mounts for the future!)
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 09 Sep 2020, 12:50 
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I agree, at this point the RCA connections and cables involved with the distribution amp I'm using are likely the issue. I thought about impedance and assumed this would be fine since the card's input is 75 ohms. However, the test point is likely not sending a signal that is matched to a 75 ohm input. Using a single 50 ohm BNC connector with 50 ohm mini coax would be best?
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 09 Sep 2020, 18:03 
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titan91 wrote:
I agree, at this point the RCA connections and cables involved with the distribution amp I'm using are likely the issue. I thought about impedance and assumed this would be fine since the card's input is 75 ohms. However, the test point is likely not sending a signal that is matched to a 75 ohm input. Using a single 50 ohm BNC connector with 50 ohm mini coax would be best?


Take whatever i say, with a grain of salt. I am no expert. I took a few DC electronics classes in high school, and have tinkered with things all my life. I never progressed into complex AC / RF though.

Impedance in an AC circuit isn't a static value, it varies over the frequency spectrum. The rating given is Nominal impedance. So let's say an RCA connector, which was designed for audio frequencies, has a "nominal impedance" of 75ohms. That might only be for the 20hz-20khz frequency range for which it was designed, though. The impedance for frequencies higher could be significantly different. BNC connectors are rated up to several Ghz, so the nominal impedance rating of a BNC connector takes that into account. This is what i have been able to gather so far (this is my best interpretation, anyone with more succinct knowledge please correct me if i am wrong).

All that being said, I was thinking about it last night, and i was wondering if maybe the LD-Decode "code" has evolved in such a way, in the direction of the DDD, that quality of the TV capture card method has diminished? This occurred to me because you said you had good results with the VHS captures with a similar setup? It may be the case, that in trying to get the best fidelity from the DDD, that fidelity from the TV card is compromised.

It looks like your luma signal is pretty good, and that the noise is in the chroma signal?
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 10 Sep 2020, 16:25 
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I was reading up on impedence the other day and basically learned the same. However, I agree and don't think that's the main cause of my poor decodes. It's a factor, but like you pointed out my VHS decodes fared much better with the same card, similar cabling, and same distribution amp. Now with VHS, the chroma signal is at around 629KHz with LaserDisc being 3.58MHz. I'll update this thread again after checking the spectrum and SNR of my VHS captures. Some of my vhs-decode samples:

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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 11 Sep 2020, 01:40 
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I actually think the main cause of the LD captures having chroma noise is the lower signal level. I also used an F type screw on connector for the VCR instead of a RCA connector, as well as a VGA splitter before my RCA video distribution amp. The VHS chroma noise isn't as prominent. Here's a spectrum graph of one of my VHS samples to the same scale, showing about 10db of difference on the luma signal over the LD luma signal:

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FFT plot showing darker traces:

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Decode with color I mixed in manually after deinterlacing (required due to a phase rotation issue between fields in each frame):

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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 11 Sep 2020, 19:01 
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Chad, would it be possible to add a feature to ld-analyse to export the frames of a TBC file as a series of PNGs? There's already a single frame export feature, hopefully it wouldn't be hard to add this in as another option. Ld-chroma-decoder does the job, but it outputs raw bitmap files that must be further muxed and processed by ffmpeg. This extra option would really help with the learning curve and speed of conversion I feel.
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 12 Sep 2020, 02:20 
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@titan91

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I set up my old tv capture card to test. this isn't great, and definitely not as good as i got before with the tv card, but it isn't as bad as yours.

Of note, when i did this a long time ago, it was the CLD-1050 straight from RF test point. This is 4300 and a 47ohm resister inline. Also, my card is cx8802.

Also, my card isn't honoring the sample rate setting, i meant to capture at 35.8, but it ended up being 28.6


Last edited by 9954tony on 12 Sep 2020, 22:54, edited 1 time in total.
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 12 Sep 2020, 17:55 
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Thanks for the additional info Tony. I guess if it's not any better with 50 ohm RG-174 coax cable with a BNC connector/adapter going straight to my card I guess it's just the output level at the test point of my player. I definitely don't intend to calibrate it due to not having the knowledge, equipment, service manual, and calibration disc. However, there is a trim pot for RF gain near the laser ribbon cable and test point. Can someone confirm if this adjusts the gain coming off of the laser itself, or just the gain for the test point? I also absolutely don't want to touch anything internally if there's a chance of damaging some circuitry.
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 14 Sep 2020, 20:16 
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@titan, the RF gain pot also effects everything downstream, because you can see a difference on the screen when you have the composite/svideo hooked up.

I think in my previous cxadc capture, i still had my physical low pass filter hooked up (that i use with the lime) which eats some of the signal. I did a new one today without it. This is 4300>47ohm resister>BNC>svideo bnc y adapter: Image

Then, i hooked up this $12 RF amp i bought on ebay years ago (i removed the SMA connectors and soldered leads to the board, i power it with a 9 volt battery). ebay amp. This was so powerful, i had to put another resister inline (20ohm) because 'leveladj' clipped at level 0. This bumped me up 5/6db in the S/N ratio:

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My card still won't honor the settings in the config file, it just does 8bit@28msps. Not sure if 35msps would help there?
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 16 Sep 2020, 01:20 
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Thanks for the info. For the 47Ω resistor, will any through hole sized resistor work? We're not talking about crazy high frequencies in terms of RF. Also for the parameters, I use the below flag when inserting the kernel module:

tenxfsc (0 or 1, default 0)
By default, cxadc captures at a rate of 8 x fSc (8 * 315 / 88 Mhz, approximately 28.6 MHz). Set this to 1 to capture at 10 x fSc (approximately 35.8 MHz).
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 16 Sep 2020, 01:31 
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It's just a regular resistor, yeah. You would only want to add a resistor if you get a more powerful amp and experience clipping i think, your signal is already weak. I added the resistor to the 4300 because i needed it for testing with the LimeSDR setup. I don't have a resistor at all on the 1050. The little amp i have adds more gain depending on voltage applied, up to 12 volts. Attaching a 9v battery was easy, so that is what i did.

Maybe that player you have just outputs low RF?

You mentioned before you had an SDR. While you probably can't use it to capture, you might be able to use it to monitor the RF signal in real time using gnuradio companion. I've done that quite a bit, testing out which things improve or degrade the signal strength.

you could turn up the RF gain too, just mark where it is before you turn it, in case it has a negative effect, then you can set it right back to where it is if it doesn't help, while checking the test point signal strength.
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 16 Sep 2020, 16:21 
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I have an RTL_SDR and HackRF One. Unfortunately in this case the HackRF is only 20MSPS max which may work for nyquist, but as stated before the minimum capture bandwidth suggested is 12.5MHz, which is too much. It's also not as sensitive as my RTL_SDR.

The kicker here is neither is going to work at all due to the minimum tunable frequency of both devices. Can you upload your 28.6MSPS sample using the amplifier from eBay?
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 Post subject: Re: (WIP) Laserdisc software image decoder from raw signal
PostPosted: 17 Sep 2020, 17:40 
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titan91 wrote:
Chad, would it be possible to add a feature to ld-analyse to export the frames of a TBC file as a series of PNGs? There's already a single frame export feature, hopefully it wouldn't be hard to add this in as another option. Ld-chroma-decoder does the job, but it outputs raw bitmap files that must be further muxed and processed by ffmpeg. This extra option would really help with the learning curve and speed of conversion I feel.


This would probably be "really" low priority from them, because, as you said, you can already do this with a longer tool chain command line. I wrote a perl script that basically does this, download that. or here is the direct code:
#!/usr/bin/perl
$staticdiv = 1500000;  #very crude aproximation of frame size
if (!$ARGV[0]){
        print 'Must provide file name, number of pics, pal/ntsc as input\n';
        exit 0;
}
chomp($ARGV[1]);
$filesize = `ls -ail $ARGV[0]|awk '{print \$6}'`;
$frames = int($filesize / $staticdiv); #rough count of number of frames
for ($x=5; $x < $frames; $x=$x + int($frames / $ARGV[1])) {
                print "FRAME:$x\n";
                if ($ARGV[2] =~ /pal/ ) {
                    $command = qq(ld-chroma-decoder -s $x -l 1 $ARGV[0] -|ffmpeg -f rawvideo -r 25 -pix_fmt rgb48 -s 928x576 -i - -an -vf "yadif,scale=768:576" -y $ARGV[0]$x.png);
                    $output=`$command`;
                }else{
                    $command = qq(ld-chroma-decoder -s $x -l 1 $ARGV[0] -|ffmpeg -f rawvideo -r 30000/1001 -pix_fmt rgb48 -s 760x488  -i - -an -vf "yadif" -y $ARGV[0]$x.png);
                    $output=`$command`;
                }
}


USAGE: getpics.pl input.tbc 6 ntsc
That would output 6 .png files from input.tbc spaced equally throughout the capture. ntsc is default, so you could technically leave that off.
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