9 ms·

Capturing PAL video with an SDR, and a few dead ends

113 points by 19870213 7y ago

TazeTSchnitzel 7y ago
> [PAL colour] was designed in the 1960s to be backwards compatible with black-and-white TV receivers. Fun fact: unlike for NTSC, this technically isn't true, because there is not a preceding black-and-white format that PAL was compatible with. Rather, PAL's black-and-white format (the line count and refresh rate) was defined at the same time as the colour standard. They chose their number of lines (625) to avoid having to do NTSC's 29.97fps thing. Or so I think I have heard. Please correct me if I am wrong.
- makomk 7y ago
  It's complicated and I don't know the full history, but the reason it's 625 lines is so that the horizontal refresh rate is approximately the same as for NTSC, allowing monochrome sets to be relatively easily converted. Also, 625-line television was developed at roughly the same time as early colour but I'm not sure they necessarily arrived at the same time depending on the country.
  - EvanAnderson 7y ago
    Standupmaths has a good video on this: https://www.youtube.com/watch?v=3GJUM6pCpew https://www.youtube.com/watch?v=3GJUM6pCpew
  - TazeTSchnitzel 7y ago
    That might explain why 60Hz PAL modes (on games consoles etc) work so well.
- interfixus 7y ago
  Ah, memories! I was a teenager in Denmark in the seventies. We easily picked up TV from both of the two Germanies. The channels from the west were sleek and modern in full color, while those from the DDR were in lifesapping greyscale ("black and white", as some like to call it). I knew this was an artifact from the SECAM -> PAL conversion, but it still managed perfectly well to reinforce our hazy notions about the Eastern Bloc as a strange, grainy, not-quite-real kind of place - a slightly subconscious impression many of my age will no doubt recognize. And then there were the occasional American clips on the evening news, and sometimes even whole shows that noone had bothered to give a quality conversion: Poor resolution and loud, garish, oversaturated colors. This I also knew to be a conversion artifact, but just like the eastern greyscale, it tended to reinforce certain preconceived notions. Perception is an interesting thing to hack.
codys 7y ago
In reading this I'm reminded of some efforts to produce a software decoder for Laserdiscs by using a ADC sample the exact data on the disk [1] 1: https://www.domesday86.com/?page_id=978 https://www.domesday86.com/?page_id=978 I wonder if there's some way to tweak the now seemingly common SDR adapters to provide a plain ADC (ie: without demodulation). When folks are capturing old consoles, generally the RF output is avoided because of the extra noise injected. It would be ideal to work directly with the composite video. Working with the composite video also would have implications for archiving VHS and other tape-based analog footage: right now, folks tend to use hardware devices called TBCs (time base correctors) to correct issues in the video signal from a VHS player. These are all obsolete at this point and growing increasingly expensive, so it would be nice to replace this hardware with software.
- makomk 7y ago
  I'm not sure this is terribly practical for console capture unless you're a signal processing hacker who really doesn't want to go out and buy a capture device - decent consumer-level capture devices use some really fancy, carefully-optimized filtering to produce high quality pictures with minimal artifacting, and it'd probably take quite a bit of development and CPU time just to match the off the shelf stuff.
  - Maxious 7y ago
    A reasonably priced and good quality bodge is described in "The Best Easy Way to Capture Analog Video (it's a little weird)" https://www.youtube.com/watch?v=ZC5Zr3NC2PY https://www.youtube.com/watch?v=ZC5Zr3NC2PY
    - Mathnerd314 7y ago
      So he has a $25 RCA-to-HDMI and a $70 HDMI capture card for around $100. A fully assembled Domesday board is also somewhere $100 according to https://forum.lddb.com/viewtopic.php?p=100903&sid=a582ac9ebcad2b37d24930ac444a2311#p100903 https://forum.lddb.com/viewtopic.php?p=100903&sid=a582ac9ebc..., although I guess it could be higher without a good PCB supplier. The article's Airspy R2 SDR is $169. There's a PAL decoder in https://github.com/happycube/ld-decode/blob/3d5cdeddbaf4515b41d0dff2c43faa51d649c132/tools/ld-comb-pal/palcolour.cpp#L50 https://github.com/happycube/ld-decode/blob/3d5cdeddbaf4515b..., I'm guessing it could be adapted to the article's purposes with some work. https://twitter.com/windyoona/status/1163043586314821635 https://twitter.com/windyoona/status/1163043586314821635 says she was looking at it.
MegaDeKay 7y ago
This is the same person that was listening to a clip from a news helicopter on YouTube and heard some "weird interference" in the audio. She went on from there to do some audio processing and eventually realized it was the chopper transmitting some information. That information was its Latitude & Longitude, after which she plotted its flight path on a map. http://www.windytan.com/2014/02/mystery-signal-from-helicopter.html http://www.windytan.com/2014/02/mystery-signal-from-helicopt... Windytan has hearing that puts a bat's to shame.
- jacquesm 7y ago
  Anecdotally, there are people that can pick up RTTY by ear.
- icebraining 7y ago
  Oona is great, and her blog is the perfect example why I fid RSS readers indispensable: she posts four or fives times per year at random intervals, yet each post is worth dozens of average articles. The "firehose of information" model is simply not adequate for these cases.
- stevekemp 7y ago
  I remember when I was a young student I lived next to a park, and I could hear bats! No longer the case, and not for a long time. I've loved this blog though, since coming across it somewhat randomly - looking into decoding the radio-broadcasts that update the Helsinki tram-displays. I never got it to work, despite a few attempts, but it is still something I'd like to come back to. I did get sucked into the world of decoding random 433Mhz transmissions, and doing protocol analysis with ESP8266 devices though. So that if the company sauna gets turned on I can now post to slack..
- Narkov 7y ago
  Even crazier, she just used Perl and SoX to extract the data. It's MacGyver-like stuff. Amazing.
bt848 7y ago
This is interesting as it highlights how easily we lose arts. You could tune PAL video on cheap Brooktree ASICs 25 years ago, and code for decoding PAL video on the USRP has been available forever.
- makomk 7y ago
  Brooktree was effectively spun off as Conexant, who made direct successors to many of those ASICs for quite a while including updated single-chip USB and PCIe versions. I don't think they're still around now that analog broadcast is dead though.
kevingadd 7y ago
Really cool experiment. I'd love to see how much the performance could be improved by offloading a bunch of the filtering to the GPU, though I suspect that might be a bit tricky due to driver constraints (and in particular how bad OS X drivers can be). Typically pixel shaders are quite capable of doing filtering and convolution even if you aren't ready for compute shaders - a mid tier GPU can handle doing filters with 29 taps or more per pixel pretty easily at 60hz. Ideally that'd get the CPU and power usage way down, freeing up CPU for things like streaming.
PunksATawnyFill 7y ago
All that, and never defined what "SDR" is. Fail.

113 points by 19870213 7y ago

> [PAL colour] was designed in the 1960s to be backwards compatible with black-and-white TV receivers. Fun fact: unlike for NTSC, this technically isn't true, because there is not a preceding black-and-white format that PAL was compatible with. Rather, PAL's black-and-white format (the line count and refresh rate) was defined at the same time as the colour standard. They chose their number of lines (625) to avoid having to do NTSC's 29.97fps thing. Or so I think I have heard. Please correct me if I am wrong.

makomk 7y ago
It's complicated and I don't know the full history, but the reason it's 625 lines is so that the horizontal refresh rate is approximately the same as for NTSC, allowing monochrome sets to be relatively easily converted. Also, 625-line television was developed at roughly the same time as early colour but I'm not sure they necessarily arrived at the same time depending on the country.
- EvanAnderson 7y ago
  Standupmaths has a good video on this: https://www.youtube.com/watch?v=3GJUM6pCpew https://www.youtube.com/watch?v=3GJUM6pCpew
- TazeTSchnitzel 7y ago
  That might explain why 60Hz PAL modes (on games consoles etc) work so well.
interfixus 7y ago
Ah, memories! I was a teenager in Denmark in the seventies. We easily picked up TV from both of the two Germanies. The channels from the west were sleek and modern in full color, while those from the DDR were in lifesapping greyscale ("black and white", as some like to call it). I knew this was an artifact from the SECAM -> PAL conversion, but it still managed perfectly well to reinforce our hazy notions about the Eastern Bloc as a strange, grainy, not-quite-real kind of place - a slightly subconscious impression many of my age will no doubt recognize. And then there were the occasional American clips on the evening news, and sometimes even whole shows that noone had bothered to give a quality conversion: Poor resolution and loud, garish, oversaturated colors. This I also knew to be a conversion artifact, but just like the eastern greyscale, it tended to reinforce certain preconceived notions. Perception is an interesting thing to hack.

codys 7y ago

In reading this I'm reminded of some efforts to produce a software decoder for Laserdiscs by using a ADC sample the exact data on the disk [1] 1: https://www.domesday86.com/?page_id=978 https://www.domesday86.com/?page_id=978 I wonder if there's some way to tweak the now seemingly common SDR adapters to provide a plain ADC (ie: without demodulation). When folks are capturing old consoles, generally the RF output is avoided because of the extra noise injected. It would be ideal to work directly with the composite video. Working with the composite video also would have implications for archiving VHS and other tape-based analog footage: right now, folks tend to use hardware devices called TBCs (time base correctors) to correct issues in the video signal from a VHS player. These are all obsolete at this point and growing increasingly expensive, so it would be nice to replace this hardware with software.

makomk 7y ago
I'm not sure this is terribly practical for console capture unless you're a signal processing hacker who really doesn't want to go out and buy a capture device - decent consumer-level capture devices use some really fancy, carefully-optimized filtering to produce high quality pictures with minimal artifacting, and it'd probably take quite a bit of development and CPU time just to match the off the shelf stuff.
- Maxious 7y ago
  A reasonably priced and good quality bodge is described in "The Best Easy Way to Capture Analog Video (it's a little weird)" https://www.youtube.com/watch?v=ZC5Zr3NC2PY https://www.youtube.com/watch?v=ZC5Zr3NC2PY
  - Mathnerd314 7y ago
    So he has a $25 RCA-to-HDMI and a $70 HDMI capture card for around $100. A fully assembled Domesday board is also somewhere $100 according to https://forum.lddb.com/viewtopic.php?p=100903&sid=a582ac9ebcad2b37d24930ac444a2311#p100903 https://forum.lddb.com/viewtopic.php?p=100903&sid=a582ac9ebc..., although I guess it could be higher without a good PCB supplier. The article's Airspy R2 SDR is $169. There's a PAL decoder in https://github.com/happycube/ld-decode/blob/3d5cdeddbaf4515b41d0dff2c43faa51d649c132/tools/ld-comb-pal/palcolour.cpp#L50 https://github.com/happycube/ld-decode/blob/3d5cdeddbaf4515b..., I'm guessing it could be adapted to the article's purposes with some work. https://twitter.com/windyoona/status/1163043586314821635 https://twitter.com/windyoona/status/1163043586314821635 says she was looking at it.

MegaDeKay 7y ago

This is the same person that was listening to a clip from a news helicopter on YouTube and heard some "weird interference" in the audio. She went on from there to do some audio processing and eventually realized it was the chopper transmitting some information. That information was its Latitude & Longitude, after which she plotted its flight path on a map. http://www.windytan.com/2014/02/mystery-signal-from-helicopter.html http://www.windytan.com/2014/02/mystery-signal-from-helicopt... Windytan has hearing that puts a bat's to shame.

jacquesm 7y ago
Anecdotally, there are people that can pick up RTTY by ear.
icebraining 7y ago
Oona is great, and her blog is the perfect example why I fid RSS readers indispensable: she posts four or fives times per year at random intervals, yet each post is worth dozens of average articles. The "firehose of information" model is simply not adequate for these cases.
stevekemp 7y ago
I remember when I was a young student I lived next to a park, and I could hear bats! No longer the case, and not for a long time. I've loved this blog though, since coming across it somewhat randomly - looking into decoding the radio-broadcasts that update the Helsinki tram-displays. I never got it to work, despite a few attempts, but it is still something I'd like to come back to. I did get sucked into the world of decoding random 433Mhz transmissions, and doing protocol analysis with ESP8266 devices though. So that if the company sauna gets turned on I can now post to slack..
Narkov 7y ago
Even crazier, she just used Perl and SoX to extract the data. It's MacGyver-like stuff. Amazing.

bt848 7y ago

This is interesting as it highlights how easily we lose arts. You could tune PAL video on cheap Brooktree ASICs 25 years ago, and code for decoding PAL video on the USRP has been available forever.

makomk 7y ago
Brooktree was effectively spun off as Conexant, who made direct successors to many of those ASICs for quite a while including updated single-chip USB and PCIe versions. I don't think they're still around now that analog broadcast is dead though.

kevingadd 7y ago

Really cool experiment. I'd love to see how much the performance could be improved by offloading a bunch of the filtering to the GPU, though I suspect that might be a bit tricky due to driver constraints (and in particular how bad OS X drivers can be). Typically pixel shaders are quite capable of doing filtering and convolution even if you aren't ready for compute shaders - a mid tier GPU can handle doing filters with 29 taps or more per pixel pretty easily at 60hz. Ideally that'd get the CPU and power usage way down, freeing up CPU for things like streaming.

PunksATawnyFill 7y ago

All that, and never defined what "SDR" is. Fail.