
Capturing PAL video with an SDR, and a few dead ends - 19870213
http://www.windytan.com/2019/08/capturing-pal-video-with-sdr-and-few.html
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MegaDeKay
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-
helicopt...](http://www.windytan.com/2014/02/mystery-signal-from-
helicopter.html)

Windytan has hearing that puts a bat's to shame.

~~~
icebraining
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.

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codys
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.

~~~
jacobush
The domesday project is much more important IMHO for VHS and Beta captures,
than for console captures. Consoles tend to have a stable timebase.

Tapes tend to, especially if copies were made tape-to-tape, be _very_ jittery.
These could be almost flawlessly fixed in the domesday software, much better
than any TBC.

Since the capture is at the "almost RF" stage early in the tape player, the
overall quality is very good too. There is the potential to view VHS in better
quality than anyone has ever seen it, even back in the day.

~~~
bscphil
Is there anyone using this hardware for VHS or Beta captures? Seems the
original project itself is very targeted, and I'm having difficulty searching
up different uses of the project. I'd considering purchasing one to do high
quality VHS transfer, but I'd want a community I could turn to with problems.

~~~
jacobush
One of the committers have submitted VHS patches, so there is a community of
one so far. :-)

Currently it can decode to black and white I think.

I am very interested also. I bought a high end consumer JVC deck unopened in
box, just for this purpose in the future. But the thing is, only the
mechanical transfer needs to be good, and the heads OK, the rest of the
machine need not be high end at all. It really has the potential to get better
VHS rips than any pro lab today in existence can achieve.

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TazeTSchnitzel
> [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
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
Standupmaths has a good video on this:
[https://www.youtube.com/watch?v=3GJUM6pCpew](https://www.youtube.com/watch?v=3GJUM6pCpew)

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kevingadd
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.

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bt848
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
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.

