An interactive look at how a video signal is made

Cool thing about analog video was the ability to cheaply store a Lot of data using common VHS deck. Japanese started recording Digital PCM on Video tapes in 1977, long before CDs. This is why CD audio has 44.1 sampling rate - common divisor for fitting 3 samples per line of video in both 50 and 60 field rates.<p><a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=DLS9sQUxVlI" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=DLS9sQUxVlI</a><p><a href="https:&#x2F;&#x2F;youtu.be&#x2F;-ZJmQQ9OCtI?t=7240" rel="nofollow">https:&#x2F;&#x2F;youtu.be&#x2F;-ZJmQQ9OCtI?t=7240</a><p>In 1992 enterprising Russian company released a line of ISA controllers turning VHS into a streamer <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;ArVid" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;ArVid</a> up to 4GB per tape at the time 4GB hard drive cost over $1000.

This is pretty cool.<p>For about two to three years now, I&#x27;ve leaned very heavily into analog video signals as a hobby. It culminated into me making not just a software PAL decoder, but my own fully compliant PAL &quot;graphics card&quot; with an FPGA and analog circuitry. PAL might be one of the most complicated analog video signals next to SECAM (though PAL is not that far from NTSC if you understand it well already), and I wanted to really understand every step in it, including all of the math.<p>I of course test everything with the once ubiquitous[1] PM5544 test pattern, which is described in noticeable detail here in case you were every wondering why it looks the way it does: <a href="https:&#x2F;&#x2F;web.archive.org&#x2F;web&#x2F;20190907170308&#x2F;http:&#x2F;&#x2F;www.radios-tv.co.uk&#x2F;Pembers&#x2F;Test-Cards&#x2F;Test-Card-Technical.html#PM5544" rel="nofollow">https:&#x2F;&#x2F;web.archive.org&#x2F;web&#x2F;20190907170308&#x2F;http:&#x2F;&#x2F;www.radios...</a><p>It&#x27;s satisfying to really understand analog TV now. I watched a lot of TV through my life, and always had the thought that while I use it so much, I had no good idea how the TV in front of me actually worked. It&#x27;s also a bit bittersweet, because while generating NTSC or PAL is still useful in existing composite video situations (the motivator for the PAL graphics card was another project where I needed composite output), there are almost no broadcasts left, and it seems rather outdated in general.<p>One day I might make a website that explains how NTSC and PAL work in detailed but understandable way, but I keep putting it off indefinitely.<p>[1] If you are from Europe or other PAL countries. In the US, I think simple SMTPE color bars were more common, and a lot of the elements in the PM5544 test card don&#x27;t matter for NTSC.

This is a great visualization. I remember playing with generating VGA through bit-banging an Arduino - it was my course project for a class in the first year of my PhD. I sadly never wrote up a detailed explanation of it, but a picture of the setup survives as my Twitter header: <a href="https:&#x2F;&#x2F;pbs.twimg.com&#x2F;profile_banners&#x2F;20347234&#x2F;1463010760&#x2F;1500x500" rel="nofollow">https:&#x2F;&#x2F;pbs.twimg.com&#x2F;profile_banners&#x2F;20347234&#x2F;1463010760&#x2F;15...</a><p>The pixel clock (640x480 at 60fps) is supposed to be ~25 MHz, while the little Arduino Nano only runs at 16 MHz, so the implementation cheats a bit and gets the timings somewhat wrong (plus, it smears the pixels a bit, but I hid this by having relatively blocky graphics). Some TVs I tested with didn&#x27;t like this signal much, but luckily the projector that I did the final demo on didn&#x27;t mind the spec violation. For the actual demo I had a simplistic version of Tetris running (with sound!); all the computation happened during hblank and vblank because the main picture time was just a handwritten assembly loop shoving pixels out.

Ben Eater has a great video[1] about building a video card from simple gates and other basic components. It goes into some detail about how a VGA signal is generated.<p>[1]: <a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=l7rce6IQDWs" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=l7rce6IQDWs</a>

I always want to make a nice-looking infograph for Wikipedia on the TV color bars, with colors, labels, and explanations of the staircase waveforms, black level, color burst, etc (basically combining all annotations in a textbook to a single image). Most people have only seen the color bars as an image, but the more interesting aspects can only be seen on a TV waveform monitor, if the signal is properly adjusted, you can see the &quot;staircase&quot; waveform align to the etched mark on the CRT (not a particularly good graph: <a href="https:&#x2F;&#x2F;www.maximintegrated.com&#x2F;content&#x2F;dam&#x2F;images&#x2F;design&#x2F;tech-docs&#x2F;734&#x2F;di39fig06.gif" rel="nofollow">https:&#x2F;&#x2F;www.maximintegrated.com&#x2F;content&#x2F;dam&#x2F;images&#x2F;design&#x2F;te...</a>). Currently, Wikipedia articles on NTSC&#x2F;PAL don&#x27;t have any explanation on how an analog video signal is made. Too bad that I don&#x27;t know anything about image editing (I did export a waveform from the ADS simulator, waiting to be visualized indefinitely).<p>Also, if anyone has a high quality photo of the Philips PM5544 video signal generator, please upload it to Wikipedia. This machine is an important artifact of popular culture, yet photos of the signal generator itself is uncommon on the web (and many people mistakenly believed the PM5544 is just a test card, not a signal generator), but so far there&#x27;s no high-quality photo under a free license. (Or leave a comment if you have the actual machine, I&#x27;d pay $1000 for that. If I ever get the machine, I&#x27;ll take a photo and upload it, and write a blog post about how the circles and lines are drawn by the analog circuitry). Finally, manuals, manuals and manuals: I&#x27;m willing to buy any documents about the Philips PM5544 (or any notable signal generators) to get them digitized. Currently the only document on the web is an issue of <i>Philips Electronic Measuring and Microwave Notes</i> [0] that only briefly mentions a tiny bit of its inner working.<p>[0] <a href="https:&#x2F;&#x2F;frank.pocnet.net&#x2F;other&#x2F;sos&#x2F;Philips_PM5544_PM3400_Publication.pdf" rel="nofollow">https:&#x2F;&#x2F;frank.pocnet.net&#x2F;other&#x2F;sos&#x2F;Philips_PM5544_PM3400_Pub...</a>

Very interesting. I&#x27;m just learning about VGA. On the oscilloscope, I assume the front and back porch is clearly visible on the left and right hand side. What&#x27;s the fuzzy bit on the left, just before the picture starts?<p>Edit: and which bit is the horizontal retrace? The bit on the far left hand side, before the front porch?

What causes the &quot;spikes&quot; in the trace on all the lines? First They&#x27;re regular but in different positions - first I thought it was a grid pattern. If my math works out, it is a ~53kHz pulse.