Do you know whether it would be feasible to decode an 15 kHz RGB signal (from a video game console) using a sound card ?

Providing such input can be captured finely enough by a 192 kHz sound card, the decoding process would be done by a software.

I've been trying to find out more information about the encoding of such format such as these diagrams for PAL encoding but I couldn't find any. The only thing that I'm sure about is that since a CVBS signal carrier runs at about 6 MHz there's absolutely no hope of feeding a sound card with it since it is way beyond 192 kHz. But no idea about an RGB signal coming out from the SCART. (a simple thought tends to make me think it would be possible : it runs at 15 kHz)

My questions :

  • Does an RGB signal really runs at 15 kHz ?

  • If yes then would some simple components such as resistors be all that is needed to lower the voltage to a value that is acceptable to the sound card input? Or have I missed something?


Let's have a look at the VGA (RGB) timing. A good intro is here: http://www.xess.com/blog/vga-the-rest-of-the-story/

So, a 'line' takes 25.17us. Assuming, say, a 800x600 picture, that's 25.17/800 us per pixel. Assuming an ideal Shannon limit, you'd need to sample at at least half that time to recover the signal. So you sampling frequency needs to be at least 1/(12.6e-6/800) Hz, which is 63MHz.

From Wikipedia:

For most common VGA mode 640x480 "60 Hz" non-interlaced the horizontal timings are:

  • Pixel clock frequency 25.175 MHz
  • Horizontal frequency 31.4686 kHz

So, not going to work I'm afraid.

  • 1
    \$\begingroup\$ Sorry - forgot the /800 on my first go... See revised answer. \$\endgroup\$
    – RJR
    Apr 17 '14 at 0:18
  • \$\begingroup\$ Ah yes right I forgot that this value is about the horizontal frequency X) Thank you :D \$\endgroup\$
    – aybe
    Apr 17 '14 at 1:18
  • \$\begingroup\$ Extrapolating down - you might get away with it if you lower your resolution to 3x2 (pixels, that is...) :-) \$\endgroup\$
    – RJR
    Apr 17 '14 at 3:17

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