Pal decoder

Posted on Feb 7, 2014

After some years i started using the NMS8280 again for HAM radio purpose. When sending a signal in the air, radio amateurs should always identify by giving their callsign, this is also the case for sending a video signal. I wrote a program using the WBASS assembler which shows a scrolltext, this can be downloaded here. Unfortunately the incoming

Pal decoder
Click here to download the full size of the above Circuit.

video signal is first converted to RGB, and converted back later after superimposing. The PAL decoder in this MSX is not very good, the encoder to convert RGB to CVBS is quite okay. At first sight the picture improved by placing a capacitor over the filter just before the PAL decoder, this filter separates the colour (c) (4. 43MHz) signal from the luminance (y) signal. For the y path, the video signal is passed through a lowpass filter which starts attenuating around 3MHz. This gives loss in sharpness. I tried to improve this filter, but it`s not easy to keep the delay of the y and c path equal, and too high frequencies will give aliasing while digitizing. Another work around is to simply switch between the unconverted inputsignal and the signal coming from the MSX video processor after the PAL encoder. This can be done by a 4053 for example. Normally the internal PAL encoder from the MSX will switch between the RGB signal of the MSX and the RGB version of the incoming video signal. The MSX video processor (VDP) determines when to display the internal or external signal. When this signal is used to switch the CVBS signals with the 4053, we are done for 80%. The select signal from the VDP is related to the RGB output of the VDP, after converting to CVBS the signal will be delayed for about 200ns. A 74HC74 is used to delay this digital select signal 2 clock cycles of the DHCLK of the VDP, otherwise the 4053 would switch too...

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