May 27, 2014

Bitmaps and Fonts

Filed under: Hobby Electronics, Raspberry 3.14, linux — Tags: , , , — admin @ 5:54 am

A 10×10 chars bitmap font ( 100×100 px , .bmp )

A 16×16 chars bitmap font ( 160×160 px , .bmp )
For easy display of hexadecimal 0..255 using a single char per byte.

Here is a sample of the resulting look of a HiM using SDL on the raspberry PI :


December 14, 2012

Videopac C-52 : The SCART way ( R.G.B peritel Mod )

Filed under: Hobby Electronics — Tags: , , , , , , , , , , , — admin @ 4:39 pm

Inside the videopac c-52 ( bottom cover removed ) :

The VHF modulator is the board at the bottom of the picture.
It has to be replaced with a little electronic board, to convert the signals incoming from the main cpu ( the one at the center).

The RGB adapter board takes Red, Green, Blue , Composite synchronisation, Luminance, and BF (audio) signals from the CPU.

  • The composite sync signal has to be inverted before it can be sent to the TV set thru the scart (peritel) plug. To do so, we pick up the Vertical and Horizontal Sync signals on the pcb (they are allready inverted at that particular points) then use a classic diode based OR gate.
  • R,G,B and sound signals have to be lowered before getting into the scart.
  • RGB signals, also, have to be mixed with the luminance signal, to get the correct RGB levels at the peritel’s intputs (in the right proportions ).Depending on the combination of said color signal added to the luminance signal , the RGB output voltage will have the following values : 0mv, 200mv, 400mv , 600 mv (approximatively)
  • The RGB signals have to be blanked during the Horizontal blank and Vertical Blank time.
    That point is important ::
    The front porch/back porch for lines is necessary. If the RGB signal is not correctly blanked, the TV set will not show permanently coloured lines ( for example, a blue background of a game ).
  • Pin 8 of the scart can be left ‘floating’, but if you want your TV set to automaticaly switch to the scart, you ‘ll have to put a 12V to that pin. (Unfortunatly, it’s not present at the pins on the edge of the CPU board. Only 5V is available there.) So you will have to turn your TV to ’scart’ mode manually with the remote, or add a diode-pump doubler electronic assembly ( for example you could use the line sync as the working clock for it …)
  • Pin 16 of the scart must be set to the correct voltage to switch the TV to RGB input instead of Composite Video input .
    If  pin 16 is driven between 1 and 3V , the TV set will switch to RGB input. (good )
    If pin 16 is lower than 1V , the TV set will switch to Composite Video Input (bad)
    ->> We have 5V coming from the videopac’s CPU board, then a simple Resistor bridge will be sufficient to lower that input voltage to ~3V .
  • For the audio part, a resistor bridge (divider) is enough to lower the level of the Audio coming from the CPU board then feed it into pins 2 and 6 of the SCART plug ( left + right will play the same MONO signal coming from the videopac ), while pin4 (audio ground) can be joined to the other ground pins.

All scart’s pin name terminated with ‘ground’ can be joined to a common ground, connected to the ground of the Videopac’s CPU board , allthough in about all TV sets, that pins are tied together inside the TV, at pcb level.

Now, here is the schematic  of the C-52’s CPU board, viewed from solder side (bottom cover removed).
I have added important signal’s names for the CPU on this figure.

Now, for information, the SCART plug pinout ( VGA pinout here for comparison ) ( french named PERITEL )

The RGB adapter can be built on a prototype PCB. that’s what i did to create a correct electronic adapter from scratch. Here is how mine looks : (yes, it looks ugly with some components out, but that was a prototype, remember ! )

That module fits in to the bottom left of the videopac, right in place of the removed uhf modulator :

And finally, the result you get : the RGB adapter @ work . (the moire is not visible, actually. it’s just a photo shoot artefact ). The Colours are vivid and fine ! let’s play…

Following a request , here is the .PDF file of the schematic :
Hope this will help you into moding the Videopac: rgb_videopac_c52_adapter-78v (pdf)

  • Notice : IC4 ( U4:A on the schematic ) , on my pcb version, was replaced by a 2 x 1n4148 diodes based OR gate, so the final circuit has only 2 TTL IC on it, and VSYNC/HSYNC signals are taken from IC669 instead of IC652 , as you can see on the picture of the cpu’s bottom pcb.
  • Also, IC652-pin27’s connection (sound) is directly available at P6-3, as you may have noticed.
Finally, here is a partial schematic , extracted from the JET25 service manual. the pcb/IC , as i can judge, is similar to the one on the C-52.It may help you understanding the how-why of the SCART adapter described on this website.


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