Dragon32Issue2aSecam

Dragon 32 Issue 2a (SECAM) Reproduction

This repository contains the KiCad project files to reproduce the SECAM variation of the Dragon 32 Issue 2a main board.

This design requires the SECAM power board to operate correctly and will not work with the standard board. See https://github.com/jimbro1000/Dragon32PowerSecam

Notes

The board incorporates some factory bodges to simplify assembly efforts.

A front and back ground plane fill has also been added along with effort to make sure that a ground plane is present across most of the board.

Video Circuit Changes

Compared to the original Dragon 32 (in PAL or NTSC form) the SECAM version of the design requires a significant level of additional work to generate a viable composite signal.

The most significant change is that the board does not make use of a modulator chip (the LM1889N is used on the PAL version). Instead the generation of the FM based composite signal is generated through discrete components.

The board is effectively split into two areas. The regular PAL design (with a few small tweaks) and the SECAM composite generation - located in the north section of the PCB.

Power Board Changes

While the same 9 wire interconnect is present on both designs, the SECAM version repurposes the 5 non-voltage connections making the power board incompatible with the PAL/NTSC main board and the SECAM main board incompatible with the PAL/NTSC power board.

The original pin out provided ground, composite, audio, 5v and a bias voltage.

The SECAM board uses the same pins as audio, composite, luma, phiA and phiB (the last 3 forming a YPrPb signal). By passing the raw output of the VDG to the power board there is space to perform additional transformation into a genuine RGB output that is sorely missing from the original.

RGB Calculation

The transformation is performed in two stages - first to homogenise the signal levels of the VDG output (while it broadly follows the YPrPb format it doesn't meet the requried signal voltages). The second stage performs some analogue computation to create R-Y, G-Y and B-Y components. These then can be added to the homogenised Y signal to create the three R, G and B signals.

An exta calculation is needed to isolate the sync signal that appears on the G channel.