Re: V-Box

From: Richard Atkinson (
Date: 2000-09-02 13:45:28

On Fri, 1 Sep 2000, Marko Mäkelä wrote:

> Could you tell us more about the SECAM C64?  Is all the magic in the RF
> modulator, or does the also main circuitry differ from the PAL C64?

The SECAM C64 uses a standard Rev B 12V circuit board. A large hole is
drilled through the PCB in one of the ground planes and a patch lead
soldered onto the solder side of the board in various places. There are
some track cuts on the PCB too. The patch lead ends in a header which is
plugged into a SECAM encoder board. This board does not appear to have
been manufactured by Commodore. The board is screwed to the underside of
the keyboard using two long versions of the normal keyboard screws. The
PCB is not in fact held flush with the black keyboard rail but rests on
several large components on the board instead. Overtightening of the long
screws could have disastrous consequences!

I believe the board works by decoding the PAL signal to RGB and then
re-encoding to SECAM. It could work by decoding to YUV but the board has a
PAL delay line and I think the chips work with RGB. There are absolutely
LOADS of adjustments on the board which again suggests complete RGB
decoding and encoding. The signal that passes to the SECAM encoder
therefore has all the PAL colour mixing artifacts present in a PAL TV.
This is then encoded as SECAM by missing out R-Y and B-Y alternately, and
a proper SECAM decoder has a 1 line delay (not required to be as accurate
as a PAL delay) to return the missing colour signal from the line above on
each line. Because the C64 outputs an even number of lines, the SECAM
lines assigned to R-Y and B-Y are static, thus SECAM encoding artifacts
can also be seen on top of PAL encoding artifacts.

Vertical colour edges would normally bleed 1 line on a PAL machine;
however on the Commodore SECAM C64 (when displayed on a proper SECAM TV)
the colour edges sometimes bleed 1 line, other times 2 lines, depending on
the colour. This can be seen quite easily when viewing blue characters on
a black background compared with red characters on a black background. In
truth, almost all colours necessarily bleed 2 lines, but the strength of
the 2nd line of colour bleed depends on the amplitudes of R-Y and B-Y on
that line and the line above, and can be so small as to not be visible. Or
it can be as strong as the 1st line of colour bleed, depending entirely on
the colour at those points. SECAM artifacts are very strange things

The horizontal colour resolution on my setup is *absolutely appalling*,
but this may be due to my TV using poor SECAM decoding techniques rather
than the C64 itself. Having said that, SECAM uses FM for its colour
carrier rather than AM, which stretches the FM system to its limits in
this application. I would not be at all surprised if the horizontal colour
resolution of the SECAM C64 is poor due to the inherent weakness in the
SECAM encoding system. Why they didn't just use non-quadrature AM is
beyond me...


Richard Atkinson
Software Engineer
Tenison Technology EDA Ltd

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