Re: C16/Plus4

From: Richard Atkinson <>
Date: Tue, 1 Nov 2011 10:55:17 -0000
Message-ID: <EA9860B9C91D43F5AA19F76731E28664@abion>
I have a C64 with the Procep PAL to SECAM transcoder.

This board works by decoding the PAL signal from the 6569 VIC-II chip, 
including delay line averaging of adjacent lines of chrominance, and then 
re-encoding it as SECAM. SECAM encoding works by alternately throwing away 
one of R-Y and B-Y and encoding the other as an FM signal. The receiver has 
a one line store (another delay line) which it uses to decode SECAM back to 
RGB using R-Y from the current line and B-Y from the previous line, and vice 

This means the C64 display using Procep's transcoder has both PAL artifacts 
(so PAL colour mixing works) and SECAM artifacts on top of those. The 
transcoder board has two adjustments on the SECAM side of things to set the 
FM carrier centre frequencies for R-Y (Dr) and B-Y (Db). These two carriers 
are defined separately in the SECAM standard(s), but they are not defined as 
accurately as PAL or NTSC subcarriers, so the Procep transcoder has two 
tuned circuits rather than crystals.

It's difficult to be sure how this system would have looked on a true French 
SECAM television because I don't have one, but on the multistandard Sony I 
have here it looks awful! Not because of the SECAM delay line artifacts, but 
because the chrominance signal is very soft and has lost much of its high 
frequency definition. I think the FM encoding system needs greater bandwidth 
than AM (or QAM) for the same ~1MHz chrominance bandwidth. There's also a 
lot of noise and the colour decoding drifts in and out of operation, 
possibly as the tuned circuits drift.

There are also VIC 20s with this PAL to SECAM transcoder circuit, and C64s 
with a PAL to RGB (RVB - rouge vert bleu) decoder board. The PAL to RVB 
boards are a lot smaller and give similar picture quality to a PAL S-video 
connection, because the PAL to RVB board has access to the S-video signals 
internally. Those PAL to RVB C64s must be the only C64s with RGB signals on 
their video sockets.


From: "Anders Carlsson" <>
Sent: Monday, October 31, 2011 10:40 AM
To: <>
Subject: Re: C16/Plus4

> Didier Derny wrote:
>> In 1984/1985 it was a problem, no room left to insert a quite large pal 
>> to SECAM interface.
> On the topic of SECAM, is it true that a C64 equipped with PAL to SECAM 
> interface outputs a worse RGB picture than what S-Video would do on a PAL 
> monitor?
> Perhaps if the TED chip had built-in RGB output, it would have been more 
> popular in France etc, but then again on a cheap low-end European market, 
> not too many users in 1983/84 would have a TV with RGB SCART input 
> anyway.. at least not outside France. However it is true that e.g. Acorn 
> Electron has a TTL RGB output just like its big brother, the BBC Micro. 
> For some reason, I consider the Commodore 16 and Acorn Electron classmates 
> in terms of release date, capacities and relation to more expensive 
> machines (Commodore 64 and BBC Micro, although of course the Plus/4 is 
> more directly a big brother to the C16).
> Best regards
> -- 
> Anders Carlsson
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Received on 2011-11-01 11:00:37

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