Re: Procep SECAM Re: C16/Plus4

From: Richard Atkinson <>
Date: 02 Nov 2011 17:42:49 +0000
Message-ID: <>
Bonjour Didier,

I remember being astonished at the complexity of the Procep PAL to SECAM
transcoder. Procep had to drill a hole in the 250407 REV B C64
circuit board and make several track cuts and solder many wires to the
board. It would have been a huge job and it had to be done to every single
C64 SECAM they sold. It must have added enormously to the cost of the C64 in

Do you have any info on why the VIC 20 (cost reduced version) was available
only in SECAM versions whereas the C64 was available in both SECAM and RVB

To do a SECAM version of the VIC-II would have been quite impractical, if 
the Atari FGTIA is anything to go by. The best option probably was to do a 
board level conversion, i.e. something like Procep's PAL to SECAM 
transcoder board integrated onto the same PCB as the Commodore 64 itself. 
This would have required the delay line and most of the other components on 
the Procep board, just without the hole drilling, track cutting and extra 


On Nov 2 2011, didier derny wrote:

>I'm not a specialist of video, I just remember that Commodre said they 
>could not produce a
>VIC II Secam due tu the delay line.
>we hoped that we would get a secam vic II chip, converting the vc20 and 
>c64 to secam
>was not an easy task, first we had to find a technology not to expensive
>(some converter already existed but almost at  the price of a C64.)
>The PS200 was the first solution found but pour quality and relatively 
>then the integrated secam board less expensive, better quality but 
>needing a modification
>of all c64 sold.
>I was not  a specialist of the C64 either, I juste used it in 1984/85 to 
>a minitel emualtor for the C64  (written in aztec C and 6502 assembly)
>Le 02/11/2011 14:12, Richard Atkinson a écrit :
>> You needed glass delay lines to decode PAL or SECAM, not to encode 
>> them. The
>> two Procep decoders contain PAL delay lines because they first have to
>> decode PAL before they can output RVB or encode SECAM. The glass delay 
>> line
>> is the light green coloured box near the TDA3510 chip in the Procep 
>> boards.
>> This is not something that could be integrated, it's a large analogue 
>> part.
>> BTW I found pictures of the Procep PS 2000 external PAL to SECAM 
>> transcoder
>> box. They're on, which you have to register for to see them
>> (title C64 externer Secam Decoder von Procep). One of the pictures 
>> shows the
>> circuit board inside the PS 2000. It has a 220V power supply, a TDA PAL
>> decoder chip and a glass delay line, a SCART socket and - curiously - 
>> what
>> looks like two ASTEC RF modulator boxes. It's difficult to see for 
>> sure, but
>> it looks like both RF modulators are coupled onto the same RF output.
>> France's SECAM-L system for RF channels is rather unusual, using
>> positive modulation for the AM vision carrier (instead of negative
>> modulation) and AM for the sound carrier (instead of FM). I wonder if 
>> there
>> wasn't a native System L RF modulator available at the time, and what we
>> have here instead is two separate modulators being used for the vision 
>> and
>> sound aspects of SECAM-L.
>> The big problem with how you encode SECAM is not delay lines but how you
>> generate accurate enough FM chrominance carriers. By contrast, both 
>> NTSC and
>> PAL use QAM (quadrature amplitude modulation) where both the phase and 
>> the
>> amplitude of a carrier carries information. The key thing here is that 
>> the
>> frequency never changes, therefore you can have crystal generated 3.58MHz
>> and 4.43MHz carriers. There's a Commodore patent on how the VIC-II chip
>> achieves NTSC and PAL chrominance signals; basically it takes 4x 
>> subcarrier
>> frequencies (the 14.31818MHz and 17.73447MHz crystals we know and 
>> love) and
>> divides them by four, using both rising and falling edge triggered flip
>> flops, to generate four phases of subcarrier (0, 90, 180 and 270 degrees)
>> which can then be added in pairs using different weights to produce the
>> VIC-II colours. (Incidentally as yet another aside, I wonder how the
>> 6560/6561 VIC-I chrominance encoding works, as it predates this 
>> patent, and
>> in the case of the 6561 it only receives PAL subcarrier frequencies at 1x
>> subcarrier frequency)
>> Back to SECAM. To generate SECAM chrominance subcarriers, you have to
>> generate two FM carriers, each on alternate lines. The frequency of these
>> carriers varies according to the information they are encoding, therefore
>> you can't use a crystal with a fixed frequency. There is a SECAM 
>> version of
>> the Atari 800 colour chip GTIA, called FGTIA, which shows how Atari 
>> did it
>> using 1984 technology. A block diagram is on page 23 of the PDF and the
>> explanatory text starts on page 18.
>> The FGTIA chip has seven (!) external outputs to control an external 
>> voltage
>> controlled oscillator (VCO) as part of a phase locked loop (PLL) circuit.
>> Three of them are a colour value output, to produce different frequencies
>> from the VCO, and four are control signals. One of the luminance 
>> outputs of
>> the GTIA had to be removed in order to free up pins for the VCO 
>> interface,
>> so the SECAM Atari 800/XL/XE only has eight luminance levels (128 
>> colours)
>> rather than the sixteen (256 colours) of the NTSC and PAL GTIAs. Other
>> inputs on the original GTIAs were multiplexed to free up pins (diagram on
>> page 30, explanatory text on page 29). It clearly required a lot of
>> re-engineering!
>> Richard
>> --------------------------------------------------
>> From: "Segher Boessenkool" <>
>> Sent: Wednesday, November 02, 2011 2:16 AM
>> To: <>
>> Subject: Re: C16/Plus4
>>>> Apparently after what I've heard, integrating the delay lines was a  
>>>> huge problem
>>>> Perhaps that at that time it was not something that could easy done.
>>> A digital delay line would add 50% to the die area, and that's a
>>> low estimate.  Not going to happen :-)
>>> Segher
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Received on 2011-11-02 18:00:04

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