Re: Procep SECAM Re: C16/Plus4

From: didier derny <>
Date: Wed, 02 Nov 2011 16:50:36 +0100
Message-ID: <>
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 16:00:28

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