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 expensive) 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 write 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 forum64.de, 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. > > http://ftp.pigwa.net/stuff/collections/nir_dary_cds/Tech%2520Info/FGTIA.PDF > > > 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" <firstname.lastname@example.org> > Sent: Wednesday, November 02, 2011 2:16 AM > To: <email@example.com> > 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 >> >> >> Message was sent through the cbm-hackers mailing list > > > Message was sent through the cbm-hackers mailing list Message was sent through the cbm-hackers mailing listReceived on 2011-11-02 16:00:28
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