Re: Procep SECAM Re: C16/Plus4

Hi Segher,

Going from a hypothetical 625 line NTSC to 625 line SECAM (or 525 line NTSC 
to a hypothetical 525 line SECAM) would not have required a glass delay 
line. It's a component which is required to decode PAL properly. I found a 
picture of one opened up. It's an ultrasonic device which relies on the 
speed of propagation of pressure waves through a medium (in fact quartz).

http://www.flickr.com/photos/anachrocomputer/2840365262/

But the hard part remains providing an accurate FM carrier (or pair of 
carriers). Procep's circuit carries out frequency modulation entirely in the 
analogue domain. The colour difference signals R-Y and B-Y are available in 
analogue form after PAL decoding (including cancellation of phase errors 
using the quartz delay line). These are used to modulate FM carrier 
oscillators. By the look of the Procep board, there are two oscillators, one 
for R-Y and one for B-Y, and a switch selects which output forms the SECAM 
chrominance signal every line.

By contrast, Atari's FGTIA circuit is a hybrid of digital and analogue 
techniques. The FGTIA chip controls a single external FM carrier oscillator, 
using three bits for colour information. Within that three bit encoding is 
the information to distinguish the R-Y lines from the B-Y lines. The 
datasheet acknowledges that the resulting R-Y and B-Y carriers will not be 
exactly compliant to SECAM spec; fortunately SECAM televisions themselves 
are quite forgiving as to the exact R-Y and B-Y centre frequencies.

Richard

--------------------------------------------------
From: "Segher Boessenkool" <segher@kernel.crashing.org>
Sent: Wednesday, November 02, 2011 8:58 PM
To: <cbm-hackers@musoftware.de>
Subject: Re: Procep SECAM Re: C16/Plus4

> Hi Richard,
>
>> You needed glass delay lines to decode PAL or SECAM, not to encode  them.
>
> [snip]
>
> Ah.  So it would have been much simpler for your vic->secam external
> circuit if the VIC did not do the PAL alternating thing?  They could
> have done a chip like that easily.  Or do you still need a lot of
> conversion circuitry that way?
>
>
> Segher
>
>
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