[stella] Closed Captioning

Subject: [stella] Closed Captioning
From: "Eric Ball" <eball@xxxxxxxxxx>
Date: Wed, 16 Oct 2002 13:45:15 -0400
FYI: The document I have been working with is Chapter 8 from Video

To summarize what I've learned out so far and what it means for a 2600
based CC generator.

CC is based on a 32x Fh clock, while the 2600 uses colourburst (227.5x Fh,
though the 2600 has 228 per line).  Although these aren't directly
compatible, it appears that the playfield registers could be used for
almost all of the necessary CC cycles, with the sprite registers handing
the CC cycles which fall across PF boundaries.  That's the good news.

The bad news is the active region for CC is 26/32 cycles (or 81.25% of Fh),
while the 2600 only has 70% active and a black border.  Based on some
assumptions and calculations, this means that the 2600 cannot display the
first and last 2 CC cycles (4 CC cycles lost).  The first 2 CC cycles are
part of the 7 CC cycle run in used for clock synchronization and are
probably not too critical, but the last 2 CC cycles are D6 and parity (odd)
for the second data byte.  This significantly restricts the characters
which can be put in that position and the 2 byte control codes which can be

In particular, although the Text Restart command (used to initiate full
screen text mode) is available, the carriage return command (used to go to
the next line in text mode) is not.  Also, the NULL character cannot be put
in the second position (due to parity), which may cause problems if the CC
decoder requires the parity of both bytes to be correct in order to display
the characters.

The alternatives would be to drop all 4 CC cycles from the clock run-in, or
squeeze more CC cycles into the 2600 active region.  Unfortunately, these
may not be compatible with standard CC decoders, which probably assume the
standard 32x Fh clock and timing.

None the less, I hope to code some test programs RSN.

PS  Line 1 in Z26.log is NTSC line 4.  This is because VSYNC generates the
serration pulses.

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