I've been looking at the waveform I provided earlier
(http://www.biglist.com/lists/stella/archives/200208/msg00173.html) and
have come to the conclusion that it isn't possible for the 2600 to create
the necessary waveform for closed captioning for the following reasons:
1. The active portion of the 2600 screen is only 44.698uS, while the CC
waveform requires 51.143uS. There is also no way on the 2600 to control
the time between the sync pulse and the first pixel. Still, it might be
possible to generate a waveform which a liberal CC decoder could be able to
handle iff
- the length and timing of the clock run-in were modified to put it
at the beginning of the 2600 active screen (with the data bits comprising
the last 121 pixels of the active screen)
- the delay between the end of the clock run-in and the start bit was
shortened
2. Although it is possible to generate an approximation of the clock run-in
using the 2600 player graphics, generating the actual data bits would be
impossible. This is because the data bits are modulated using the same
frequency as used for the clock run-in. Each cycle spans 7.11 pixels, each
bit is 17/9 cycles long and 13.43 pixels wide. Obviously the 2600 is not
up to the task of creating even an approximation of such a complex signal
(each pixel of the data bits would need to be controlled).
- If a different frequency were used then the task becomes somewhat
easier. However, it may still be impossible to create even a comparable
signal since the player graphics would have to be used. How the decoder
determines the start and end of each bit also becomes very important.
- The only other possibility is if the CC decoder doesn't required
the bits to be modulated. Then the playfield graphics could be used, as
has been previously hypothesized, to generate the data bits.
Note: I am assuming that the 2600 is based on a standard 3.5795MHz
colorburst clock, although that means that each line generated by the 2600
is 63.695uS rather than the NTSC specified 63.5555uS.
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