Date   : Wed, 04 Mar 2009 12:53:40 +0100
From   : kortink@... (John Kortink)
Subject: Fwd: New Screen Modes with ARM7 Co-Processor?

On Wed, 4 Mar 2009 12:33:11 +1100, "Richard Wilson"
<richard@...> wrote:

>[...]
>
>> >And gives no chance of getting the fields the wrong way round -
>> >requiring user intervention is bad!
>> 
>> That I agree with. Assuming the frame type really is
>> impossible to determine or force by software.
>
>
>I would think that determining which field Odd or Even would be relatively
>easy since the VSYNC is delayed by (R0 + 1) / 2 on the odd frames only. It's
>a relatively simple task to measure the distance between VSYNC pulses in a
>frame by reading CA1 on the System VIA.

That's not how it works.

The length of the vsync and display periods are always
exactly the same. The actual discriminator is in the
distance between end end of vsync and first hsync, or
(therefore) last hsync and start of vsync. For even
frames, the former distance is half a line shorter.
This is how e.g. video sync separators like the LM1881
work.

I remember having to write my own detector 20 years
ago (for my video digitiser project). The LM1881 I
used didn't always reliably detect frame polarity
for video recorder signals. Fortunately, it did
detect hblank and vsync reliably, so I was able to
determine frame polarity on my own with a bit of
carefully crafted code.

Unfortunately we can't detect hsyncs with the standard
beeb hardware. An extra wire would be enough, but that's
probably a deal breaker in most scenarios.

>[...]


John Kortink

-- 

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