Re: Open hardware AV to digital conversion

From: Ingo Korb <>
Date: Thu, 08 Jan 2015 22:40:56 +0100
Message-ID: <> writes:

> That's the problem. The first stage I want to do is to keep the 64 being
> a 64, meaning it can still be connected to whatever it was before,
> /plus/ those to which it couldn't over the same pins/cables.

I do not think this is a reasonable constraint. S-Video is already quite
far on its way out, composite seems to be following (see below) and I
refuse to consider analog RF. While composite is still an option for
now, it's not the best quality you can get from the C64. Once
again... Why settle for something okay-ish when you have the chance to
build something great?

If you just keep the existing signals without modifications on the
existing outputs and add a new one you gain the flexibility to choose
output video formats without being constrained to SD and without being
limited to the lowest common connector.

> I do, because I want it to remain 100% compatible with what was there
> before.

If it worked before, it can just continue working by not modifying the
signals that it used before.

> and I am not having RGB at this stage anyway. I have (sampled)
> luma/chroma and if I scan-double this, I bet (haven't tried) the same or
> even more devices won't accept the 576p on composite/s-video lines as
> the non-interlaced 288p.

Correct, as far as I know there has never been a signal standard that
uses PAL/NTSC color encoding at resolutions beyond SD.

I didn't expect that you would even consider outputting composite or
S-Video signals though because the AD video decoder chip that was
mentioned previously included color decoding, so its output would be RGB
or possibly YCbCr. What is your plan to turn that back into the
S-Video-like signals the VIC outputs?

>> Which reminds me: How do you propose to shift the signal timing enough
>> that you can add[1] half a line to change the timing from 240p to 480i?
> The same as the "regular" devices do.

Yes, but... "regular" devices have a slightly different video timing. If
I'm not mistaken the C64 outputs half a line less per frame, exactly
that half line that is needed to signal an interlaced picture.

> I count the fields and I delay each line (out of VBLANK area) as much
> as is required for processing whenever I process/output odd fields and
> I do the same plus properly timed half-line period for lines
> constituting even fields. At a flip of a bit I skip the extra delay
> for even fields.

I think if you do that you end up with a signal where each of the two
fields has a different number of lines. Are you sure that such a signal
is acceptable to modern TVs? I haven't tried something like that with
interlaced signals yet, but when I accidentally generated a 480p signal
where two consecutive frames were differing by one or two lines, I
mostly saw the familiar "No Signal" message.

> I think because I can't output 576p over composite/s-video pins of the
> 64, can I? And I can't expect the devices to swallow that kind of
> signal.

No, and I wouldn't even consider going in that direction anyway - both
composite video and S-Video appear to be on their way out, S-Video
faster than composite. My current TV does not have a dedicated composite
AV input anymore, instead it re-uses the luma channel from the component
input for composite if you select AV. It doesn't support S-Video on
SCART either.

> If we skip VGA, I'd have to go purely digital, which would mean there
> is no "cheap" internal variant to which one could add an HDMI module

You don't have to, analog Component video (YPbPr) is still
there. If you have the resources to spare for the color space
conversion, a video DAC that allows Component video can usually also be
used for VGA just by removing the sync offset from the luma

For a C64 you probably wouldn't even need a full 24-bit video DAC (~6
USD in single quantities at Digikey), a lower-resolution R2R DAC would
be sufficient.

> later on and would mean either dropping the analogue output or making
> an external device. Such are available (for $400+) already.

You always have the option to make a cheaper external device. It would
also increase your possible market size, thus increasing the number of
units sold and reducing the cost of the parts if you can order them in
larger batches.

Fun little idea on the side: Are "C64-incompatible" displays also
usually incompatible with consoles like the SNES or Megadrive/Genesis?
Those two also use a 240p/288p video mode in the majority of games, but
if their signal is acceptable to a display while the one from the C64 is
not, simpler approaches may be viable like regenerating just the sync
pulses without modifying the video signal itself.

-i 'the constraints are getting weirder ;)' k

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Received on 2015-01-08 22:00:03

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