Re: NTSC Emulation

[Sheldon Simms <wsspammy@gmail.com>]

On 2011-01-23 03:23:53 -0500, Michael J. Mahon said:

> Sheldon Simms wrote:
> > MDJ mentioned NTSC video emulation yesterday and that piqued my
> > interest because I have been working on Apple II video emulation by
> > NTSC decoding/demodulation in software.
>
> Very nice--it was just such an emulation that I envisioned being
> the method of populating a table of 4096 32-bit color entries,
> each corresponding to the NTSC response to the previous 12 bits
> of (14MHz) Apple video signal.

In order to then use a table lookup to emulate the Apple II screen?

My intention has been to use the NTSC emulation directly in an Apple II
emulator. The CPU load is acceptably low on any relatively recent
computer: about 50% of one core on my 2.0 Ghz Core 2 Macbook.
By being careful about only calculating new output when it is needed,
the load drops to near zero for mostly static screens.

> Something looks a little off in your colors, however.  What filter
> bandwidths are you using for Y, U, and V signals?  Then there's
> the delay in the Y signal to compensate for the U and V filters...
> Gamma may also be an issue...

Gamma may be an issue and I don't know about you, but I have
a dim LCD screen on my computer and it doesn't saturate like an
old CRT.

Here's a comparison of a screen capture with a photo I just took
of my real Apple IIe with AppleColor Composite Monitor IIe.

http://wsxyz.net/compare.html

I'm doing YIQ decoding. I am not a DSP expert and I picked up
what seemed to work after a little bit of introductory reading, so
the filtering is very simple - I pretend I'm sampling the Apple II
picture signal at 14 Mhz and run that through a very simple ~3.58
Mhz low pass filter to extract Y. Then I subtract that out from the
input signal to get chroma, which I then demodulate and filter
each component with the same filter adjusted to half the
bandwith (i.e. ~1.79 Mhz)

The real code is kind of hard to follow because I use fixed point
math and combine and simplify expressions for performance
reasons, but it generally goes like this:

Y = Y + (signal - Y) / 4;
IQ = signal - Y;
isignal = IQ * cos(phase);
qsignal = IQ * sin(phase);
I = I + (isignal - I) / 8;
Q = Q + (qsignal - Q) / 8;
RGB = /* calculate from YIQ */
phase = next(phase);