Apple II timing vs. NTSC (was Re: Good Composite->VGA converters for classic computers (& video games...)

Eric Smith eric at brouhaha.com
Sun Jun 1 16:08:21 CDT 2008


I wrote:
> The problem with the Apple II is that it does NOT produce NTSC video,
> but rather something close enough to fool an analog television set or
> monitor.  The timings are deliberately wrong, especially the frequency
> and phase relationships of the color subcarrier to the horizontal
> sync.

Jim wrote:
> While I knew about the slightly goofy signal, the phrase "DELIBERATELY
> wrong" is new.  Why was it wrong on purpose?

NTSC requires that the horizontal rate be 227.5 times the color
subcarrier frequency, and phase-locked to it.  The color carrier
thus alternates in phase 180 degrees on consecutive lines within a
field.

The Apple II horizontal rate is 288.0 times the color subcarrier
frequency, so that the color phase is the same on every line.

This is also why every 65th cycle of the processor clock is longer
than the other 64.  The Apple timing is derived from a 14.31818 MHz
oscillator, four times the color burst frequency.  For NTSC there
should be 910 cycles per scan line (227.5 * 4).  The CPU clock is
normally the oscillator frequency divided by 14, to get 1.023 MHz.
However, if that timing was used consistently, the color phase would
alternate as per the NTSC spec.  Instead, every 65th processor cycle
is divided by 16, rather than 14, so that there are a total of 912
oscillator cycles per scan line, to get the constant color phase.
This makes the average CPU clock slightly lower, at 1.020 MHz.
     (14.31318 MHz * 65) / (64 * 14 + 16)

This caused terrible problems for anyone that wanted to use an
Apple II as a source for video broadcast; the signal couldn't be
used even with most genlocks.  I worked in a studio that used an
Apple II with a VB3 Microkeyer from Video Associate Labs.  This
was a two-board set that installed in the Apple II to provide a
genlock and proc amp.  It gave a 100% compliant video signal by
replacing a fair bit of the Apple II circuitry.  You had to pull
about a dozen TTL chips out of the Apple and replace them with
ribbon cables to the two cards.  One card went into a slot, and
the other sat above the power supply.

Naturally this wouldn't work on an Apple IIe.

Many years later Apple introduced their own video overlay card.
Rather than modifying the circuitry of the base Apple, it completely
replicated the video circuitry on the card, but provided standard
NTSC timing.

Eric




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