Re: Reccomending a monitor?

[mjmahon@aol.com (Michael J. Mahon)]

Kelli Halliburton replied:

>Michael J. Mahon wrote:
>> Kelli Halliburton wrote:
>>
>>> Michael J. Mahon wrote:
<David Stark wrote:>
>>>>> The other issue is if the 80 columns are interlaced or not.  The
>>>>> standard composit output from the Apple is 30 frames (not 60)
>>>>> interlaced.
>>>>
>>>> No, the Apple II video is 60 fps, non-interlaced.
>>
>> Actually, David Stark wrote the >>> part.  I wrote the >> part.
>
>Yeah, I got that. I was responding to the parrt with ">>", you may have
>noticed.
>
>> And 60 fps in the case of non-interlaced video is "frames",
>> since for non-interlaced video there are no "fields" (or the
>> field is identical to the frame).
>
>Right, but Apple II video is NOT 60 frames per second. And the video it
>produces IS interlaced.
>
>>> No. NTSC video is 30 frames per second interlaced, which works out
>>> to 60 FIELDS per second. The reference to 60 fields per second (an
>>> interlaced frame consists of two fields) is probably what inspired
>>> the earlier poster's correction.
>>>
>>> Apple II video is NTSC video, because it can be output to any NTSC
>>> composite, or monochrome monitor.
>>
>> Apple video is _close_ to NTSC, in the sense that it will work with
>> an NTSC-compatible monitor, but it is _not_ NTSC, since it is
>> non-interlaced.  It also differs from the NTSC standard in its
>> blanking and sync pulse timing--it's just "close enough" to
>> work on almost anything that NTSC works on.
>
>It IS interlaced. An NTSC monitor will NOT scan at 60 frames per second.

All NTSC monitors scan at 60Hz.

>The Apple II simply produces the same image for both fields, which is what
>is called a "scan-doubled" image.

There are 60 refreshes per second, but they precisely overlap, so
there is no interlace, and no "scan doubling".  Just look at a diagonal
line of pixels and you will see that this is true.

>>> And I use the term "NTSC monochrome" advisedly, to indicate 525-line,
>>> 30-frame-per-second, interlaced monochrome video. I would also use
>>> the term "PAL monochrome" in the same context to indicate 625-line,
>>> 25-frame-per-second monochrome video. Of course, we could always
>>> adopt HD nomenclature, and say 525/60i and 625/50i.
>>>
>>> HDTV terminology deals with the smallest vertical refresh unit,
>>> which is a frame in progressively scanned video, but a field in
>>> interlaced video, and indicates the method of scanning used as the
>>> last character, so a progressively-scanned image at the same
>>> resolution and frame rate would be 625/30p.
>>
>> I agree that this is the standard terminology.
>>
>> The OP's major error was in thinking that the Apple II video was
>> interlaced, which it is not.

Kelli, you need to review how NTSC television works and how
Apple video works.

I've provided the information, but you clearly don't believe me.

For example, you might want to look at:

http://www.fact-index.com/n/nt/ntsc.html

There you will see that the vertical scan frequency for NTSC monitors
is approximately 60Hz, and that the 30Hz _frame_ rate results from the
combination of two successive 1/60 sec. _fields_, one containing the
odd lines and the other containing the even lines.

In other words, the vertical scanning frequency _is_ the field
frequency.  In a 2:1 interlaced display, the _frame_ frequency is
always one-half the vertical scan frequency.

The Apple II generates approximately the same vertical and horizontal
scan frequencies as NTSC, but does _not_ interlace the lines of
successive fields, producing instead 60 frames per second of
non-interlaced video with approximately 262 total scan lines, and
192 active scan lines.  This scheme, BTW, is common to almost
all digital displays that use an NTSC monitor.  Interlaced computer
displays, while not unheard of, are quite uncommon because of
the severe inter-character flicker in text, since many characters
have single-horizontal-line features, and in an interlaced display
they would be refreshed only every other field, or at 30Hz, which
is quite visible with standard phosphors at close range.

A normal TV picture does not contain single-line features, and
is usually viewed a distance where individual lines are not resolved,
so that the flicker is less visible.

You may be interested to know that the reason that 60Hz was
chosen as the vertical scan frequency is so that power line
interference, either from stray magnetic fields or from imperfectly
filtered power supplies, would not produce a visually annoying
"beat" with the vertical scan.  (Instead, we see a less-prominent
slow "creep" of line-synchronized noise up or down the raster.)

-michael

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