Re: Reccomending a monitor?

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

David Stark replied:

>Michael J. Mahon wrote:
>
>> No, the Apple II video is 60 fps, non-interlaced.
>
>I do not agree with this.  NTSC is 30 frames per second, in two fields. 
>  60 fields per second where each field is every other scan line.  If 
>the Apple did not agree with this format you could not use a TV as a 
>monitor, with or without a modulator.  I used it with a TV and modulator 
>for many years, so I am certain of this.  Perhaps you have some sort of 
>video card which is different.
>
>> 
>> The vertical scan rate does not determine the bandwidth required--
>> it determines the number of horizontal lines.
>>
>
>If you assume that the number of scan lines are the same, at 60 frames 
>per second you have half the time for each scan line.  To have the same 
>horizontal resolution you have to double the bandwidth.  Or are you 
>saying that the number of lines is cut in half at 60 fps?  You have a 
>262 scanline monitor?  It must be made by HP.

That is exactly what I mean.  Any scanned interlaced monitor
displays only 262.5 lines per field, with the 525 lines resulting from
the fact that the second field is scanned in between the lines of the
first--or displaced by 0.5 line of the 262.5-line field.

Interlace is not a property of scanning frequencies.  It is a
consequence of the timing of the vertical sync pulse.  If the
vertical sync pulse occurs at an odd multiple of half the line
scan period (like 262.5) then the display is interlaced.

If the vertical sync pulse occurs at a multiple of the line
scan period, then successive fields' lines exactly overlap,
and the display is non-interlaced.

The Apple II display is 262 lines, refreshed 60 times per second,
and any NTSC-compatible scanning monitor can do that.

>It might be worth pointing out that NTSC is 30 frames per second.  When 
>you change the frame rate you are no longer NTSC compatible, strictly 
>speaking.  So if the frame rate changes I wouldn't be surprised if the 
>number of lines was also changed, but these are arbitrary decisions made 
>by the equipment manufactuer.
>
>OK, I looked up NTSC.  A scan line 63.5 usec.  The line rate is 15750 
>scan lines per second.  A screen is 525 lines.  So 15750 / 525 = 30 frames.
>
>The horizontal blank is 0.16 of the line duration, so the picture has 
>(0.84)(63.5 usec) = 53.4 usec for the line.  If you have 7 pixels per 
>character, and 80 characters per line, then you need a resolution of 
>560.  53.4 usec / (560/2) = 0.1907 usec, or about 5.25 MHz.  This would 
>be an absolute bare minimum.

In the Apple II, to allow for overscan common in TV monitors, only
40 microseconds of each line are used for display.  Since you are
correct that 560 pixels are needed for the 80-column display, the
dot clock is 14.3MHz, as I stated.

BTW, only 192 of the 262 lines are used for display on the Apple II,
also to deal with overscan.

There has never been an 8-bit Apple II that was capable of more than
192 lines of native resolution.  (There have been some add-on cards
that increased it.  For example, the Video Overlay Card generates
both 525-line interlaced video or 200-line SHR, like the IIgs.)

>If everything else is the same (525 lines) but you double the frame rate 
>the time per pixel would be cut in half.  You would need 10.5 MHz. 
>Which agrees with your previous range of 10-14 MHz.  (You need to give 
>some extra bandwidth or you may get some interference from adjacent 
>pixels, plus transition bands for the filters)

Only if you double the number of lines in a "field".  As long as
there are only 262 lines being refreshed in 1/60th of a second,
doubling the number of lines by interlacing doesn't affect the
bandwidth.  That is the entire motivation for interlacing in the
first place--to increase the vertical resolution without increasing
the bandwidth.

>> Don't worry--all 80-column cards produce frame rates which are
>> well within the lock range of a nominal 60Hz monitor.  And all that
>> I know of use non-interlaced video, since the display of characters
>> with single-line features creates annoying flicker if an interlaced
>> display is used without an unusually long-persistence phosphor.
>>
>
>I'm not worried at all.  It is possible that the flicker is a problem. 
>Interesting that it was never an issue at 40 columns on an interlaced 
>display.

But no Apple II video _is_ interlaced--look it up if you don't
want to believe me.  ;-)

>  But I think TVs have longer persitance than the typical 
>monitor.  I should look up the actual video circuit.  The Apple had 
>something line 192 rasters so you should get more than 2 scan lines per 
>pixel.  Maybe that had something to do with it.  Big fat blurry pixels. 
>  It amazes me how much these guys did with not much more than TTL.

You are confused.  Read my description of Apple II video above,
or look it up for yourself.

The Apple II video generation is, indeed, a nice engineering job.
The cleverest parts were getting artifact NTSC color for "free" by
playing with the clock frequency and phase, and using the video
refresh cycle to simultaneously refresh the DRAMs.

-michael

Check out amazing quality sound for 8-bit Apples on my
Home page:  http://members.aol.com/MJMahon/