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Re: Apple video bandwidth question

MdntTrain wrote:

For Apple II Hires 280x192 monochrome video, isn't the monitor
bandwidth needed 3.59mhz with a dot clock of ~7mhz?

I thought we'd posted here some time ago that dot clock was twice the
bandwidth requirement, but I was reading in a Sencore computer monitor
analyzer manual today that to calculate a monitor's bandwith, you do
the following:

Example for Apple:
horiz scan = 15.7khz
total scan time = 1/15.7khz = 63.69us
active video time = 40/65 * 63.69us = 39.19us
display time per pixel = 39.19us / 280 pixels = 0.14us
bandwidth required for single pixel = 1/0.14us = 7.144 MHZ.

Yes, the dots would surely need to be clocked out at 7.144mhz, but
something seems wrong with their method as it doesn't to take into
account half cycles.

Or does dot clock equal bandwidth?  I just forgot! lol.

 jS
The NTSC TV had the luminance bandwidth of less than 3.57 MHz because the color subcarrier was at 3.57. No comb filters at the time either. Better monitors could turn the filter off if no color was detected and extended the bandwidth to say 5-6 MHz.
So in practical terms 3.57 is enough to see every pixel in 280x192 mode, but in theory the spectrum of a square wave is infinite. So you need infinite bandwidth to pass it with a 100% accuracy. Fortunately a lot less will work just fine.
BTW the 80 col mode uses 14 MHz pixel clock and so it doesn't look good on a color TV but looks OK on a monochrome monitor with a bandwidth of about 6 MHz or so. 

-Alex.