Immortal Nephi wrote:
On Saturday, October 20, 2012 5:02:49 PM UTC-5, Kevin wrote:My scope is almost as old as my apple, but here is what I did... I used the following script to create a low res grid on the apple 2 10 GR 20 COLOR = 7 30 FOR Y = 0 TO 39 STEP 2 40 FOR X = 0 TO 39 STEP 2 50 PLOT X,Y 60 NEXT X 70 NEXT Y and here is a picture from the scope http://cheesefactory.us/apple2/PA200009.JPG hope it helpsThank you for demonstrating the screen of oscilloscope. It appears to be IRE A, but not IRE B as I drew IRE picture earlier. The screen of oscilloscope does not look very clear. Is it possible you can please zoom from 63 microseconds to 70 nanoseconds? After 70 nanoseconds is reached, the square wave looks very clear. The color burst is to be sine wave or triangle wave. You are right as Apple II video DOES NOT generate luma wave *AND* chromosome wave. Let's talk about luma wave while color burst is NOT enabled. How do NTSC monitor interpret square wave into monochrome? I suspect all 15 colors run at 7 MHz as true grayscale as monochrome. If NTSC monitor's frequency is higher enough to sustain 14 MHz instead of 7 MHz, then grayscale does not show and true black & white monochrome shows. Are you positive sure monochrome does not use luma wave? What is the filter name do NTSC monitor use to display grayscale as monochrome?
Alex actually gave a great, if brief, explanation.First, realize that luminance and chrominance signals are simply interpretations of the composite video signal. When a composite signal is created conventionally, the luminance (monochrome level) and chrominance (hue and saturation) signals are actually formed separately and added together to make the composite signal.
When a monitor decodes a composite signal, it uses a bandpass filter (or a comb filter, if it's either a modern or an expensive monitor) centered on 3.58MHz to extract the chrominance signal. The bandwidth of this filter is typically 1MHz since color resolution is relatively low. The output of this filter will be any 3.58MHz signals and sidebands in the composite signal. The instantaneous amplitude of the 3.58MHz signal is the saturation of the color, and the phase of the 3.58MHz signal with respect to the color burst is the hue of the color.
The luminance signal is typically low-passed to less than 4MHz and its amplitude directly represents the instantaneous brightness.
Apple II video consists of not-too-far-from-standard synchronization signals, plus a 3.58MHz sinewave color burst on the "back porch" of the horizontal blanking pulse. The active part of each video line is 40 microseconds, and the video signal is only black or white on every part
of the active line.Hi-Res colors are created by displaying either isolated or alternating pixels of white and black. The pixel rate is 7.16MHz, so that changing pixel values create a 3.58MHz signal whose phase depends on whether the pixel sequence is 1010... or 0101... and on whether the sequence is delayed by 1/4th cycle of 3.58MHz by setting the "color set" bit in memory.
Double-hi-res colors and lo-res colors are created by displaying patterns of black and white with a pixel rate of 14.3MHz, allowing for four video values per cycle of 3.58MHz. The sixteen different combinations of white and black during each cycle produce 16 different combinations of average luminance, chroma amplitude, and chroma phase. (Some of these combinations are almost indistinguishable to the eye, since 1010 and 0101 both display as 50% gray.)
Of course, a monitor doesn't know anything about Apple pixels or dots, so its decoding will be a running average of the luminance (with about 4MHz bandwidth for the 14MHz dot sequence) and the hue and saturation (with about 1MHz bandwidth), regardless of where the Apple's 4-bit boundaries are, resulting in smooth transitions between colors, or blending of adjacent colors. Apple II graphic artists used these effects to produce the effect of a larger color palette, sometimes with quite artistic results.
All this is done simply with sequences of black and white levels generated at a 14.3MHz or 7.16MHz rate. This method of composite video generation is sometimes referred to as "artifact color", since color is created as an artifact of the luminance signal changing rapidly at a multiple of the chroma subcarrier frequency.
-michael NadaNet 3.1 for Apple II parallel computing! Home page: http://home.comcast.net/~mjmahon/ "The wastebasket is our most important design tool--and it's seriously underused."