Right, so I got curious about this and hauled out the FM-7 to do some experimentation. The theory I now have for the FM-7 display is:
- The system always generates 640Ă200 video.
- The standard glyphs are stored as a 6Ă7 dot matrix.
- The sub-CPU, when asked to print a character at a certain spot on the screen, renders it as-is in
WIDTH 80 (80-column) mode and double-width in WIDTH 40 mode.
I worked this out by using the SYMBOL (x,y),"c",hscale,vscale command in FM-BASIC, which plots character c at (x,y) on the screen (x ranging from 0-639 and y ranging from 0-199) scaling it horizontally by hscale and vertically by vscale. Scale factors of 1,1; 2,1; and 8,4 look like this:
(This image was photographed from on my 450-line PVM-9045Q, and under different conditions from the photos in my previous post, so donât use that to compare resolution, just use it confirm the character dot matrix.)
The ăšăšăšă©ăȘă«ăŹăă line at the top was printed using the standard (40-column) text output so you can see the standard inter-character spacing, which I canât do with SYMBOL since that plots only one character at a time at arbitrary co-ordinates. It looks like those add one column of pixels for spacing, giving 7 pixel-wide characters, but they should be 8 pixels wide for 80 chars on a 640 pixel line. I have no idea where the extra pixel went.
Anyway, going back to my previous postâs image:
I canât find the âIâ youâre referring to, but if we have a look at the exclamation point, which is one pixel wide, and the double quote and octothorp to the right of it (the !"# character sequence), we see pretty clearly that each character is getting about three lines of horizontal resolution on the 250-line PVM-9042Q, and can see the adjacent red, green and blue phosphors in each line:
I think youâre right that thereâs a âreaction timeâ thing going on here, but I donât think itâs anything to do with the phosphor: itâs simply that the analog signal, continuously varying across the scan line, is (partially) âdigitizedâ by the aperture grille, i.e. you see only the sections of signal where itâs not been blocked by the grille. (There are actually three signals being displayed, for R, G and B, but since the composite output is monochrome, R, G and B will all be at the same level.) So if you have several adjacent horizontal pixels lit up that fully cross the R, G and B phosphor columns in front of the grille, youâll get a nice bright white, as you see for the horizontal lines in the octothorpe. But when only one pixel is being lit by the computer, thatâs not enough to cover the full width of an aperture grille column, so youâll see that section only partially lit up because the signal is âonâ for part of it and âoffâ for part of it. (Actually, it is truly analogue; there will be a rise and fall time there, but I imagine that the column will simply average out the total energy value of the signal as it varies over that column.) Thus the double-quote, pattern .â..â. , ends up more or less evenly lighting all three columns.
This also seems clear in the sequence of âăšă©ăȘă«ăŹăâ where there are eighteen vertical slices of the aperture grille across the six characters, and though I suspect the beam can resolve this, the aperture grille cannot:
Hereâs the 80-column character sequence on the 450-line PVM-9045Q.
Iâm not sure how reliably these can be compared, because my camera doesnât have manual controls for me to get consistent images, but if we extract the same âăšă©ăȘă«ăŹăâ sample to show it here full size:
I count something like 35 vertical lines there, which isnât too far off from the calculating 18*450/250 = 32.4 lines. The individual coloured columns donât come out in that image, or any others Iâve taken, though. I donât know if thatâs an issue with the photography or if somethingâs different about the monitor.
Now that Iâve done all this, Iâm not sure if Iâve become less confused or more confused about this. But at least I know to check for lines of horizontal resolution on colour monitors and that 450 lines is kind of a bare minimum for 80-colum work. :-)
