Jaguar Video and Clock Speeds


The video system of Jaguar is programmable to within the precision of the
supplied video clock.  In the Jaguar game console the video clock is chosen to
allow an inexpensive modulator system.  In the interest of saving money the
processor clock speed is the same as the video clock speed.  This requires a slightly
different clock speed for NTSC and PAL system.  These numbers are as follows
(Please note, more digits will be available later but these will probably not
change.):

	PAL	26.594 MHz
	NTSC	26.591 MHz

From this video clock the system then produces the pixel (or dot) clock.  The ratio
between video and pixel clock is determined by high order bits of the VMODE
register.  The possible values for the ratio are in the range 1-8.  The attached spread
sheet contains the number of pixels that will fit within two different sized areas of
a video screen.  The numbers are the same for NTSC and PAL.

For both PAL and NTSC the "safe" video area is about 40 us wide.  The area
required to guarantee overscan is about 50 us.  The spreadsheet gives the number
of pixels that can be displayed within these times for all available pixel clock
dividers.  Note that these numbers are not integers.  The numbers are also not even
"nice" computer numbers like 320 or 256.

We recommend that ALL software for the Jaguar console overscan both vertically
and horizontally so for the rest of this discussion we will restrict ourselves to the
OVERSCAN column.

The first row (divisor of 1) requires that the object processor be started twice each
line and produces a ridiculous resolution for a TV, it will be ignored.  The rest of
the rows are all usable but two of them are special.  A divisor of three gives a non
overscanned resolution of about 320.  This is a good match for many computer
systems.  A divisor of four gives pixels that are about square.  Square pixels are a
great advantage for art creation and we recommend their use.

Let's look at the specific case of an overscanned game using square pixels.  This
uses a pixel divisor of 4.  In both NTSC and PAL this allows for about 332 pixels to
be displayed.  Choosing 320 gives us a <4% error.  Of these 320 pixels we should
only count on the middle 266 being visible at all times.  This means that there is a
border of 27 pixels on each side that are drawn but should not contain essential
game information.

The other pixel clock divisors that are of likely interest are 3 and 5.  In both of
these cases the number of overscanned pixels is usably close to a blittable width:
448 for a divisor of 3 and 256 for a divisor of 5.

Note: The number to use as display width in VIDINIT is the one for a divisor of 1.
Use 1330 for an overscanned display.

To overscan vertically we suggest a screen height of 240 lines for NTSC and 288
PAL.  This will alow for both PAL and NTSC users to see a fully overscanned image
both vertically and horizontally.  The guaranteed visible region within which
crucial game information is restricted is 200 lines for NTSC and 240 lines for PAL.
Using 200 lines of critical video for both systems is a significant, and acceptable,
simplification.


	Pixel Divisor		Not Overscan	Overscan

		1			1063.6		1329.5
		2			531.8		664.8
		3			354.5		443.2
		4			265.9		332.4
		5			212.7		265.9
		6			177.3		221.6
		7			151.9		189.9
		8			132.9		166.2



The End.

;=========================================================================================================

The following is an example of equates we use to set up our video:

;*********** VIDEO ***********************************************

vclk		equ	376

horiz_per	equ	317778
sync_per	equ	46050
eq_per		equ	23500
front_porch	equ	17450
line_blank	equ	109500
disp_width	equ	1347			;(pixel width + 1) * 3
disp_height	equ	240			;this is in lines

; Horizontal computations

n_hp		equ	horiz_per/vclk
n_hbb		equ	((horiz_per-sync_per-front_porch)/vclk)+$400
n_hbe		equ	(line_blank-sync_per-front_porch)/vclk
n_hs		equ	((horiz_per-sync_per)/vclk)+$400
n_hvs		equ	(horiz_per-(2*sync_per))/vclk
n_heq		equ	(horiz_per-eq_per)/vclk
n_hde		equ	((disp_width/2)-1)+$400
n_hdb1		equ	((n_hp)-(disp_width/2))
n_hdb2		equ	n_hdb1

n_vp		equ	523
n_vee		equ	6
n_vbe		equ	40
n_vdb		equ	n_vbe-2			;n_vbe+4
n_vde		equ	n_vdb+(disp_height*2)
n_vbb		equ	n_vde
n_vs		equ	n_vp-10
n_veb		equ	n_vs-n_vee

SCRN_TOP	equ	n_vdb			;ypos of first line at top of screen
SCRN_BOTTOM	equ	n_vde			;xpos of first line below screen
SCRN_RIGHT	equ	448			;xpos of first column off right side of screen

;*******************************************************************