FOCUS Enhancements FS453 - Manual (Page 12)

FS453/4 AND FS455/6 DATA SHEET: HARDWARE REFERENCE
4. Scaling and Positioning Notes
The FS453 graphics converter does not use a frame memory. Therefore, the FS453 input video frame
rate must be synchronous to and match the output video field or frame rate. In SDTV modes, the FS453
uses internal line memories in order to perform horizontal and vertical scaling. This imposes certain
requirements on the scale and position settings.
4.1 Vertical Scaling
Because the frame/field rates are synchronous, and no frame memory is available, the ratios of input to
output VTOTAL and input to output VACTIVE must match. (See Figure 4 below.) In this sense, the
output VACTIVE is not necessarily the total active lines of the selected TV standard, but is the number of
TV lines that will contain active video information from the input source material. If the output VACTIVE
value is smaller than the value specified by the TV standard, then the FS453 will place borders and below
the image. TV_VTOTAL and GCC_VACTIVE in the VTOTAL equation are determined by the selected
TV standard and graphics mode. TV_VACTIVE is selected to set the desired number of TV lines
containing video information. The Vertical Scaling Coefficient is programmed in register 06h. The ratio of
input to output VTOTAL also determines the vertical scaling factor used. Note that calculations are done
using the output frame size, even though the output is interlaced, because interlacing is done after
vertical scaling.
G C C _ V A C T IV E / G C C _ V T O T A L = T V _ V A C T IV E /T V _ V T O T A L
F o r d o w n sc a lin g , V S C = (T V _ V T O T A L / G C C _ V T O T A L ) * 6 5 ,5 3 6
F o r u p sc a lin g , V S C = (T V _ V T O T A L / G C C _ V T O T A L – 1 ) * 6 5 ,5 3 6
Figure 4: Equations for VTOTAL and VSC
Notes:
GCC_VACTIVE: The number of active lines of computer graphics in a frame.
GCC_VTOTAL: The total number of lines in a computer graphics frame, including active and blanking.
TV_VACTIVE: The number of lines in a TV video frame that will contain scaled graphics data.
TV_VTOTAL: The total number of lines in a TV video frame. PAL has 625 lines. NTSC has 525 lines.
For example, consider a case where the input graphics active area contains 600 lines and the selected
TV standard is NTSC. In NTSC, TV_VTOTAL is 525 lines per frame and the full-size active area is 487
lines per frame. To program the FS453 to scale the GRAPHICS image to fit on 400 lines of TV video (for
example, to fit the image within the TV bezel), set TV_VACTIVE to 400. This sets three of the four
parameters in the equation, and solving for VGA_VTOTAL results in a value of 787.5. Because values
must be integers, set VGA_VTOTAL to 788. The scaled image will still occupy approximately 400 lines.
Given these VTOTAL values, the vertical scaling factor is 0.6662, and the VSC register will be set to
43,663 (0xAA8F).
TV_VTOTAL
VGA_VTOTAL
VGA_VACTIVE
TV_VACTIVE
Figure 5: VTOTAL and VACTIVE ratios must match
JANUARY, 2005, VERSION 3.0 12 COPYRIGHT ©2003-4 FOCUS ENHANCEMENTS, INC.
FOCUS Enhancements Semiconductor