PCA84C646 Datasheet, PDF (55/72 Pages) NXP Semiconductors – Microcontrollers for TV tuning control and OSD applications

Philips Semiconductors
Microcontrollers for TV tuning
control and OSD applications
Preliminary specification
PCA84C646; PCA84C846
14 OSD CLOCK IN DIFFERENT TV STANDARDS
14.1 Maximum number of characters per row
The number of characters per row is a function of the OSD
clock frequency and the TV standard used.
The active video signal period of a horizontal line is
53.5 µs (see Fig.44). However, in order to reduce the
jittering of the screen edge, overscan is normally applied
by the TV manufacturer and this reduces the visible video
signal period to ⁹⁄₁₀ × 53.5 µs = 48.15 µs.
The examples given below show how the number of
characters per row and the character width may be
obtained for the NTSC 525LPF/60 Hz TV standard using
different OSD clock frequencies.
14.1.1 EXAMPLE 1
• For the OSD clock frequency f_OSD = 6 MHz; clock
period = 0.166 µs. The number of visible dots on one
horizontal line is 290 (48.15 µs/0.166 µs).
• Start of the first character dot is roughly 45 dots after
HSYNC (see Section 9.2; command B, C, D).
Therefore 290 − 45 = 245 dots are visible.
• Since each character is composed of 12 × 18 dots, the
maximum characters displayed on a row is 20 (245/12).
• On a 19" TV screen, the width of a horizontal line is
approximately 370 mm and this gives a character width
of 18.5 mm (370 mm/20).
14.2 Maximum number of rows per frame
The number of rows per frame is a function of the number
of active lines per display field and the number of vertical
dots in the character matrix (which is 18).
The number of rows per frame (N) is calculated as shown
below.
N = ^{n----u---m-----b---e----r---o----f---a---c---t--i-1v---8e-----l-i--n---e---s-----p---e----r---f--i-e----l-d--}
The three examples shown below illustrate how the
maximum number of rows per frame is obtained for each
TV scanning standard.
14.2.1 EXAMPLE 1; NTSC 525LPF/60 HZ
The number of active lines per field for this standard is
between 241.5 and 249.5H (see Fig.45). If the value of 241
is used then the maximum number of rows per frame is 13.
14.2.2 EXAMPLE 2; PAL 625LPF/50 HZ
With this standard it is not necessary to divide HSYNC by
two as both the horizontal and vertical frequency are
doubled. The maximum number of rows per frame is 15.
14.1.2 EXAMPLE 2
• For the OSD clock frequency f_OSD = 10 MHz; clock
period = 0.1 µs. The number of visible dots on one
horizontal line is 481 (48.15 µs/0.1 µs).
• Start of the first character dot is roughly 45 dots after
HSYNC (see Section 9.2; command B, C, D).
Therefore 481 − 45 = 436 dots are visible.
• Since each character is composed of 12 × 18 dots, the
maximum characters displayed on a row is 36 (436/12).
• On a 19" TV screen, the width of a horizontal line is
approximately 370 mm and this gives a character width
of 10.3 mm (370 mm/36).
1995 Jun 15
55