Television – Synchronization – Sync generation
Reexamination Certificate
1999-11-12
2003-02-04
Miller, John (Department: 2614)
Television
Synchronization
Sync generation
C348S536000, C348S537000, C348S542000, C348S555000
Reexamination Certificate
active
06515708
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a clock generator adapted to generate different dot clock signals correspondingly to formats, respectively, of more than one video signal, formatted differently from each other, an image displaying apparatus and method in which the clock generator is used.
2. Description of the Related Art
The video signal formats adopted for the purpose of image display include those used in the television systems, such as HDTV (high-definition television) system and NTSC (National Television Standards Committee) double-speed system, which can implement a high resolution of image display with more than 1000 horizontal scanning lines (525 to 625 scanning lines in the conventional TV systems).
The above HD signal and NTSC double-speed signal are video signals, respectively, formatted as shown in
FIGS. 1 and 2
, respectively. The HD signal has a total of 1125 vertical lines per frame as shown in
FIG. 1
, while the NTSC double-speed signal has a total of 1050 vertical lines per frame as shown in FIG.
2
. The horizontal scanning frequency depends upon a total number of vertical lines per field and a vertical scanning frequency. So, when the vertical scanning frequency is 60 Hz, the horizontal scanning frequencies of the HD and NTSC double-speed signals are as follows:
HD signal
1125/2 × 60 = 33.75 kHz
NTSC double-speed signal
1050/2 × 60 = 31.5 kHz
For displaying images of different horizontal scanning frequencies, a deflection system is necessary for each of the different horizontal scanning frequencies. To cope with this problem, the number of vertical lines in the NTSC double-speed signal is increased up to 1125 as shown in
FIG. 3
for the NTSC double-speed signal to have a horizontal scanning frequency of 33.75 kHz. Namely, whichever a video signal supplied for display is, HD signal or NTSC double-speed signal, it will be displayed on the assumption that it has a horizontal scanning frequency of 33.75 kHz. Note that an NTSC double-speed signal whose number of vertical scanning lines is increased as mentioned above will be referred to as “magnified NTSC double-speed signal” hereinafter.
In the HD signal shown in
FIG. 1
, the ratio of 1920 effective horizontal pixels to the total of 2200 horizontal pixels (1920/2200) is approximately 0.87, while in the NTSC double-speed signal shown in
FIG. 2
, the ratio of 1440 effective horizontal pixels to the total of 1716 horizontal pixels is about 0.84. Since the ratio of the effective horizontal pixels to the total of horizontal pixels in the HD signal is larger than that of the effective horizontal pixels to the total of horizontal pixels in the NTSC double-speed signal, the effective horizontal pixels will occupy a larger part of the total of horizontal pixels in the HD signal than in the NTSC double-speed signal, which means that the retrace ratio for the HD signal is smaller than that for the NTSC double-speed signal.
The deflection system provided for the above-mentioned reason and adapted to operate correspondingly to the HD and NTSC double-speed signals, respectively, adds to the manufacturing costs of the TV set or image displaying apparatus because of the difference in retrace ratio between the HD and NTSC double-speed signals. To avoid such an additional cost, it has been proposed to increase the total number of horizontal pixels in the HD signal so that the retrace ratio for the HD signal is nearly the same as that for the NTSC double-speed signal.
The total of horizontal pixels in the HD signal, required for the above purpose, will count 2280 in number because 1920/total number of horizontal pixels is 0.84 as in the above. Thus, with the retrace ratio taken in consideration, a video signal is adopted which is formatted to include a total of 2280 horizontal pixels as shown in FIG.
4
. The video signal formatted as shown in
FIG. 4
will be referred to as “magnified-retrace HD horizontal signal” hereinafter.
Thus, a TV set in which 33.75 kHz is fixed as the horizontal deflection frequency, will display images represented by an HD signal as shown in
FIG. 1
, magnified NTSC double-speed vertical signal as shown in
FIG. 3 and a
magnified-retrace HD horizontal signal as shown in
FIG. 4
, respectively.
Frequencies of dot clock signals necessary for the three different formats of the HD signal, magnified NTSC double-speed vertical signal and magnified-retrace HD horizontal signal, respectively, are as will be described below. In Table 1 below, “dot clock” is a signal having a frequency derived from multiplication of the horizontal scanning frequency by the total number of horizontal pixels.
TABLE 1
Horizontal
Output format
deflection frequency
Dot clock
HD signal
1920 (2200) ×
33.75 kHz
74.25 MHz
1080 (1125)
Magnified NTSC
1440 (1716) ×
33.75 kHz
57.915 MHz
vertical signal
960 (1125)
Magnified-retrace
1920 (2280) ×
33.75 kHz
76.95 MHz
HD horizontal
1080 (1125)
signal
Since the horizontal deflection frequency (33.75 kHz) of the magnified NTSC vertical signal is the same as that of the HD signal, a clock generator is used to generate a dot clock signal by multiplying a horizontal scanning frequency by a total number of horizontal pixels.
However, many of the conventional TV sets use the PLL circuit as shown in
FIGS. 5A
to
5
C. In
FIG. 5
, the PLL circuit is generally indicated with a reference
100
. To generate a dot clock signal for each of video signals formatted differently from each other, the PLL circuit designed as shown in
FIG. 5
selects a frequency division ratio in a frequency divider
101
according to the video signal format. More particularly, in the PLL circuit
100
, the frequency division ratio is changed to 2200 as shown in
FIG. 5A
to generate a dot clock signal for display of the HD signal as an image, to 1716 as shown in
FIG. 5B
to generate a dot clock signal for display of the magnified NTSC vertical signal, and to 2280 as shown in
FIG. 5C
to generate a dot clock signal for display of the magnified-retrace HD horizontal signal. For this changing of the frequency division ratio, a select signal is supplied from outside to the frequency divider
101
.
Thus, for display of the HD signal, the PLL circuit
100
generates a dot clock signal of 74.25 MHz from the reference signal of 33.75 kHz. For display of the magnified NTSC vertical signal, the PLL circuit
100
generates a dot clock signal of 57.915 MHz from the reference signal. For display of the magnified-retrace HD horizontal signal, the PLL circuit
100
generates a dot clock signal of 76.95 MHz from the reference signal.
Since the frequency divider
101
in the PLL circuit
100
shown in
FIG. 5
has a high frequency division ratio, however, a dot clock signal generated as in the above is likely to have many jitters. Also, in the PLL circuit
100
, the reference signal supplied to a phase comparator
102
has a frequency as low as 33.75 kHz. Therefore, the PLL circuit
100
cannot generate a reference signal by a crystal oscillator. For generation of a stable reference signal of 33.75 kHz, a crystal oscillator is used to generate a signal of about 10 to 25 MHz, for example, in frequency and the signal is divided in frequency. Also the PLL circuit
100
is designed to generate a horizontal deflection frequency by further passing the dot clock signal through other frequency divider. The other frequency divider for generation of a signal having the horizontal deflection frequency has a frequency division ratio L which is determined based on a number of scanning lines in all horizontal directions.
Next, determination of a frequency division ratio N in the PLL circuit for each of three video signals having different formats such as the HD, magnified NTSC vertical signal and magnified-retrace horizontal signal, will be described below by way of example.
The total numbers of horizontal pixels in the formats of the magnified NTSC double-speed vertical signal, HD signal and magnified-retrace HD horizontal signal are 1717, 220
Desir Jean W.
Frommer William S.
Frommer & Lawrence & Haug LLP
Miller John
Smid Dennis M.
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