Television – Image signal processing circuitry specific to television – A/d converters
Reexamination Certificate
1999-05-27
2002-11-19
Hsia, Sherrie (Department: 2614)
Television
Image signal processing circuitry specific to television
A/d converters
C348S571000, C348S708000, C348S712000
Reexamination Certificate
active
06483550
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a video signal processing system, and particularly to a device for converting an analog television signal into a signal in compliance with a certain standard for digital encoding.
(2) Description of the Prior Art
In recent years, appliances capable of recording low noise video images by digitally processing video signals, such as DVC (Digital Video Cassette) and the like, have become available in the market. In line with the development of such appliances, a device for converting an analog television signal to a digital signal in compliance with a certain digital encoding standard has been increasingly demanded.
A standard for the analog television signal, for example in NTSC system, is specified in EIA standard RS-170. The signal according to this standard comprises, as shown in
FIG. 1
, a video signal portion that includes a luminance signal and a carrier chrominance signal, and a synchronizing signal portion that is used for synchronizing operation to reproduce a picture in a television receiver. The signal levels for each of the signals are specified to be that, when a signal level of the synchronizing signal is 40, the video signal has a signal level difference between a black level and a peak white level of 100.
A standard for a digital encoding is specified, for example, in ITU-R Recommendation BT601 as a digital encoding standard for a component signal. This standard specifies a sampling frequency, a signal level for quantization, etc. in converting an analog luminance signal and an analog carrier chrominance signal to digital signals. For NTSC system, for example, it is stated that no synchronizing signal is used, and that in the luminance signal a sampling frequency for sampling analog data is 13.5 MHz, and signal levels for quantization are a black level being 16/255 (=2
8
−1), and a peak white level being 235/255 when one sample is expressed (converted) by eight bits.
As described above, between these two standards, there is a difference in a video signal level as shown in FIG.
2
. In
FIG. 2
, on the left is a video signal according to the EIA standard RS-170, and on the right is a video signal according to the ITU-R Recommendation BT601. Thus, a need to adjust the difference arises in converting analog video signals to digital signals.
In conjunction with the above, a prior art device for converting an analog television signal to a digital signal in compliance with a digital encoding standard is now described with reference to the figures.
FIG. 3
shows a construction of a prior art analog-to-digital converter. In the figure, there are shown an upstream A-D converter circuit
111
, a downstream A-D converter circuit
112
, a Y/C separating circuit
12
, an upstream synchronizing signal detecting circuit
131
, a downstream synchronizing signal detecting circuit
132
, a D-A converter circuit
31
, an amplifying circuit
32
, and a synchronizing signal converting circuit
14
. The operation of this prior art device is as follows.
An analog television signal, such as a signal for NTSC system, is supplied from an input terminal
10
into the upstream A-D converter circuit
111
, and converted therein to an 8- digital signal. As for signal levels of the output signal from this A-D converter circuit, the black level is made to be, for example, 64/255 and a peak white level is 210/255, as shown in FIG.
4
. The upstream synchronizing signal detecting circuit
131
receives the 8-bit digital signal and detects a synchronizing signal from the 8-bit digital signal. The Y/C separating circuit
12
separates a luminance signal and a carrier chrominance signal from the 8-bit digital signal using the detected synchronizing signal, and thereby a composite signal, which contains a synchronizing signal, a luminance signal, and a carrier chrominance signal, is converted into a component signal in which a signal containing the luminance signal and a synchronizing signal is separated from a carrier chrominance signal. The separated luminance signal is converted back to an analog signal by the D-A converter circuit
31
, and amplified by the amplifying circuit
32
so that the signal level between the black level and the peak white level becomes 1.5 times larger than that of the input signal. The amplified luminance signal is then converted back to an 8-bit digital signal by the downstream A-D converter circuit. As for signal levels of the output signal from the A-D converter circuit
112
, the black level is made to be 16/255 and the peak white level is 235/255, as in the waveform shown on the right of FIG.
2
. The downstream synchronizing signal detecting circuit
132
detects a synchronizing signal from the output signal from the amplifying circuit
32
. The synchronizing signal converting circuit
14
uses the synchronizing signal received from the synchronizing signal detecting circuit
32
to fix a synchronizing signal portion of the luminance signal to be, for example, 16/255, and output a video portion of the luminance signal without changing. Finally, the carrier chrominance signal and the luminance signal are separately output to a display (not shown), and thus a better picture is attained than a picture obtained in the case where the output signal to the display is such that the carrier chrominance signal and the luminance signal are combined.
According to such a construction described above, an analog television signal, such as a signal for NTSC system, can be converted to a digital signal in compliance with a digital encoding standard. However, the above-described circuit construction has such a drawback that, after converted to a digital signal by the upstream A-D converter circuit
111
, the input signal is required to be converted back to an analog signal by in the D-A converter circuit
31
and converted back again to a digital signal by the downstream A-D converter circuit
112
, thereby causing the deterioration of a signal-to-noise ratio. Furthermore, to change the signal level of the luminance signal, a plurality of A-D converter circuits as well as a D-A converter circuit are required, therefore the cost is increased.
In view of the above drawbacks, a video signal processing system having an improved signal-to-noise ratio, having a small circuit size, and being cost-effective, is desired. In addition, although the foregoing discussion pertains primarily to a signal processing in an A-D conversion for television pictures according to NTSC system, such as normally used in U.S.A. and Japan, there remain problems caused by amplifying the signals to adjust a difference between the signals each based on a different standard for digital encoding even under other systems. Therefore, there also exists a need to develop a technique that can be suitably applied to such cases.
In addition, as a matter of course, qualities of resulting pictures must not be deteriorated as a result of signal adjustments by amplification and the like. Accordingly, it is also desired to develop a technique that can prevent the deterioration of picture qualities caused by such adjustments and the like processing.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a solution to the foregoing problems and drawbacks. According to the present invention, video signal conversion is performed digitally, unlike prior art circuits in which such conversion is performed with analog signals.
This and other objects are accomplished in accordance with the present invention by providing an analog-to-digital converter for converting a video signal comprising:
an A-D converter circuit for converting an analog television signal to a 2
n
-bit digital signal,
a synchronizing signal detecting circuit for detecting a synchronizing signal in a digital signal received from the A-D converter circuit,
a Y/C separating circuit for separating a luminance signal and a carrier chrominance signal from the digital signal received from the A-D converter circuit utilizing a synchronizing
Miyoshi Toshihiro
Murata Hisaji
Hsia Sherrie
Parkhurst & Wendel L.L.P.
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