Television – Image signal processing circuitry specific to television – Gray scale transformation
Reexamination Certificate
1998-08-06
2001-08-28
Kostak, Victor R. (Department: 2611)
Television
Image signal processing circuitry specific to television
Gray scale transformation
C345S215000
Reexamination Certificate
active
06281945
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to an automatic gain control system and more particularly to an automatic gain control system for a High Definition Television.
Currently, there are two ways to transmit data to a television (TV) receiver, the terrestrial transmission for the conventional TV and the cable transmission for Cable TV. Particularly, both the terrestrial and cable transmission use the vestigial sideband (VSB) modulation, by which the data is transmitted by multilevel symbols. The established standard data levels for the VSB signals are 2VSB with 2 symbol levels, 4VSB with 4 symbol levels, 8VSB with 8 symbol levels, and 16VSB with 16 symbol levels. The data level with the higher symbol levels can transmit greater data. However, as the data level increases, the number of symbols and the interspace between the symbols correspondingly decrease, resulting in a low signal to noise ratio (SNR). Particularly, the 16VSB with 16 symbol levels has the smallest interspace for each symbol and thus the lowest SNR.
The High Definition TV (HDTV) follows the suggested terrestrial and cable broadcasting standard using a VSB transmission of the 2, 4, 8, and 16 VSB. Accordingly,
FIG. 1
shows a general block diagram of a HDTV incorporating an automatic gain control (AGC) system and
FIG. 2
shows a block diagram of the AGC circuit.
Referring to
FIG. 1
, a HDTV automatic gain control system includes an antenna
10
receiving the transmitted signals; a tuner
12
tuning and converting the transmitted signal to an intermediate frequency (IF); an IF/carrier recoverer
14
converting the intermediate frequency to an analog-based signal, adjusting the gain factor according to the gain signal received, and outputting a gain control signal to the tuner
12
; an A/D converter
16
converting the analog-based signal to a digital signal; and an AGC
18
determining the gain factor from the digital signal and outputting a gain control signal to the IF/carrier recoverer
14
.
As shown in
FIG. 2
, the AGC
18
further includes an operator
180
taking the absolute value of the digitized signal and outputting the positive value, an adder
182
which sums the outputs from the operator
180
and the data level, an integrator
184
integrating the output of the adder
182
, and a gain selector
186
which outputs a positive gain control signal if the integrator
184
outputs a negative value and outputs a negative gain control signal if the integrator
184
outputs a positive value. The adder
182
receives the positive signal value and assigns a negative polarity to the data level and outputs the summed value of the positive and the negative value.
In operation, the tuner
12
receives the input signal from the antenna
10
and converts the signal to an intermediate frequency. The IF/carrier recoverer
14
receives the intermediate frequency, converts the frequency signal to an analog-based signal, and outputs the analog-based signal to the A/D converter
16
. The IF/carrier recoverer
14
also increases or decreases the gain according to the gain control signal from the AGC
18
. The A/D converter
16
digitizes the analog based signal and outputs a digital signal to the AGC
18
, wherein the absolute magnitude of the digital signal is taken and output to the adder
182
.
In the signal transmission for a HDTV, every data line includes a segment synchronization (sync) signal in the first four symbols and the segment sync signal carries the designated data level with respect to the VSB mode. The adder
182
assigns a negative value to the designated data level and sums the positive digital signal value received from the operator
180
and the negative data level value of the segment sync signal. The integrator
184
receives the summed value from the adder
182
, continually integrates the summed value, and outputs the integrated value to the gain selector
186
.
If the integrated value is negative, the current gain on the signal transmission is determined to be lower than the data level and the gain selector
186
outputs a positive gain control signal to the IF/carrier recoverer
14
. On the other hand, if the integrated signal value is positive, the current gain on the signal transmission is determined to be higher than the data level and the gain selector
186
outputs a negative gain control signal to the IF/carrier recoverer
14
. The IF/carrier recoverer
14
receives either the positive or negative gain control signal from the gain selector
186
and accordingly adjusts the intermediate frequency. However, the IF/carrier recoverer has a predetermined range for adjusting the intermediate frequency and if the range were to be surpassed by the necessary adjustment, the control of the gain on the intermediate signal is transmitted back to the tuner
12
.
Thus the gain of the analog based signal is converted by A/D converter
16
to a digital signal and the digitized signal level can be adjusted to the correct data level through the AGC
18
. Up to now however, the automatic gain control system was directed to simply correct the data level without taking into consideration the number of symbol levels. Accordingly, the signal to noise ratio (SNR) suffers when the number of symbol levels increases because the interspace between each symbol level must necessarily decrease. One way to solve this problem is to use greater transmission power to maintain channel signal separations when the number of symbol levels increase. However, higher transmission power requires a greater cost.
OBJECTIVES OF THE INVENTION
An object of the present invention is to solve at least the problems and disadvantages of the related art.
Particularly, an object of the present invention is to minimize the SNR, especially for the 16VSB mode.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, the HDTV automatic gain control system according to the present invention includes a VSB mode detector and outputs the gain control signal according to the detected VSB mode.
REFERENCES:
patent: 5764309 (1998-06-01), Krishnamurthy et al.
Jun Jung Sig
Park Hee Bok
Kostak Victor R.
LG Electronics Inc.
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