Television – Plural transmitter system considerations
Reexamination Certificate
1998-12-09
2001-05-08
Eisenzopf, Reinhard (Department: 2614)
Television
Plural transmitter system considerations
C348S607000, C348S608000, C348S614000, C348S725000, C348S723000, C375S346000
Reexamination Certificate
active
06229560
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a digital television system (DTV) and more particularly to determining co-channel interference in the DTV employing a vestigial sideband (VSB) transmission system.
2. Discussion of Related Art
According the digital television broadcasting standardized by the Grand Alliance (GA), both the digital and NTSC television systems use radio frequency (RF) signals. Also, each channel band of the digital and NTSC television system is limited to 6 MHz. However, a significant difference between the digital and NTSC television system is in the transmission spectrum of the broadcasting.
In the transmission spectrum of the NTSC system, a large video carrier frequency is always 1.25 MHz above the low end of a channel, a chrominance subcarrier frequency is 3.5 MHz above a video carrier signal, and a sound carrier signal has an unmodulated center frequency of 4.5 MHz higher than a video carrier frequency for the same channel. In contrast, the transmission spectrum of the DTV system only has a very small pilot carrier signal with an unmodulated center frequency of 0.31 MHz to assist the carrier recovery at the receiver.
Moreover, DTV signals are coded and transmitted in a different manner from the coding and transmission methods of the analog signals for NTSC system. Thus, a viewer generally cannot watch a NTSC broadcast with the DTV system. Nevertheless, a same tuner may be used since the DTV and NTSC system both use the same RF channel and the same intermediate frequency (IF) band. The DTV signal and the conventional analog NTSC television signal use the same RF channel and the NTSC system uses a portion of the channel designated as a taboo channel to prevent channel interference. Thus, the DTV and NTSC system may share an antenna and a tuner.
However, co-channel interference may occur in the DTV broadcast, when an NTSC broadcasting is utilizing the co-channel during the DTV broadcasting. The carrier powers, namely the video carriers, the chrominance subcarrier, and the sound carrier, of the NTSC system are significantly larger than the pilot signal of the DTV system, resulting in severe interference in the DTV broadcast. Generally, a comb filter is used to remove co-channel NTSC interference.
The comb filter consists of delays and subtracters. Each delay consists of 12 resistors connected in series and delays the received signal for 12 symbols to obtain the transmission characteristics suitable for filtering. The comb filter also subtracts the delayed data for 12 symbols from the current data. The carrier powers of the NTSC system are removed by the comb filter by converging to “0” (zero). Specifically, the video carrier, chrominance carrier, and sound carrier of the NTSC system are around the null point when passing the comb filter, thereby avoiding interference with the DTV system.
Using a comb filter also increases the power of the random noise resulting in a deterioration of the picture quality. However, because the deterioration of the picture quality is insignificant compared to the effect of removing the NTSC channel interference, using a comb filter is still preferable. Furthermore, since the co-channel NTSC interference does not always exist, the comb filter is used when a co-channel NTSC interference occurs in the DTV system.
FIG. 1
is a block diagram of the DTV system. When RF signals modulated according to the VSB system are received through an antenna
100
, a tuner
110
selects a desired channel frequency by tuning and converts the frequency into an IF signal. A frequency-phase locking loop (FPLL)
120
demodulates the IF signal from the tuner
110
into baseband signals I and Q to lock the output phase and frequency. The FPLL
120
is a circuit unifying both a frequency tracking loop and a phase-locking loop, but the frequency is locked prior to the phase.
An analog/digital (A/D) converter
130
converts the I signal into digital data of a given bit number (for example, 10 bits). The Q signal is used for carrier restoration within the FPLL
120
. Using the converted digital data of a given bit number, a synchronization recovery unit
140
recovers a synchronization signal inserted at the time of transmission to be used for timing recovery and equalization.
The standardized VSB transmission signal includes a specific synchronization (sync) signal pattern within the transmitted data which allows a receiver to easily recover the sync signals. Accordingly, the data incorporates consecutive data segments, wherein each segment consists of 832 symbol units and within the 832 symbol units, four symbols define the segment sync signal pattern while the other 828 symbols are data information. Also, a field consists of 313 data segments and at every 313th data segment position, a field sync signal is inserted. The segment and field sync signals are similar to a horizontal and vertical sync signals of the NTSC broadcasting signals, respectively. Because the sync signals allow an easy data recovery, erroneous detection and recovery of the sync signals adversely affects the overall system.
An equalization & error coding correction (ECC) unit
150
uses the data segment and data field signals as training signals. An equalization is performed to correct linear distortion of amplitude causing interference between symbols, ghost images, etc. Afterwards, an ECC is performed to correct errors that may have occurred during transmission through the channel. A video decoder
160
decodes the equalized and error corrected signal according to the moving picture expert group (MPEG) algorithm, allowing the TV audience to watch a broadcast through display
170
.
Before the sync recovery unit
140
outputs the recovered sync signals to the equalization & ECC unit
150
, a determination unit
190
determines whether co-channel NTSC interference exists. If co-channel NTSC interference does not exist, the data bypasses a comb filter
180
and is output to the equalization & ECC unit
150
. If co-channel NTSC interference is determined to exist, the data passes through the comb filter
180
and is output to the equalization & ECC unit
190
. The determination unit
190
controls whether the data passes or bypasses the comb filter
180
by sending a selection signal to a selection unit
200
which outputs a comb filtered data if the selection signal is received and outputs the data directly otherwise.
The DTV and NTSC broadcasting systems transmit data through space and may be significantly attenuated by various noise in space. The noise which is picked up by a channel during transmission is a Gaussian noise. The power of the Gaussian noise is generally constant, but may sometimes surge instantaneously to a large value. In such cases, the Gaussian noise may be determined to be co-channel NTSC interference causing the data to pass through the comb filter
180
before being output to the equalization and error code correction unit
150
.
Particularly, in a zone where the DTV and NTSC broadcasting system overlap, a common-channel interference such as the Gaussian noise or a co-channel NTSC interference may occur. When a noise which passes through and which does not pass through the comb filter
180
have a similar size, the corresponding data which passes or does not pass through the comb filter
180
may be incorrectly selected and output, this adversely affects the operation of equalization & ECC unit
150
. Therefore, circuits removing noisy signals contained in the incoming digital signals are affected causing a malfunction of the DTV receiver.
OBJECTIVES OF THE INVENTION
Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the related art.
An object of the present invention is to provide a DTV system with a constant Gaussian noise.
Another object of the present invention is to provide a DTV system having comb filters which correctly outputs signals regardless of the noise size.
Additional advantages, objects, and features of the invention will be set forth in unit in the d
Birch & Stewart Kolasch & Birch, LLP
Desir Jean W.
Eisenzopf Reinhard
LG Electronics Inc.
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