Television – Image signal processing circuitry specific to television – Noise or undesired signal reduction
Reexamination Certificate
2001-09-19
2004-06-01
Kostak, Victor R. (Department: 2614)
Television
Image signal processing circuitry specific to television
Noise or undesired signal reduction
C348S470000, C375S346000, C375S348000
Reexamination Certificate
active
06744474
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to video signal receivers which receive high definition television (HDTV) signals and, in particular, to employing a trellis decoder to decode a received VSB-modulated HDTV signal after it has been demodulated and comb-filtered to reject NTSC co-channel interference.
2. Background
In data transmission systems, data, such as audio and video television (TV) data, is transmitted to a plurality of receivers. In the field of television signal transmission systems, the current NTSC (National Television Systems Committee) standard of transmission is being replaced by a higher-quality system, known as HDTV, or the ATSC-HDTV standard (see United States Advanced Television Systems Committee, ATSC Digital Television Standard, Document A/53, Sep. 16, 1995). Such HDTV signals are of the VSB-modulated (Vestigial SideBand) type proposed by the Grand Alliance in the United States.
The ATSC-HDTV standard requires an 8-VSB modulated transmission system which includes forward error correction (FEC) as a means of improving system performance. Referring now to
FIG. 1
, there is shown a simplified block diagram of the forward error correcting (FEC) aspects of a HDTV transmission system
100
. System
100
contains a Reed-Solomon encoder
103
, followed by a byte interleaver
104
, and a trellis encoder
105
on the transmitter side
101
. At the receiver end
121
there is a corresponding trellis decoder
125
, byte deinterleaver
124
, and Reed-Solomon decoder
123
.
In such a system, data signals are first encoded in accordance with a given code or encoding scheme, such as a convolutional, or trellis, code, by trellis encoder
105
. The trellis code employed is a rate 2/3 TCM (trellis coded modulation) code, as described in the ATSC Digital Television Standard. This code is implemented by coding one bit using a rate 1/□, 4-state convolutional encoder, and adding an FEC uncoded bit, which is differentially precoded. Each set of three encoder output bits is then mapped to an 8-VSB modulator symbol by modulator
106
, and transmitted over a given communications channel and transmission medium
150
. For example, the modulated, encoded HDTV signal may be transmitted as a terrestrial RF signal through the air. The transmitted signal contains digital data representing HDTV image and other information in the form of multilevel symbols formatted into groups of successive fields, each field comprising a field segment, a plurality of data segments, and associated sync components.
The HDTV receiver
121
receives the transmitted signals. Demodulator
126
is used to demodulate the signal to provide a demodulated baseband signal; and trellis decoder
125
is used to decode the demodulated signal to obtain the original data.
Due to the fact that NTSC and HDTV signals will coexist in the terrestrial broadcast channel for a number of years, it is important for the receiver
121
to reject possible NTSC co-channel interference. The elimination of NTSC interference may be performed by an NTSC rejection filter, such as a comb filter, added to the demodulator. The comb filter is typically a 12-symbol one-tapped delay line with signal attenuating nulls at or near the NTSC carriers.
Thus, when the HDTV receiver detects NTSC co-channel interference, it filters the demodulated signal to remove the NTSC co-channel interference that would otherwise arise, before performing trellis decoding. In the non-NTSC interference case, to avoid unnecessary filtering and undesirable effects of such filtering, the comb filter is not applied.
When no NTSC interference is detected, the optimal trellis decoder for the AWGN (Additive White Gaussian Noise) channel is a 4-state Viterbi decoder with the Euclidean metric. See G. Ungerboeck, Channel Coding with Multilevel/Phase Signals,
IEEE Trans. Inform. Theory,
vol. IT-28, pp. 55-67, January 1982. Thus, in performing the decoding, the trellis decoder
125
typically employs an Euclidean metric, which can provide optimal decoding when there is no NTSC interference. However, when NTSC interference is present, the use of the NTSC rejection (comb) filter introduces correlation in the noise (Additive Colored Gaussian Noise), such that the optimal trellis decoder is much more complex. Therefore, an optimal trellis decoder is typically used where there is no NTSC co-channel interference, and a partial response trellis decoder is used whenever the NTSC rejection filtering is employed.
Such systems employ 12 intra-segment interleaving (deinterleaving) in the trellis encoding (decoding), in which 12 identical encoders (decoders) are used. This permits implementing the trellis decoder
202
in the NTSC interference case as an 8-state (partial response) trellis decoder, and as a 4-state (optimal) trellis decoder
203
in the non-NTSC interference case. By employing the 12 encoder/decoder interleaving, each of the identical decoders of the trellis decoder for the NTSC interference case views the comb filter with a 1-symbol delay (instead of 12). The advantage of this architecture is that the optimal trellis decoder can be implemented with an 8-state Viterbi decoder. See United States Advanced Television Systems Committee, Guide to the Use of the ATSC Digital Television Standard, Document A/54, Oct. 4, 1995.
Referring now to
FIG. 2
, there is shown a block diagram illustrating the HDTV trellis decoding performed by receiver
121
of system
100
of
FIG. 1
, with and without NTSC interference, for each of 12 sequential decoders of trellis decoder
125
. Symbol-level signal data is received from demodulator
126
(FIG.
1
). In a first (NTSC interference) path, the demodulated signal is filtered by NTSC rejection (comb) filter
201
, and this filtered, demodulated signal is decoded by partial response 8-state trellis decoder
202
. The 8-state decoder
202
receives at its input a partial-response signal plus noise, because it is comb-filtered. This partial-response signal, which is derived from 8-VSB symbols, is also known as 15-VSB since it has 15 amplitude levels. In a second (non-NTSC interference) path, the demodulated signal is not filtered, and is decoded by optimal 4-state trellis decoder
203
. Switch
204
selects the appropriate decoded signal depending on whether NTSC interference is detected.
As will be appreciated, there may be only a single trellis decoder
125
which is used to implement both 8-state trellis decoder
202
and 4-state trellis decoder
203
, depending on whether demodulator
126
detects NTSC interference or not. Or, trellis decoder
125
may include separate decoders
202
,
203
, one of which is selected depending on whether NTSC interference is detected. Further, comb filter
201
is included in demodulator
126
. It is selected, or applied, by demodulator
126
when it detects NTSC interference. Thus, when demodulator
126
detects NTSC interference, it outputs a comb filtered, demodulated signal to decoder
125
, and also instructs decoder
125
that NTSC interference has been detected so that decoder
125
can use the 8-state trellis decoder
202
. Conversely, when demodulator
126
does not detect NTSC interference, it outputs a non-comb filtered, demodulated signal to decoder
125
, and does not instruct decoder
125
that NTSC interference has been detected, so that decoder
125
can use the 4-state trellis decoder
203
. This functionality is illustrated in the process flow of FIG.
2
.
Both the optimal 4-state trellis decoder
203
, and partial response 8-state trellis decoder
202
, employ the Euclidean metric or some variation of it in current implementations. In the non-NTSC interference case of decoder
204
, this results in an optimal trellis decoder and optimal results, as described above. However, the 8-state trellis decoder
202
with the Euclidean metric has a performance degradation of about 3.0 to 3.75 dB with respect to the non-NTSC interference case. A suboptimal truncated non-Euclidean trellis decoder metric with better performance than the Euclidean metr
Kostak Victor R.
Kurdyla Ronald H.
Laks Joseph J.
Thomson Licensing S.A.
Tripoli Joseph S.
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