Television – Receiver circuitry
Reexamination Certificate
1999-09-15
2002-07-30
Miller, John W. (Department: 2614)
Television
Receiver circuitry
C348S607000, C348S470000, C348S021000, C348S735000, C348S738000, C348S736000, C375S346000, C375S348000, C375S349000, C375S350000, C375S277000, C375S285000, C455S307000, C455S315000, C455S286000
Reexamination Certificate
active
06426780
ABSTRACT:
The invention relates to digital television receivers for vestigial-sideband (VSB) digital television (DTV) signals and, more particularly, to the suppression of interference with DTV signal reception caused by the frequency-modulated audio carrier of a co-channel NTSC analog television signal.
BACKGROUND OF THE INVENTION
The VSB DTV signals that are broadcast in the United States are transmitted in the same channels used for broadcasting NTSC analog television signals. Generally speaking, the VSB DTV signals are broadcast at lower power than NTSC analog television signals are. Until such time as DTV broadcasting completely supplants NTSC analog television broadcasting, then, there is substantial possibility of co-channel NTSC interference with VSB DTV signal reception, particularly during the summer months. The video carrier modulation components of a co-channel NTSC signal overlap a VSB DTV signal in frequency, which makes the suppression of these video components by frequency selective filtering a difficult task. The current content of the video portion of a co-channel NTSC signal can be predicted quite well from its previous content, however, which facilitates comb filtering to suppress artifacts of the video portion of a co-channel NTSC signal that accompany baseband symbol code detected from the received VSB DTV signal. Prediction of the current content of the audio portion of a co-channel NTSC signal from its previous content is generally more difficult, although short-time prediction of the audio portion is possible based on considerations of continuity in a narrowband signal. The audio carrier modulation of a co-channel NTSC signal does not overlap a VSB DTV signal in frequency, which makes it more feasible to employ frequency-selective filtering to suppress the audio portion of a co-channel NTSC signal than to suppress the video portion.
There has been great concern with carefully controlling the overall amplitude and phase characteristics of the VSB DTV receiver in order to minimize intersymbol error, while at the same time rejecting interference from signals in adjacent channels. Generally, the overall response of the receiver is defined by surface-acoustic-wave (SAW) filtering done using gallium-arsenide devices in internediate-frequency amplifiers for the ultra-high-frequency (UHF) band or using lithium-niobate devices in intermediate-frequency amplifiers for the very-high-frequency (VHF) band. Getting flat amplitude response within ±1 dB over a bandwidth of 5.5 to 6 MHz, while maintaining acceptable group delay characteristics, requires SAW filtering with a great number of poles and zeroes to define the receiver bandwidth. It is difficult and expensive to implement such SAW filtering for a VHF band, such as 41-47 MHz. Also, the insertion loss is quite high in a VHF band, typically 15-17 dB for the 41 to 47 MHz band. The SAW filtering to define receiver bandwidth is more easily implemented for a UHF band, such as at 917-923 MHz, as long as care is taken to drive the SAW filter from the optimal source impedance specified by its manufacturer. This is because the &Dgr;f/f ratio of 6 MHz to 920 MHz is substantially lower than the &Dgr;f/f ratio of 6 MHz to 44 MHz. Insertion losses also tend to be lower in a UHF band, typically 10-12 dB for the 917 to 923 MHz band.
The cost of a SAW filter used in a UHF or VHF I-F amplifier is substantially increased if it is designed to provide trap filtering for the modulated audio carrier of the co-channel NTSC signal. Since the co-channel NTSC signal is 250 kilohertz from the edge of the television channel, and since critical VSB DTV information extends to within 310 kilohertz of that edge of the television channel, the SAW filter response must reach substantial attenuation in less than a 60 kilocycle range. This is a very difficult requirement to fulfill.
Consequently, trap filtering for the modulated audio carrier of the co-channel NTSC signal is omitted in some DTV receiver designs, and the artifacts of co-channel interference from this carrier are suppressed to some extent by comb filtering performed in the digital regime for reducing artifacts of co-channel interference from the video portion of the NTSC signal. Co-channel interference from the audio portion of the NTSC signal is not suppressed very much unless the comb filtering used to suppress co-channel interference from the video portion of the NTSC signal subtractively combines samples differentially delayed by twelve symbol epochs. Often, however, comb filtering of different type would better suppress co-channel interference from the video portion of the NTSC signal.
So cheaper and better filtering for suppressing co-channel interference from the audio portion of the NTSC signal is a desideratum. It is pointed out in this specification that such filtering is possible in DTV signal receivers in which a low-band final I-F signal with its uppermost frequency in the low or mid high-frequency band is generated and synchrodyned to baseband for recovering symbol code. RLC analog filtering of such an I-F signal can be done to suppress co-channel interference from the audio portion of the NTSC signal. Examples of suitable RLC analog filtering are Butterworth-Thomson transition filters, bridged-T trap filters and bifilar-T trap filters.
Non-uniform group delay attends RLC analog filtering, so designers eschew LC analog filtering in favor of digital filter designs with which uniform group delay can be easily obtained. The non-uniform group delay from LC analog filtering can be substantially compensated by non-uniform group delay designed into SAW filtering used for the VHF I-F signal or SAW filtering used for the UHF I-F signal, however. The inclusion of such delay compensation into such a SAW filter is not expensive. Remaining non-uniformity in group delay is easily remedied by the adaptive channel equalization filtering customarily incorporated into DTV signal receivers. So combinations of LC analog filtering low-band intermediate-frequency DTV signals with preceding SAW filtering of higher-intermediate-frequency DTV signals designed to result in overall group delay that is reasonably uniform can provide the desired cheaper and better filtering for suppressing co-channel interference from the audio portion of the NTSC signal.
RLC filters with maximally flat amplitude characteristics are described by S. Butterworth in a paper “On the Theory of Filter-Amplifiers” in EXP. WIRELESS AND WIRELESS ENG., Vol. 7, p. 536, October 1930. RLC filters with maximally flat delay characteristics are described by W. E. Thomson, M. A., in a paper “Networks with Maximally-Flat Delay” in WIRELESS ENGINEER, Vol. 29, pp. 256-263, October 1952. RLC filters with characteristics intermediate to those of Butterworth and Thomson filters, which transitional filters have better transient response characteristics than Butterworth or Thomson filters are described by Y. Peless and T. Murakami in a paper “Analysis and Synthesis of Transitional Butterworth-Thomson Filters and Bandpass Amplifiers” in the March 1957 issue of RCA REVIEW pp. 60-94. The bridged-T trap filter is described by F. E. Terman, Sc. D., in RADIO ENGINEERS” HANDBOOK, 1
st
. Ed., §13, §§7, pp. 918-920, copyright 1943 to McGraw Hill Book Co., Inc. of New York and London. The procedures for designing bifilar-T traps are known to television engineers from a licensee bulletin LB-961 titled “An Analysis of the Bifilar-T Trap Circuit” supplied by Radio Corporation of America through its Industry Service Laboratory to television receiver licensees on Sep. 16, 1954.
The conversion of very-high-frequency intermediate-frequency VSB DTV signal to a band below 10 MHz for digitization and its subsequent demodulation in the digital regime are described in U.S. Pat. No. 5,479,449. This patent entitled “DIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVER” issued Dec. 26, 1995 to C. B. Patel and A. L. R. Limberg. Demodulation in the digital regime is performed in U.S. Pat. No. 5,479,449 by converting the digitized int
Limberg Allen LeRoy
Patel Chandrakant B.
Miller John W.
Natnael Paulos M.
Samsung Electronics Co,. Ltd.
LandOfFree
DTV receiver with low-band final I-F signal filtered for... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with DTV receiver with low-band final I-F signal filtered for..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and DTV receiver with low-band final I-F signal filtered for... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2868485