Abstract:
A circuit for developing correction signals for on-line controlling a precorrection arrangement in an HDTV 8VSB digital signal transmitter. The transmitted signal is sampled, sorted, smoothed and sliced to determine the symbol amplitude levels of the transmitted multilevel symbols. The centers of the sliced clusters of sampled symbol levels are used to determine the symbol amplitude levels. Correction signals for compensating the precorrection arrangement are developed based upon the determined symbol levels and the known symbol levels.

Description:
BACKGROUND OF THE INVENTION AND PRIOR ART 
     This invention relates generally to transmitters and in particular to transmitters that are used to transmit digital HDTV signals. 
     Television transmitters generally use vacuum tube amplifiers because of their very large power handling ability. Vacuum tube amplifiers have a non linear operating characteristic, which causes signal distortion, especially at high power levels, which are the most efficient levels for the transmitter operators. For conventional NTSC analog transmissions, the distortion is generally tolerable and it is not unusual to operate such vacuum tube amplifiers at a 60 kilowatt level. In most instances, an attempt is periodically made to minimize the effects of the distortion caused by the power amplifier by sampling the output of the transmitter while transmitting a known training signal and manually performing corrective measures on the transmitter IF amplifier to compensate for distortions introduced into the training signal. This is a trial and error approach which is time consuming and highly dependent upon the skill of the technician. It also must be repeated periodically and is generally done “off-line”, i.e. when the transmitter is off the air. This is highly inconvenient for continuous transmitters. 
     With the imminent advent of HDTV television signal broadcasting, the distortion problem has taken on much greater significance. The newly adopted digital HDTV signal is an 8VSB signal consisting of multilevel symbols. When such a signal is processed by state of the art vacuum tube power amplifier tubes, the operating signal power is on the order of −12 dB from the normal 60 kilowatt level for NTSC signals. This power limitation is necessary because of the impairment suffered by the digital signal in non linear transmitters operating near the 60 kilowatt level, and results in severe operating inefficiencies for the transmitter operator and a significant curtailment of the transmission range of the HDTV signal. The distortion introduced is also non linear in that the gain of the amplifier changes with changes in input signal amplitude. Such distortion may be compensated by non linear precorrection of the input signal to offset the amplifier-introduced distortion. 
     The present invention is directed to developing an on-line correction signal for a non linear precorrector in the transmitter. The invention permits conventional transmitter power amplifiers to be operated with acceptable distortion at their normal 60 kilowatt levels with an HDTV 8VSB multilevel symbol digital television signal. 
     OBJECTS OF THE INVENTION 
     A principal object of the invention is to provide a novel technique for precorrecting non linear distortion in a transmitter power amplifier. 
     Another object of the invention is to provide an improved transmission system for digital television signals. 
     A further object of the invention is to provide a digital television transmitter with automatic on-line precorrection of signal distortion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the invention will be apparent from reading the following description thereof in conjunction with the drawing, the single FIGURE of which is a simplified block diagram illustrating the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The digital television signal recently adopted for use in the United States is an 8VSB multilevel symbol signal that consists of two fields of 313 data segments each, with each data segment being headed by a four symbol segment sync signal and the first segment of each field comprising a field data segment signal. There are eight equally spaced data levels for the symbols and distortion in the signal causes great difficulty in determining these symbol levels. Because of the nature of digital signals, it is essential that the symbol levels be recovered with minimum error. Non linear distortion at higher power levels is more of a problem with an 8VSB signal because the degradation in symbol level recovery occurs more abruptly as opposed to non linear distortion in an analog NTSC signal where the degradation is more gradual and causes fewer harmful effects on analog NTSC demodulation. 
     As mentioned above, the present invention is performed on-line and includes a technique for deriving correction information for application to a non linear precorrector in the transmitter. 
     Referring to the drawing, a source of 8VSB HDTV signal  10  supplies an I channel filter  12  and a Q channel filter  14 , both, in turn, being coupled to a complex multiplier  28  that processes both the I and Q components of the signal. While the IF frequency supplied to the RF upconverter is 46.69 MHz, the digital processing is performed at a lower frequency, namely 6.33 MHz. The output of the complex multiplier  28  is supplied to a non linear precorrector  30 , which is a controllable amplifier, the gain of which is related to a correction signal. The output of the precorrector  30  is supplied to a D/A (digital-to-analog) converter  32  where the signal is converted to analog form and applied to a multiplier  34  that is also supplied with a 40.36 MHz local oscillator signal for generating the desired 46.69 MHz IF signal. The IF signal is supplied to an RF up-converter  38  which produces the desired channel output signal. This signal is applied to a power amplifier  40 , which as discussed above, comprises a high powered vacuum tube amplifier that has a typical non linear distortion curve. The power amplifier  40  supplies a transmitting antenna  42 , and a suitable tap on the transmitter supplies the output signal to an HDTV receiver that includes a tuner  50  and an IF/detector  52 . The demodulated signal from IF/detector  52  is applied to a block  56  that may include a DC removal circuit, an AGC circuit, clock/sync recovery circuitry and a comb filter. The signal from block  56  is supplied to a block  58  that includes an equalizer, a phase tracker and a slicer. The output of block  58  is coupled to a block  60  that includes a trellis decoder, a symbol/byte converter, a convolutional deinterleaver, an R-S decoder and a derandomizer, all of which are well known in the digital VSB transmission and receiving system. The output of block  60  is supplied to a block  62  that provides data recovery and further receiver processing. 
     A sync and timing block  54  develops appropriate timing signals from the demodulated signal from IF/detector  52  and supplies a sampler circuit  66  that samples the demodulated signal. The output of sampler  66  is supplied to a sorter and smoother  68 . In the preferred embodiment, about 32,000 samples of the demodulated signal are taken or captured from the input of the equalizer in block  58 . These samples provide a distribution of data that clusters around the amplitude levels of the 8VSB signal. The samples are placed in 10 bit, two&#39;s complement form and sorted in ascending order from −512 to +511. A histogram of the sorted data would show a statistical clustering around the eight peaks that are defining the 8VSB levels. In practice, the x variable ranges from about −300 to about +300 and the Y variable ranges from 0 to about 4000 to show the number of samples at each point in the x variable range. A “shell” sort technique may be used to sort the individual data. The smoothing function that is applied to the sorted data smooths the rough or noisy edges that appear on the distribution of the data. A technique that may be employed is to take a moving average of the sorted data. 
     The sorted and smoothed data is supplied to a slicer  70 . The slicing level for the samples is chosen so that the smoother regions of the peaks defining the 8VSB levels are above the slicing level. The sliced data is applied to a centroid determiner  72  which determines the center of each peak, which becomes the estimate of the 8VSB level for each peak. The output of the centroid determiner  72  is applied to a deviation determinor  74  that also receives an input from a known levels block  76 . The output of deviation determiner  74  is applied to a look-up table  78  that supplies the correction information to non linear precorrector  30 . 
     In practice, the eight “determined symbol levels” from centroid determiner  72  are essentially compared with the eight known symbol levels in the original 8VSB signal. Deviation determiner  74  accesses look-up table  78  that stores the necessary correction data for controlling the gain of non linear precorrector  30  to compensate for the non linear distortion effect of power amplifier  40  on the symbol levels of the 8VSB signal. 
     It will be appreciated by those skilled in the art that the levels of the 8VSB symbols are known and therefore, deviations from those levels in the demodulated signal may readily be determined so that correction signals for compensating for the deviations may be developed and applied to the precorrector. It is recognized that numerous changes in the described embodiment of the invention will occur to those skilled in the art without departing from its true spirit and scope.