MXPA98004528A - Sync compensated agc system for vsb receiver - Google Patents

Abstract

A technique and apparatus for developing AGC voltage in a receiver capable of receiving VSB signals that do not have the average magnitudes of the data symbol levels and the sync symbol levels related so as to produce the same average level. One technique eliminates the sync symbols from determination of the AGC voltage. A preferred technique corrects for the different symbol levels (56, 58 and 60) of the sync by multiplexing (54) in an offset at the proper time or multiplying the sync symbols by a compensating factor.

Description

AUTOMATIC GAIN CONTROL SYSTEM COMPENSATED BY SYNCHRONIZATION FOR RESIDUAL LATERAL BAND RECEIVER DESCRIPTION OF THE INVENTION This invention relates to the signal system VSB (residual sideband) and specifically to the methods and apparatus for developing AGC voltages (automatic gain control) in digital VSB signals. The recently adopted standards for digital terrestrial VSB signals establish certain data levels and synchronization symbol. In the document of the ATSC (Committee of Advanced Television Systems), the levels of data that should be used for the transmission systems 8VSB (coded by Trellis) and 16VSB (ATSC) are specified. The VSB transmission system is not restricted to over-the-air transmission (terrestrial) and Zenith Electronics Corporation has specified three additional modes that can be used for cable or MMDS systems. These VSB modes were identified as VSB cable modes 8/4/2. The two 8VSB modes differ only in the amount of data that is transported. As discussed in full in U.S. Patent No. 5,508,748 entitled SELECTION FROM THE DATA LEVEL TO THE MULTIPLE LEVEL VSB TRANSMISSION SYSTEM, the data levels and REF levels: 27003 synchronization in the different VSB cable modes can be selected so that they have a desired relationship with each other, which results in a large Simplification and reduced costs in data division and error correction. This relationship also allows the easy production of AGC voltages of the VSB signal. Unfortunately, that desired relationship is not present during synchronization of the two-tier segment and the block synchronization of the standards adopted by the ATSC and does not need to be present in the VSB signals that can be used. With the desired ratio, the average value of the magnitude of the data symbols and the average value of the magnitude of the synchronization symbols is the same. In some of the mentioned signals, that relationship is not present as the average of the magnitude of the data levels nor is it the same as the average of the magnitude of the reference synchronization levels. Therefore the production of AGC potentials based on the average of the data and the amplitude of synchronization is prone to errors. The present invention solves the problems created by the undesirable relationship between synchronization and data for generation of AGC in numerous ways and allows a VSB receiver to generate AGC voltages for all VSB mode signals in a relatively simple manner.

A feature of the invention is that it provides a novel method and apparatus for generating AGC voltages for VSB mode signals. The present invention therefore provides a method for operating an AGC system in a receiver that receives a plurality of digital signals having different levels of average data symbol and average synchronization symbol levels, the method includes the step of processing a signal received to determine an average symbol level while preventing any differences in the synchronization symbol level from affecting the determined average symbol level, and using the determined average symbol level to develop an AGC voltage. Another feature of the invention is that it provides a novel VSB receiver that can easily produce the AGC voltage for all VSB mode signals in a simple manner. The present invention further provides a receiver for developing an AGC control voltage for any of a plurality of received digital signals having different average symbol levels and average synchronization symbol levels, the receiver includes means for processing the received signals to determine a average symbol level, means to prevent differences in synchronization symbol levels from affecting the determined average symbol level, and means to use the determined average symbol level to develop the AGC potential. These and other features and advantages of the invention will become apparent upon reading the following description of a preferred embodiment of the invention in conjunction with the drawings, in which. FIGURE 1 is a simplified partial block diagram of a prior art VSB receiver; and FIGURE 2 is a block diagram of an AGC generation system for the circuit of FIGURE 1 embodying the invention. Referring to FIGURE 1, an RF signal (which may be cable or over the air) is applied to an IF tuner and the demodulator 10 where it is processed in a well-known manner to develop an analog band signal base. The demodulated signal is converted to a digital form in an A / D (analog to digital) converter 12 and is applied to a block 14 that includes the appropriate circuits for removing DC, developing AGC voltages of increasing gain and falling gain, recovering information from clock and synchronization signals, operate a comb filter and develop a signal of VSB mode. The VSB mode of the received signal is also determined at this point. The signal is applied to a compensator 16 which in turn is supplied to a phase tracker 18, which is operated in accordance with the teachings of U.S. Patent No. 5,406,587, entitled "ERROR TRACKING CIRCUIT". The phase tracker is supplied to a divider 20 which operates as described in the above-mentioned patent to recover the symbols in the received signal. The divider 20 feeds a block 22 which includes a symbol / byte converter, convolutional deinterleaver circuit, trellis decoder, R-S decoder and descrambler, all of which are well known in the art. The output data is applied to a well-known television circuit or data processor (not shown) to display / use the data. FIGURE 2 represents an AGC generating circuit constructed in accordance with the invention. The A / D signal 12 is coupled to a DC remover circuit 30 where the DC due to the pilot and other DC deviations in the signal VSB are removed. The absolute value of the signal is taken in a circuit 32 and applied to a positive input of an adder 34. The adder 34 supplies the signal to an accumulator 36 which, in turn, supplies it to a tilting circuit 50 (which acts as a record) . The output of the jogger 50 is coupled to a pulse width (PWM) modulator 52 which generates the AGC voltage and activates any of its ups or downshifts.A series of deviations are produced by the blocks 35, 56 , 58, 60 and 62 and are supplied to the corresponding inputs A, B, C, D and E of the multiplexer 54 which is operated by a collective conductor that selects the mode For the selected size 5 of those signals the deviations are +144 for a 4VSB signal, +120 for an 8VSB signal and +108 for a 16VSB signal The output of the multiplexer 54 is coupled to the negative input of the adder 34. In all VSB modes, the average of the data level JLO symbols is 96 (for a selected size of the signal.) This is also the average of the two-level sync symbols in the 2VSB signal, for example, and the development of an AGC voltage by sampling all or portions of such a signal is not is affected s i the synchronization symbols are sampled together with the data symbols. For other signals where that relationship is present, some compensation should be made for the synchronization of the different levels. It should be appreciated that the AGC voltage can be developed by sampling only the synchronization symbols; sampling only the data symbols; or by sampling a combination of synchronization symbols and data symbols. By lengthening the portion of signal that is sampled and averaged, the AGC will be more accurate, but it will be slower.

Averaging shorter samples, although less accurate for the AGC. In operation, the DC of the pilot and any other deviations of DC are removed in the DC stirring circuit 30 and the absolute value of the rest of the sampled signal is applied to the adder 34 where it is combined with a selected one of the deviations of the circuits 35, 56, 58, 60 and 62 by the operation of the multiplexer 54. The operating signal selected by the multiplexer 54 is determined from the VSB mode of the signal and the type, ie, whether the signal is cable or is on the air. The collective bus driver selects the appropriate MUX input. During the data portions, the signal input A of the MUX 54 is selected. The other inputs are selected based on the relationship of the synchronization levels and data in the other modes. The circuit for determining the selected signal is well known and is not part of the present invention. In signals where the desired relationship between the data symbol levels and the synchronization symbol levels is present, the data deviation developed in block 35 is supplied at all times4 This deviation is indicated, at the beginning +96, the which results in the level of the signal at the entrance to the nuisance 36 being zero under ideal conditions. It should be appreciated that the deviation values are based on the particular signal levels and equipment used and are not limiting of the invention. If a 16VSB signal is received, the deviation of +108 could be supplied by the block 60 during the synchronization symbols and +96 during the data symbols to compensate for the higher synchronization symbol levels. If an 8VSB signal was received, a deviation of +120 from block 58 of the sampled signal could be subtracted during the synchronization time periods and +96 could be subtracted during the data times. Similarly, for a 4VSB signal, a deviation of +144 during synchronization and +96 during data could be subtracted. A 2VSB signal obviously has the same levels of synchronization and data and a deviation of +96 could be subtracted during synchronization and data. The deviation in block 62 illustrates that the system is not limited to the discussed signals but can easily be extended to other VSB signals that share the common data levels, but include synchronization levels that differ from any of the signals discussed. It should also be noted that the timing signals can be used to completely eliminate synchronization signals during sampling. In such a system, the size of the synchronization symbols will not affect the derived average symbol level at which the AGC voltage is based. This is an aspect of the invention, namely to correct for the differences in the levels of synchronization between the different VSB signals, as described, eliminated all synchronization symbols from consideration in the development of the AGC potential. This solution involves more elaborate timing circuits and may also involve some sacrifice in the speed of the AGC. The preferred implementation is to use the correct synchronization levels indicated with the different deviations. In this implementation, the effect of segment synchronization has been ignored since segment synchronization occupies only a very small portion-unlike block synchronization. It should also be appreciated that the multiplication technique can be used to correct the synchronization symbol levels in VSB modes over air. Such a technique is similar to the adder arrangement shown, but could involve multiplying the synchronization symbol levels by an appropriate constant, depending on the type of signal received, to compensate for the error introduced by the synchronization levels. What has been described is a novel method and apparatus for developing AGC potentials in a receiver that is capable of receiving different types of VSB signals in which the desired relationship between the data symbol levels and the synchronization symbol levels are not present It is recognized that those skilled in the art will contemplate numerous changes in the embodiments of the described invention without departing from the true spirit and scope thereof. The invention should be limited only as defined in the claims.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (8)

1. A method for operating an AGC system in a receiver that receives a plurality of digital signals having different levels of average data symbol and average synchronization symbol levels, the method is characterized in that it includes the steps of processing a received signal to determine a average symbol level while avoiding any differences in the synchronization symbol level affecting the determined average symbol level, and using the average symbol level determined to develop an AGC voltage.

2. The method according to claim 1, characterized in that the plurality of received digital signals includes a pilot and DC deviations, the method includes the step of removing the DC pilot and the DC deviations of the received signal before processing the signal received to determine an average symbol level.

3. The method according to claim 1 or 2, characterized in that the average data symbol level differs from the average synchronization symbol level in the received signal and the method includes the step of adjusting the synchronization symbols to produce a level of symbols Average synchronization corresponds to the average data symbol level.

4. The method according to claim 1 or 2, characterized in that the average data symbol level differs from the average synchronization symbol level in the received signal and the method includes the step of multiplying the synchronization symbols by a factor to bring the symbol level of average synchronization to a correspondence with the average data symbol level.

5. The method according to claim 1 or 2, characterized in that the processing step comprises using timing signals to sample only data symbols in the received signal.

6. A receiver for developing an AGC control voltage for any of a plurality of received digital signals having different average data symbol levels and average synchronization symbol levels I, the receiver is characterized in that it includes means for processing the received signals to determine an average symbol level / means to prevent differences in the synchronization symbol levels from affecting the determined average symbol level, and means to use the determined average symbol level to develop the AGC potential.

7. The receiver according to claim 6, characterized in that the plurality of received digital signals includes a DC pilot and DC deviations, and the receiver includes means for removing the DC pilot and the DC deviations of the received signals before the reception of the signals received by the means of processing.

8. The receiver according to claim 6 or 7, characterized in that the average data symbol level differs from the average synchronization symbol level in the received signal, and the receiver includes means for adjusting the synchronization symbols of the received signal to produce a correspondence between the average synchronization symbol level and the average data symbol level.