MXPA99003343A - Receiver for adjusting sync levels in vsb systems - Google Patents

Abstract

A receiver for receiving a cable and terrestrial digital signals of differing VSB modes where the data levels of the various signals have a desired relationship that enables simple data level slicing and error determination. Some of the signals have sync levels that do not conform to the desired relationship and the receiver includes a comb filter for reducing interference when receivingan 8VSB terrestrial signal. The mode of the received signal is determined and a correction factor is applied to the syncs of the signals as needed to reestablish the desired relationship. Operation of the comb filter results in additional levels and a disruption of the desired relationship. For such cases, the comb filtered data levels are modified to make them a subset of the VSB 16 data levels. The modifications are accomplished in feedback paths of the phase tracker after the circuit equalizer and do not affect the data levels passed to the next stage.

Description

RECEIVER TO ADJUST SYNCHRONIZATION LEVELS IN VESTIGIAL SIDEBAND SYSTEMS DESCRIPTION OF THE INVENTION This invention is concerned with the signal systems of VSB (vestigial sideband) and specifically with methods and apparatus for processing the synchronization and data symbols in the digital VSB signals. The recently adopted standards for digital VSB terrestrial signals establish certain levels of data symbols and synchronization. In the document ATSC (Committee on Advanced Television Systems), the data levels to be used for the transmission systems of 8VSB (trellis coding) and 16VSB (ATSC) are specified. The VSB transmission system is not restricted to airborne (terrestrial) transmission and Zenit Electronics Corporation has specified three additional modes that can be used for cable or MMDS systems. These VSB modes are identified as 8/4/2 VSB modes. The modes coded by trellis of 8VSB and 8VSB are identical except for their data transmission speeds. As fully discussed in the Patent REF .: 29436 North American No. 5,508,748 entitled DATA LEVEL SELECTION FOR MULTILEVEL VSB TRANSMISSION SYSTEM, data levels and synchronization levels in the various modes of VSB can be selected to carry a desired relationship with each other, which results in a greater simplification and reduced cost in data segmentation and error correction. Unfortunately, that desired relationship is not present during bilevel segmentation and frame reference synchronization in standards adapted by ATSC and do not need to be present in the VSB signals that can be used. Zenith trellis 8VSB coded mode also contemplates the use of a comb characteristic filter in the receiver to minimize the interference of the NTSC co-channel signals. When the comb characteristic filter is active, it increases the number of data levels, which also results in a deviation from the desired relationship mentioned above. The present invention solves the problems created by the above air signals and allows a VSB receiver to process all the VSB mode signals in an efficient and cost effective manner.

A feature of the invention is to provide a new method of apparatus for processing VSB mode signals in a simple manner. Accordingly, the present invention provides a method for operating a receiver to process digital signals in different ways having a desirable data level relationship, the synchronization levels of certain of the digital signals deviate from the desired relationship in known aspects. , the method includes the steps of determining the mode of the digital signal that is received and altering the levels of synchronization to conform to the desired relationship. The present invention further provides a receiver for receiving a plurality of different digital mode signals having a desirable data level relationship, the synchronization levels of certain of the digital signals deviating from the desirable relationship, such receiver includes means for determining the mode of the digital signal that is received, means for altering the levels of synchronization, of certain signals to conform to the desirable relationship, and means for activating the means of alteration in response to the means of determination when one of the signals is received.

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 receiver of VSB of the prior art; and Figure 2 is a diagram showing the data and synchronization levels for the VSB 16/8/4/2 signals; and Figure 3 is a block diagram of the phase tracker of Figure 1 comprising the invention. With reference to FIGURE 1, an RF (radio frequency) signal (which can be by wire or air) is applied to an IF tuner and demodulator 10 where it is processed in a well-known manner to develop a signal analogous baseband. The demodulated signal is converted to digital form in an A / D (analog to digital) converter 12 and applied to a block 14 that includes appropriate circuits to carry out the DC removal, develop an AGC potential, retrieve registered synchronization information. of symbols and synchronization signals and put into operation a comb characteristic filter.

The VSB mode of the received signal is also determined at this point. The signal is applied to an equalizer or balancer 16 which in turn feeds a phase tracker 18, which is put into operation in accordance with the teachings of US Patent No. 5,406,587 entitled ERROR TRACKING LOOP. The phase tracker feeds a segmenter 20 which operates as described in the aforementioned patent to recover the symbols in the received signal. The segmenter 20 feeds a block 22 which includes a trellis decoder, a symbol / byte converter, convolution de-interleaver circuit, an R-S decoder and a desaliatoriser, all of which are well known in the art. The output data is applied to well-known data processing or television circuits (not shown) for display / use of the data. The diagram in Figure 2 illustrates the data levels of the VSB signals 16/8/4/2. It will be appreciated that these levels will not be considered a limitation of the invention. The corresponding synchronization levels are also indicated for the different VSB mode signals. Note that the synchronization levels are coincident with two of the data levels in their corresponding signal, but they differ for each of the signals. Those levels of synchronization deviate from the desired relationship discussed in the aforementioned US Patent No. 5,508,748. The number in the parentheses for each VSB mode signal indicates the factor by which the synchronization levels for that signal will be multiplied to restore the desired relationship between the data and the synchronization in that signal. Figure 3 depicts the phase tracker 18 and includes a gain block 24 that feeds a complex multiplier 32, by means of a delay circuit 26 and a displacement control 28 and also through a Hilbert FIR filter 30. The complex multiplier 32 feeds a channel I output that constitutes the recovered data and a Q channel output that feeds a quantizer 36. The quantizer 36 feeds a LUT 38 (look-up table) that feeds an error accumulator 34 to develop signals of sine and cosine correction for the complex multiplier 32, a control signal for the shift control 26 and a control signal for the gain control block 24. The circuits described above operate as discussed in U.S. Patent No. 5,406,587 and are not part of the present invention.

The channel I data is applied to a synchronization corrector 40 which is also fed with the VSB mode signal and a frame synchronization signal. The synchronization corrector 40 has four inputs A, B, C and D. As indicated A represents a gain of 1, B a gain of 2/3, C a gain of 4/5 and D a gain of 8/9. A receives a control signal and is activated when the 2VSB signal is received, B is activated for a 4VSB signal, C is activated when an 8VSB signal is received and D is activated when it receives a 16VSB signal. These are the signals when the synchronization levels deviate from the desired relationship. The output of the synchronization corrector 40 is applied to the input A of a multiplexer 44 which is controlled by an enable signal at its SEL input when a signal encoded in the trellis code 8VSB is received with the comb characteristic filter that is activated during the data. Input B of multiplexer 44 receives the corrected data as will be discussed. The output of the multiplexer 44 is fed to a weighted error circuit 46 which in turn feeds the LUT 38. The weighted error circuit 46 evaluates the difference between the actual data received and the closest valid data level for the VSB mode. Then this data error I is weighted to reduce its sensitivity to white noise (amplitude noise). For example, if the distance between the two adjacent data levels is d ', the range for any data level is individual (+ d / 2, -d / 2). Data errors up to (+ d / 4, -d / 4) are not modified. The magnitudes of error greater than (d / 4) are weighted in such a way that an error of (d / 2) is zero. The weighted channel I error and the quantized Q channel data are fed to the LUT 38. The LUT 38 provides the desired correction signal to correct any phase error, offset error and gain error present in the received data. In order to correct the data levels in terms of their deviation from the desired ratio when the comb characteristic filter is active, a comb character data corrector 42 is added. The comb characteristic data corrector receives the data from the I channel and a frame synchronization and adds (or arrests) 16 to the recovered data levels. This compose data levels 15, created from the 8VSB signal during feature filtering in comb, a set of 16VSB data levels to thus restore the desired relationship between data levels. As the output of the comb characteristic data corrector 42 is applied to the B input of the multiplexer 44, this data is used only when the selected input of the multiplexer 44 receives an input in response to a coded signal of trellis 8VSB which is received and having the comb feature filter activated. Note that the corrections are made to the signal only in the phase tracking circuit feedback path and do not affect the data levels that are passed to the next stage. The synchronization levels in Zenith's 16/8/4/2 VSB modes should ideally be +128 and -128 for an appropriate scaling factor. In the mode coded by trellis 8VSB, the corresponding synchronization levels have been set to +160 and -160. To correct them, that is, to bring them to the +/- 128 levels, a multiplication factor of 4/5 is applied to the synchronizations when they are corrected by the synchronization corrector 40. When corrected, the synchronizations of the trellis encoded signal 8VSB carry a specific desired relationship with the rest of the signal. Similarly, synchronization levels of the ATSC signal of 16VSB are + 1-144. (The data levels are the same). Therefore, a multiplier of 8/9 is applied to the synchronizations in the ATSC signal of 16VSB, which restores the desired relation to the signal. For a 4VSB signal a correction factor of 2/3 is applied and a 2VSB signal does not need a correction factor. It will be appreciated that the selection of 2VSB synchronization levels as the basis at which the other synchronization levels are corrected is arbitrary. The specific levels and correction factors are not limiting of the invention. It should also be appreciated that the implementation shown is preferred although other implants may be used. For example, a look-up table that correlates the levels of synchronization with error information for the least significant bits in each mode could be used to obtain the desired relationship specified in the aforementioned US Patent No. 5,508,748. Also, the known synchronization values could be restored and the synchronization values received subtracted from them to obtain the error that require compensation for that tracker. These modifications are clearly within the scope of the present invention. Correction of feature data in comb has been discussed briefly. When the comb characteristic filter operates with the 8VSB level eight signal, fifteen signal levels result, with different levels for sixteen of the corresponding levels of a 16VSB cable signal. The fifteen levels of the characteristic signal in the comb can be made to correspond to fifteen of the sixteen levels of the 16VSB cable signal when adding up (or subtract) 16 for each feature level in comb. This composes the fifteen levels of the characteristic signal in comb to a set of the 16VSB signal and again restores the desired relation. What has been described in a new method and section to correct the synchronization levels of different VSB mode signals to restore a desired relationship between synchronization levels and data levels, thereby enabling the use of a circuit simple segmentation and cash in cost. Similarly, the changed data levels of a trellis-encoded signal of 8VSB filtered into comb characteristic are converted to a set of the sixteen levels of a 16VSB signal. It is recognized that numerous changes in the disclosed invention will be apparent to those skilled in the art without deviating from its true spirit and scope. The invention will 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 the conventional one for the manufacture of the objects or products to which it refers.

Claims (10)

1. CLAIMS Having described the invention as above, the content of the following is claimed as property * 1. A method for operating a receiver to process digital signals in different ways having a desirable data level relationship, the synchronization levels of certain of the digital signals deviate from the desired relationship in known aspects, the method is characterized in that it includes the steps of determining the mode of the digital signal that is received and altering the levels of synchronization to conform to the desired relationship.
2. 2. The method according to claim 1, characterized in that the different modes comprise VSB signals by cable and terrestrial modes of different VSB and wherein the alteration step further includes the steps of determining a correction factor for the levels of synchronization of certain signals and apply the correction factor to the levels of synchronization to certain signals.
3. 3. The method according to claim 1 or 2, characterized in that the receiver includes a comb characteristic filter selectively operable, the operation of the comb characteristic filter changes the desired ratio of the data levels and the method includes the step to modify the changed data levels to reestablish the desired relationship.
4. The method according to claim 3, characterized in that the comb characteristic filter is used to process a signal from a given vestigial sideband mode and wherein the modification step comprises changing the data levels of the filtered signal in comb characteristic to convert it to a subset of data levels of a higher vestigial sideband mode signal.
5. The method according to claim 4, characterized in that the given vestigial sideband mode is 8 and the highest bastigial sideband mode is 16 and wherein the modification step comprises raising or collapsing the filtered characteristic levels in comb by a fixed amount such that they correspond to the adjacent data levels of the vestigial sideband signal of mode 16.
6. 6. A receiver for receiving a plurality of digital signals in a different way having a desirable data level relationship , the synchronization levels of certain of the digital signals deviates from the desirable relationship, the receiver is characterized in that it includes means for determining the mode of the digital signal that is received, means for altering the levels of synchronization of certain signals to conform to the desirable relationship and means to enable the means of alteration in response to the means of determination when a a of certain signals is received.
7. The receiver according to claim 6, characterized in that the signals in different modes include VSB (vestigial sideband) signals by wire and terrestrial signals of different VSB (vestigial sideband) mode and wherein the alteration means include means for generating a correction factor for the synchronizations of the certain signals and means for applying the correction factor to adjust the synchronizations of the certain signals in response to the determination means.
8. The receiver according to claim 6 or 7, characterized in that it includes error correction means for correcting the errors received at the received data levels and wherein the corrected synchronization levels are fed to the error correction means.
9. 9. The receiver according to claim 6 or 7, characterized in that it includes comb characteristic filter means to reduce the interference in a terrestrial vestigial sideband mode signal, the comb characteristic filter means change the data levels and alter the desired relationship; and the receiver includes means sensitive to the means of determination, to modify the changed data levels to re-establish the desired relationship when the comb feature fr media is active. The receiver according to claim 9, characterized in that the modification means converts the data levels of the comb-like fred signal from the vestigial sideband mode given to a subset of the data levels of a signal mode. of VSB (vestigial sideband) higher.