Abstract:
A communications system, having a combination Reed-Solomon encoder and a Turbo-Code encoder Data frame configuration which may be changed to accommodate embedded submarkers of known value are embedded in with the data order to aid synchronization in the receiver system, by providing strings of known symbols. The string of known symbols may be the same as the symbols within a training header that appears at the beginning of a data frame. Frame parameters may be tailored to individual users and may be controlled by information pertaining to receivers, such as bit error rate, of the receiver. Additional headers may be interspersed within the data in order to assist in receiver synchronization. Frames of data may be acquired quickly by a receiver by having a string of symbols representing the phase offset between successive header symbols in the header training sequence in order to determine the carrier offset. Phase lock to a signal may achieved after determining carrier offset in receivers by correlating successive symbols in successive headers. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or the meaning of the claims.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of patent application No. 09/729,442 entitled INTERSPERSED TRAINING FOR TURBO CODED MODULATION filed on Dec. 4, 2000 now U.S. Pat. No. 6,693,566 which claims priority from provisional application No. 60/168,808 entitled INTERSPERSED TRAINING FOR TURBO CODED MODULATION filed on Dec. 3, 1999, the contents of which is expressly incorporated by reference as though set forth in full. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to digital signal reception and, in particular, signal coding which assists in the synchronization of receivers with turbo decode capability. 
     BACKGROUND OF THE INVENTION 
     In recent years, transmission of data via satellite has increased considerably. Recently, the number of personal satellite receivers have also been increasing. As large satellite receiving antennas and expensive receivers are replaced by smaller and less expensive equipment, the demand for such systems continues to rise. As the demand for satellite communication systems rises, systems which have increased performance have a distinct market advantage. Improving designs and increasing the level of system integration within satellite receivers can offer the dual benefits of decreasing system costs and increasing performance. Accordingly, there is a need for improved satellite communication systems within the art. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a method of creating a data sequence includes placing a training sequence at a beginning of a data frame, placing a plurality of the blocks of turbo encoded data within the data frame following the training sequence, and interspersing a plurality of submarkers within the turbo encoded data blocks. 
     In another aspect of the present invention, a training sequence and submarker insertion apparatus includes an input adapted to receive a plurality of turbo encoded data blocks, and an inserter adapted to insert a training sequence before the turbo encoded data blocks and insert a plurality of submarkers within the turbo encoded data blocks thereby creating a data frame. 
     In yet another aspect of the present invention, a training sequence and submarker insertion apparatus includes receiving means for receiving a plurality of turbo encoded data blocks, insertion means for inserting a training sequence before the turbo encoded data blocks, and inserting a plurality of submarkers within the turbo encoded data blocks thereby creating a data frame. 
     In a further aspect of the present invention, a transmitter includes a forward error correction device having a turbo encoder and a training sequence and submarker insertion device coupled to the turbo decoder, the training sequence and submarker insertion device comprising an input adapted to receive a plurality of turbo encoded data blocks from the turbo encoder, and an inserter adapted to insert a training sequence before the turbo encoded data blocks and insert a plurality of submarkers within the turbo encoded data blocks thereby creating a data frame, and a modulator coupled to the forward error correction device to modulate the data frame. 
     In yet a further aspect of the present invention, a method of creating a data sequence includes turbo encoding data into a plurality of turbo encoded data blocks, creating a data frame comprising a first portion and a second portion, the first portion preceding the second portion in time, placing a training sequence in the first portion of the data frame, placing the turbo encoded data blocks in the second portion of the data frame, and interspersing a plurality of submarkers within the turbo encoded data blocks. 
     It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only embodiments of the invention by way of illustration of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  is a graphical representation of an example environment in which embodiments of the present invention may operate. 
         FIG. 2  is a graphical illustration of a data format as may be used with embodiments of the present invention. 
         FIG. 3  is a graphical illustration of frequency versus amplitude graphs representing a band of signals with different center frequencies (carrier offsets). 
         FIG. 4  is a graphical representation of header symbols being transmitted through a communications channel. 
         FIG. 5  is graphical illustration of a mechanism that may be used to search for given sequence of symbols, such as those found in a header. 
         FIG. 6  is a graph illustrating correlation values versus frequency utilizing a system for a short sequence illustrated in FIG.  5 . 
         FIG. 7  is a graph of correlation value versus frequency when a long sequence is being correlated. 
         FIG. 8  is a graphical illustration of a step in the process, which can find the proper demodulation frequency of a signal, without the necessity of a series of cut and try steps. 
         FIG. 9  is a graphical representation of a differential correlator. 
         FIG. 10  is a graphical illustration relating header data to differential correlator output and further relating header position to a counter, which counts cycles of a receiver clock. 
         FIG. 11  is a graphical illustration of the process by which the frequency of the receiver clock may be synchronized to an incoming data stream, using a clock counter. 
         FIG. 12  is a block diagram illustrating the parts of a common correlator as may be used to establish phase lock to a received signal. 
         FIG. 13  is a graphical illustration of a serial correlator as may be used in embodiments of the present invention. 
         FIG. 14  is a block diagram representation of subsystem circuitry, which comprises the modulator and forward error correcting sections of a transmitter system according to an embodiment of the invention. 
         FIG. 15  is a block diagram illustrating the component and signal flow which comprise an exemplary embodiment of forward error correction according to an embodiment of the invention. 
         FIG. 16  is a graphical illustration of a generalized data frame, according to an embodiment of the invention. 
         FIG. 17  is a graphical illustration of a frame of data having three submarkers disposed therein. 
         FIG. 18  is a graphical illustration of an exemplary arrangement of data blocks and submarkers within a frame. 
         FIG. 19  is a block diagram of a set top box according to an embodiment of the current invention. 
         FIG. 20  is a block diagram of a receiver according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary Communication System 
     In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown, by way of illustration, specific embodiments illustrating ways in which the invention may be practiced. It is to be understood that other embodiments may be realized, as the inventive concepts disclosed herein may be used in the design and fabrication of other embodiments, without departing from the scope and spirit of the inventive concepts disclosed herein. 
     Accordingly, embodiments of the present invention relate, generally, to satellite communication systems. However, for the purposes of simplifying this disclosure, the embodiments are described herein with relation to direct broadcast satellite systems. Although the described exemplary embodiments disclosed herein are directed to direct broadcast satellite systems, there is no intent to limit the invention to the example embodiments. The exemplary embodiments are intended to illustrate inventive aspects of the present invention, which are applicable to a wide variety of electronic systems. 
     Satellite communication systems in general comprise three parts. The first part is a transmit system also known as a ground station. The transmit system may receive data from a variety of sources such as cable companies, Internet providers, etc. The received data is then coded, modulated and provided to a transmitter for broadcast. Coding generally includes a forward error correcting code. A forward error correction and modulation section of a transmitter accepts digital data and then constructs a signal that will be relayed, via a satellite, to the user. The second element in the satellite communication system is a satellite, which is illustratively a geo-synchronous satellite. The third part of the satellite communication system is a receiver system. The receiver system generally comprises an outdoor unit and a receiver which receives the signal from the outdoor unit, demodulates it and decodes it to recover the original signal. The original signal is then available for use in a device such as a television or computer. 
       FIG. 1  is a graphical representation of an example environment, in which the described exemplary satellite communication system may operate. Within the environment of  FIG. 1 , a data source  101  provides data to a ground station  100 , which then broadcasts the data to a satellite  109 . The satellite in turn re-broadcasts the data to a receiver system  110  thereby providing it to a user device  117 . 
     In  FIG. 1  the data source  101 , such as a television cable signal, represents an example of one of a number of various types of data signals, which may be conveyed by the system. For example, the data signals may include, but are not limited to, television channels, music channels, or data from Internet websites. The data signals are provided, by the data source  101 , to a modulator and forward error correction (FEC)  103 . Within the modulator and FEC  103 , the data is modulated on one or more carrier waveforms. The modulator and FEC  103  translates the data from data source  101  into a form suitable for transmission. The modulated data is further coupled to a transmitter  105 . In the transmitter  105 , the data stream from the modulator and FEC may be further amplified and coupled, for example, to a dish antenna  107  for transmission to the satellite  109 . 
     The satellite  109  accepts data transmitted from dish antenna  107 . Satellite  109  is commonly a geo-synchronous satellite, in which the satellite&#39;s position is a constant location above the earth, but is not limited to such. In a geo-synchronous satellite, orbital rotation of the satellite is one day exactly matching the rotational speed of the earth, thereby maintaining the satellite at a constant position above the earth. The data transmission is accepted by the satellite  109 , amplified and rebroadcast to the receiver system  110  on earth. 
     A user antenna  111 , accepts the data transmission from the satellite  109 . The received signal is then further coupled into a LNB (low noise block)  113  where it is amplified. The LNB  113  then provides the signal to a set top box  115 . Within the set top box  115 , the signal is demodulated and converted into a form which may be used by the user device  117 , such as a television or computer. The LNB  113  and the antenna  111  are collectively referred to as the ODU (outdoor unit)  119  because they are typically located outdoors. 
     Transmission of Data Between the Transmit and Receiver System 
       FIG. 2  is a graphical illustration of a data format as may be used with embodiments of the present invention. The data stream is divided into a series of successive concatenated frames. Three successive concatenated frames are illustrated in  FIG. 2  at  301 ,  303  and  309 . A typical frame  301  may comprise a header  305  followed by data  307 . Coupling a header  305  along with data  307  in a discrete frame  301  may provide particular advantages. For example, the frame  301  may have a different modulation scheme than a successive frame  303 . Frame  301  may contain QPSK symbols and frame  303  may contain 8PSK symbols, and each frame may be intended for different receivers. Additionally a different format can be used for the header and the data, for example a QPSK header may be used with 8PSK data. The header may be encoded so as to indicate the format of the data within the block, or of future blocks. Additionally the header may contain a “training sequence” to assist the receiver in synchronizing to the transmitted signal. A great variety of combinations of data partitioning are possible. Each frame may be intended for different users and hence may have different types of modulation and data formats in successive data frames. By using such a flexible scheme for data delivery, a variety of user needs can be accommodated. 
     Compensating for Frequency Offsets 
       FIG. 3  is a graphical illustration of frequency versus amplitude graphs representing a band of signals with different carrier (center) frequencies having differing carrier offsets. The band-pass curve  705  represents a nominal case in which F c  (the desired center frequency) is situated in the center of the bandwidth of the signal  705 . In actual practice, the frequency band may be offset from the desired center as illustrated in bandwidth curves  707  and  709 . This offset may be due to a variety of factors, such as drift in the transmitter or receiver front end, drift in the relay satellite, and in non Geo-stationary systems due to Doppler effects. In curve  707 , the center frequency has been displaced and is actually no longer equal to F c  but is equal to F c +ΔF. In curve  709 , the carrier frequency is actually lower than F c , the actual center frequency for band-pass curve  709  is located at F c −ΔF. In practice, offsets can cause considerable displacement of a signal&#39;s center frequency. Offsets in carrier frequency typically must be accounted for to assure proper reception. Therefore, finding the actual carrier frequency is commonly an early step in locking a receiver system to a transmitted signal. 
     In an exemplary embodiment of the present invention, header data is identified and, in the process of identifying the header data, the frequency offset of the communications channel carrier frequency is found. One method for determining the carrier frequency offset at the receiver system is to mix the incoming signal with series of frequencies one at a time until the correct frequency is found. By mixing the incoming signal across a series of frequencies and correlating the resultant signal (for example a resulting baseband signal), the actual center frequency of the incoming signal can be determined. Mixing the incoming signal with a series of frequencies, however, can take considerable amount of time. The number of frequency offsets that may have to be applied to the incoming signal, before the correct frequency offset is found, can be considerable. It is desirable to be able to determine the offset of the center frequency without going through a process of trial and error, incrementing the mixing frequency and using the incremented frequency to mix with the incoming signal. 
       FIG. 4  is a graphical representation of header symbols being transmitted through a communications channel. A sequence of symbols  801  is represented by A N . The sequence A N  contains a number of symbols H L  which is equal to the header length. A N  represents a sequence of header symbols for N=0 to N=(H L −1). 
     The header symbols A n  are coupled into communications channel  803 . The symbols are then accepted from the communication channel by a receiver system. The received symbols R N  form a set of symbols  805  that have had noise and/or distortion added to them as a consequence of transmission through the communications channel. The sequence of symbols R N  can be represented by a modeling mathematical expression A N e −jwt +N. Term N ( 809 ) represents the noise added by the communications channel and term e −jwt ( 807 ) is a frequency component, representing the frequency offset of R N &#39;s carrier. The communications channel  803  generally comprises everything between the transmitter and the user&#39;s receiver system. 
     For a given frequency offset represented by term  807 , H L  (the length of the sequence A N ) determines how large of a frequency offset can be tolerated and still form a proper correlation. The longer the sequence of symbols that are being detected in a correlator, the less frequency offset that can be tolerated in the center frequency. Therefore, in order to tolerate a large frequency offset, a short sequence is desirable. On the other hand, a long sequence will result in more gain when correlated with the sequence to be detected. So from the standpoint of correlator gain, a long sequence is desirable. Generally, correlator gain is more important than being able to tolerate a large frequency offset, because the correlator gain may be used to offset the noise within the channel. 
       FIG. 5  is a graphical illustration of a mechanism in the receiver system that may be used to search for a given sequence of symbols, such as “training symbol” found in a header. Training symbols found in the header are known symbols, which a receiver system may look for in order to synchronize or “lock to” a received transmission. The output of the communications channel  803  is provided to a demodulator  903  in the receiver system. The demodulation frequency F D    901  is also coupled into the demodulator  903 . It is F D    901  that will be mixed with the incoming signal and that must be properly adjusted to translate the incoming data symbol stream, from the communications channel  803 , into a baseband sequence of received symbols R N ,  805 . The baseband symbol stream R N , ( 805 ), is coupled into a correlator  905 . The correlator  905  will search for a match between the header sequence A N  and the received symbols R N . The correlator is clocked by a clock  905  which controls the comparison between the known header A N  and the received symbol stream R N . The comparison of A N  and R N  within the correlator  905  is can be a bit by bit comparison. The bits that match each other are typically added to produce a correlation value  919 . The correlation process can be used to ascertain the center frequency of the carrier of the received symbol R N  data stream. For example, F D    911  may be changed in steps (swept) and the correlation value, which results, saved. Once the demodulation frequency has been swept across the range of possible values, the correct demodulation frequency can be ascertained by observing which frequency step gives the highest correlation value. 
       FIG. 6  is a graph illustrating the correlation value for a short correlation sequence versus frequency for the correlation system illustrated in FIG.  5 . The maximum correlation value is given by point  1009 . The correlator output  1005  tends to have a wider curve as the sequence correlated becomes shorter. However, the maximum output  1009  tends to become smaller as the sequence becomes shorter, thus yielding a lower gain. 
       FIG. 7  is a graph of correlation versus frequency offset for a long sequence being correlated by a system such as illustrated in FIG.  5 . When a long sequence is correlated, the curve produced  1103  is much narrower and its peak value is higher than a short sequence. This means that the correlation has more gain, but that finding F D    901  is generally of greater concern. In other words, a shorter sequence will allow larger steps in F D    901  to be used in finding the correct F D , than will a longer sequence. Therefore, to search for a longer sequence, the frequency steps that are used as the demodulator frequency  901  should be closer in value and, hence, may comprise more steps than a corresponding shorter sequence. Even if a shorter correlation sequence can be tolerated, the process of trial and error in order to determine the correct frequency of demodulation  901  can be cumbersome and time-consuming. It is advantageous to eliminate the trial and error method of determining the necessary demodulation frequency. 
       FIG. 8  is a graphical illustration of a step in the process, which can find the proper demodulation frequency of a signal without the necessity of a series of trial and error steps. Equation  1201  represents a series of N header symbols. The series of header symbols are part of a data frame, for example  301 , as illustrated in FIG.  2 . The satellite transmission may contain a header  305  and data  307  in a frame  301  as a convenient way of packaging satellite transmissions. The header may be known to a receiver system beforehand and hence is a convenient data synchronization pattern. A known header, called a training sequence, is used to adjust the receiver system to receive the data frame. 
     From the header symbols, as given in equation  1201 , a secondary sequence  1203  can be created. The secondary sequence, represented by B N  is a sequence of symbols which represents the phase difference between successive symbols within the header. So, for example, the symbols in the sequence B N  are found by taking the phase difference between A N  and A N-1 . Symbol B N-1  is formed by taking the difference between symbol A N-1 , and A N-2 . The remaining symbols in the B N  sequence are formed similarly, as illustrated in equation  1203 . The general formula for the B N  sequence is given in equation  1205 . Because the header A N  is known beforehand by the receiver system, the sequence B N  also may be known beforehand (or computed) by the receiver system. 
     A sequence S N  is formed by taking the phase difference between successive received symbols R N . This sequence is as illustrated in equation  1207 . 
       FIG. 9  is a graphical representation of a differential correlator in the receiver system that can be used to correlate the S N  and B N  sequences of symbols. The differential correlator  1305  correlates the phase differential between successive symbols. B N  is the sequence to which the received symbol differential sequence S N ,  1303 , is compared. [The sequence B N  may be time reversed and a conjugated version of B N  (B N *)]. When the received symbol differential string S N  is compared to the known differential sequence B N , a series of header peaks  1307  are produced at the output of the differential correlator  1305 . 
       FIG. 10  is a graphical illustration relating the header data to the differential correlator output and further relating header timing to a counter, which counts cycles of a receiver system clock. The differential correlator  1305  receives the differential symbol stream S N . The differential correlator  1305  also has preloaded the B* N  sequence,  1301 . The correlation between output  1415  of the differential correlator  1305  compared to the received symbol sequence R N  is shown in graph  1419 . Once a complete header has been received and the differential sequence S N  created from the header, a differential correlator peak  1401  will be seen from the correlation between S N  and B N . 
     Concurrently with the operation of the differential correlator  1305 , a clock counter  1407  counts cycles of the receiver system clock  1413 . The clock counter  1407  is configured so that it rolls over, that is, once the clock counter  1407  achieves a maximum value, the counter is automatically reset to zero upon the receipt of the next clock pulse. The clock counter  1407  is configured so that its maximum count will correspond to a period that is longer than the combined period of the header and data. When the clock counter  1407  is free-running, i.e., counting receiver system clock cycles and resetting upon a maximum clock counter count, its output period is guaranteed to be longer than the received header plus data packet. Because the roll over period of the clock counter is longer than the longest combined period of a header plus data, it is guaranteed that, between rollovers of the clock counter, a maximum differential correlator value, for example  1401 , will occur. Because a maximum value is being determined, the system is not dependent on a particular threshold value. Because there is always a maximum value within the clock counter period, any design difficulties encountered by attempting to adjust a correlator output threshold for changing signal conditions is eliminated. The output counter value  1417  is portrayed as a saw-toothed waveform. This portrayal is for purposes of illustration and comparison only. The output  1417  of clock counter  1407  is generally a sequence of increasing integers, and therefore, the output counter value  1417 , instead of being a sawtooth as portrayed, is generally a series of discrete steps. 
     Once the maximum differential correlator value has been ascertained for the free-running clock counting period, the clock counter output value corresponding to the differential correlator peak value within the clock counter period can be determined. The output counter values  1403  and  1409  correspond to maximum differential correlator values. By knowing the values of the clock counter that correspond to successive differential correlator peaks, i.e.  1401  and  1409 , the actual period of the header plus data can be determined. 
       FIG. 11  is a further graphical illustration of the process by which the receiver system clock may be synchronized to the incoming data stream using the clock counter  1413 . Curve  1501  represents the output of the differential correlator  1305 . Initially, the receiver system clock counter  1413  is in a condition of free-run, as illustrated in curve  1503 . When the clock counter  1413  is free-running, it will count the receiver system clock pulses up to a maximum number and then roll over. The time between roll over of the clock counter is referred to as the clock counter period, for example  1525 . By observing the output of the differential correlator  1305  during a full clock counter period, for example  1525 , a time between peak correlator output values can be determined. 
     Once the peak correlator values  1509  and  1517  have been determined, corresponding values  1511  and  1519  of the clock counter  1413  can be determined. Once the values  1511  and  1519  have been determined, the difference between them can be determined. The free-run frequency of the receiver system clock, and hence the free-run frequency of the clock counter  1413 , which counts the cycles of the receiver system clock, can be adjusted so that the difference between  1511  and  1519  is zero. When the receiver system clock  1413  has been adjusted to a point where values  1511  and  1519  are equal, the receiver system clock has been synchronized in frequency with the transmitter clock. The frequency adjustment is accomplished by decreasing the clock counter period, as illustrated by  1527  of the waveform  1505 . 
     Those skilled in the art will recognize that the preceding description is exemplary only and many variations of this scheme are possible. For example, instead of merely measuring the time difference between points  1511  and  1519 , a series of points may be measured and an average time difference may be computed. Similarly, two maximum correlator values can be selected from a number of correlator peaks in order to select, for example, the maximum value of correlation peaks over a series of multiple correlation peaks. Once the frequency has been adjusted correctly; (as illustrated in wave form  1505 ) the timing difference between  1511  and  1521 , which represents the clock counter value, at which correlation peaks  1509  and  1517  occur, should be zero. 
     Once the frequency of the receiver system clock counter  1413  has been adjusted to match the transmit clock, the receiver system clock can then be adjusted to match the phase of the transmit system. Because the differential correlator  1305  correlates the phase difference between adjacent symbols, it is essentially phase blind. That is, once a sequence has been created, such as S N  or B N , which comprises a phase differential between symbols, the actual phase information relative to each symbol is lost. To establish phase lock with respect to incoming symbols, the received sequence is correlated in real time. 
       FIG. 12  is a block diagram illustrating the parts of an exemplary correlator, as may be used to establish phase lock. An input sequence  1601  is coupled into a correlator  1603 . The input sequence  1601  is then coupled into a tapped delay line  1605 . The stages of the tapped delay line are compared in a series of comparators  1607 , to a known sequence  1609 . The output of all the comparisons are then summed in a summation unit  1611 , which produces the correlator output  1613 . One drawback of a correlator, such as illustrated at  1603 , is that it is very area intensive to fabricate such a correlator on an integrated circuit. That is, if the input sequence  1601  happens to be  128  symbols long, then  128  multipliers, for the multiplier chain  1607 , and  128  addition stages within the summation unit  1611  will need to be employed. It is desirable to find a method that is less area intensive when implemented on a integrated circuit chip. 
       FIG. 13  is a graphical illustration of a serial correlator as may be used in certain embodiments of the present invention. A series of comparison patterns, such as packet headers B N , are illustrated. The sequential packet headers are numbered  1701 ,  1703 ,  1705 ,  1707 , and  1709 . The pattern that will be compared to the header pattern is contained in a memory  1713 . Instead of comparing all of the bits at the same time, as with the exemplary correlator illustrated in  FIG. 12 , the serial correlator compares one bit at a time. For example, B O  from the first header  1709  is compared with a first bit C O ,  1715 , in the pattern to be matched. The single bits are compared in a single bit multiplier  1711 . The result of the comparison, of the individual bits in the multiplier  1711 , is provided to a summation unit  1719 . So, for example, if a pattern represented by C O    1715  through C N    1717  is being sought, the Nth bit of the header B N  represented by  1701  may be compared to the C N  bit  1717  of the pattern to be correlated. When the next header arrives, the C N -1 bit from the comparison stream  1713  can be compared to the B N -1 bit in the second header to arrive  1703 . In this manner, not all the bits are compared at once. Instead of requiring, for example, 128 multipliers and an adder chain capable of adding 128 results, the serial correlator correlates one bit at a time and then sums the result. Once 128 header bits have been compared with the correlation value  1713  the correlator output  1721  becomes valid. Although the serial correlator can compare only one header bit at a time, the actual delay in achieving phase lock may not be perceptible to a user. By serializing the computation, a great savings in chip area may be realized. 
     The Transmit System 
       FIG. 14  is a block diagram representation of a modulator and FEC of a transmit system according to an embodiment of the invention. The FEC accepts data  1807  from an outside source. The data may be of any suitable type, for example, MPEG 2 (Motion Picture Experts Group) Video, DVB (Digital Video Broadcast), or PN (Pseudo-Noise) codes. In the exemplary embodiment, the FEC also provides a test data signal  1805 , which can be used as a data source input  1807  to the FEC. The data signal  1805  can be used for testing, for example, in a stand-alone mode. Because the test signal is a known data pattern, the receiver system can use the same pattern to measure bit error rate. Various parameters of the circuitry may influence the bit error rate. The built in test signal  1805  may reduce the necessity of having a dedicated piece of test equipment in order to provide a known test signal to measure the bit error rate. 
     The FEC  1801  also includes a data clock  1809 . The data clock  1809  clocks the input data  1807  and thereby controls the rate that the data is provided to the FEC  1801 . The FEC also accepts clocking signals from several numerically controlled oscillator clocks. The exemplary embodiment accepts clocking inputs from a byte clock  1811  (which may also function as a data clock  1809 ), a forward error correcting clock  1813 , and a bit clock  1815 . The FEC also accepts a control input  1825 . The control input  1825  may be used to control various parameters of the FEC, for example, but not limited to, modulation format, frame length of the data exiting the module, and coding rates. Once the FEC  1801  has processed the data  1807 , the processed data  1819  is further provided to the modulator  1803 . The modulator also communicates with the FEC via a communications bus  1817 . The communications bus  1817  generally provides handshaking and protocol to control the data transfer over the bus  1817 . The modulator  1803  also accepts an input from the control line  1825 . The control line  1825  may be used to control various parameters within the modulator, such as filters, equalizers, and symbol mappers. The modulator  1803  also accepts a clocking signal, such as a 100 megahertz oscillator  1823 . The oscillator  1823  also is coupled through the modulator and operates a digital to analog converter  1827  which accepts the data  1827  output by the modulator  1803 . 
     Overall, the data  1807  enters the FEC  1801 . The FEC codes the data and passes the coded data  1819  to the modulator  1803 . The modulator  1803  accepts the data from the FEC  1801 , modulates it, and places the modulated data on an output  1827 . The data output  1827  is then coupled into a digital to analog converter. The digital to analog converter converts the digital signal  1827  into an analog signal suitable to be accepted by the transmitter  105  and broadcast by the antenna  107  to the satellite  109  (see FIG.  1 ). 
     Forward Error Correction 
       FIG. 15  is a block diagram illustrating the components and signal flow which comprise an exemplary embodiment of the FEC  1801 . The FEC is divided into three asynchronous modules. The first asynchronous module is the input processing module, which receives the data input  1807 . The input processing module queues the received data, then provides it to forward error correcting module  1903 . It is from the functions provided in the forward error correcting module  1903  that the FEC  1801  takes its name. 
     The forward error correcting module  1903  processes the data provided by the input processing module  1935  and then further provides it to a training sequence and marker insertion module  1937 . The training sequence and marker insertion module  1937  adds a training sequence header and may add submarkers to the data. The data is then provided to an output  1819 , for acceptance by the modulator  1803 . 
     The input processing module  1935 , the forward error corrector  1903 , and the training sequence and marker insertion (TSMI) module  1935  are connected serially to each other via asynchronous interfaces such as FIFO (First In First Out) queues. In other words, the coupling between the input processing module  1935  and the forward error corrector  1903  is asynchronous, as is the connection between forward error corrector  1903  and training sequence and marker insertion module (TSMI)  1937 . In other embodiments, the TSMI module may be eliminated entirely. 
     Data  1807  is generally coupled into the FEC  1801  via the input processing module  1935 . Data  1807  is provided to a multiplexer  1907  which allows a microcontroller  1939  to choose between the data input  1807  and a PN sequence generator  1905 . The PN sequence generator  1905  may be used as a data source for the FEC  1801 , for example, for testing. It may also be used to provide a known signal, for example, for testing the receiver system performance. The PN sequence module  1905  is also coupled to an output  1805  for use outside of the module. The data chosen by the multiplexer  1907  is queued within a first-in first-out (FEFO) queue  1909  within the input processing module  1935 . Data is then further coupled from the FIFO  1909  to an input FIFO  1911  in the forward error correcting module  1903 . The forward error correcting module  1903  further receives data from its input FIFO  1911  and provides it to a Reed-Solomon encoder  1913 . The Reed-Solomon encoder, once it has encoded the data, then provides the Reed-Solomon encoded data to a turbo encoder  1917  and interleaver  1915  combination. 
     Once the Reed-Solomon encoder  1913 , the interleaver  1915 , and the turbo encoder  1917  completes the processing of a data symbol, it is then coupled into an output queue  1921 . The output queue  1921  is provided with a fullness indicator. So, for example, if the FIFO  1921  is greater than half full, it provides an indicator signal  1931  to loop filter  1929 . 
     The FIFO  1921  further provides symbol data to the training sequence and marker insertion TSMI module  1937 . An inserter  1927  may then insert a training sequence header and/or submarkers into the data stream, for example, to accommodate the needs of a particular data receiver. It is the training sequence and marker insertion module which creates the frame structure for the data. Once the training sequence and submarkers have been inserted into the data stream, the data is coupled into an output queue  1937 . The data resides in the output queue  1937  until called for by the modulator  1803 . The output FIFO  1937  also has a status indicator  1933  that indicates, for example, that it is half full. The indicator is coupled into loop filter  1929 . The FIFO level indicators  1931  and  1933  may both be coupled into loop filter  1929  to determine the rate at which a byte clock NCO  1811  runs. 
     In the described exemplary embodiment, three different clocks are used for control of the FEC  1801 . The first clock is the byte clock NCO (Numerically Controlled Oscillator)  1811 . The byte clock is used to control the flow of data into the FEC  1801 . The byte clock can be adjusted by the microcontroller  1939  and can also be adjusted by the output from the loop filter  1929 , for example, based on how full FEFOs  1921  and  1937  are. The forward error correcting module  1903  is clocked by an FEC clock  1813 . Although the FEC clock  1813  is independent of the other two clocks, it too may also be controlled by the microcontroller  1939 . The third clock  1815  is a bit clock. The bit clock is provided to the TSMI module  1937  and also provided to the digital to analog converter  2027  (as illustrated in FIG.  20 ). The bit-clock  1815  also may be controlled by the microcontroller  1939 . The control of the clocks by the microcontroller  1939  facilitates the ability to switch modulation on a symbol by symbol basis. 
     FEC Multimodulation Architecture 
     Because of the asynchronous nature of the modules within the FEC  1801 , the flexible control of the data stream, to be modulated, can be enhanced. For example, in the described embodiment, 8PSK or QPSK modulation can both be accommodated. Other modulation types can also be accommodated. The modulation can be controlled from outside the FEC, for example, utilizing the control bus  1825 . Additionally, the frame length and header training sequence composition and placement of submarkers can be varied on command. In addition, the turbo encoder  1917  can have a variety of coding rates, such as 2/3, 5/6, and 8/9. All the aforementioned parameters can be controlled from outside the FEC  1801 . In addition, the number of submarkers placed within a frame can be varied in length and number. Submarkers may be used by the receiving system to track the incoming signal or re-establish lock. The variables can be changed on a frame-to-frame basis. For example, one frame can be modulated with 8PSK and a successive frame can be modulated with QPSK. Additionally, the turbo coding rates can be switched within successive frames. The number, size and spacing of submarkers also can be changed in successive frames. 
     As the above mentioned variables are changed within the FECs, various latencies can be encountered. The FIFOs  1909 ,  1911 ,  1921 ,  1923  and  1925  enable the buffering of the symbol stream as parameters are changed, for example, on a frame-to-frame or intraframe basis. The FIFOs enable the symbol stream to be queued when parameters are changed. For example, FIFO  1911 , which provides data to the turbo encoder  1917 , can queue input data as the turbo encoder is switched between rates of 2/3, 5/6 and 8/9. In such a way, the input stream does not have to be started and stopped as might be the case in a synchronous type design. The incoming data rate, however, should be controlled or it is likely that the FIFOs will either overflow or be starved for data at some point. In order to prevent the modules from being starved for data or overflowing, the overall rate is controlled by the byte clock  1811 . The byte clock is controlled by the loop filter which receives, as its input, FIFO status signals  1931  and  1933 . The loop filter receives signals  1931  and  1933 , filters them and uses the filtered signal to control the frequency of the byte clock  1811 . It is, of course, possible to use other FIFO status signals from other FIFOs to control the data rate input to the modules. Which FIFOs are selected as control FIFOs depend on a variety of design variables, for example, individual FIFO size, and peak data rates. 
     The feedback from FIFOs  1921  and  1925  are used to control the byte clock  1811  and hence the data input rate of the data  1807 . Additionally control bus  1825  can communicate which variables within the FEC  1801  should change. The microcontroller  1939  may not only control the parameters within the different modules of the FEC  1801 , in addition it also can load the clocks, i.e., the byte clock  1811 , the FEC clock  1813 , and the bit clock  1815  with starting values. Tables relating the clock frequencies to different FEC parameters can be provided in ROM (not shown) to the microcontroller  1939 . When the microcontroller changes parameters within the FEC, it can also change the clock frequencies of the module clocks. These tables can be computed beforehand and placed in the ROM available to the microcontroller  1939  to control the parameters used to modulate the data  1807  as it arrives. 
     In a second operating mode, instead of having all the clock frequencies specified in tabular form, the microcontroller  1939  may be provided with, for example, a byte clock frequency. The microcontroller may then vary the parameters within the FEC  1801  and alter the byte clock rate  1811  to accommodate an estimated desired input data rate. The microcontroller can be provided with algorithms which examine the output rate from the input processing module  1935  and, based on the data rate estimate, set the FEC clock  1813  to an estimated frequency. The microcontroller can then further examine the output rate of the forward error correcting module  1903 , make an estimate of the bit clock frequency that is needed, and set the bit clock to that frequency. In other words, the microcontroller controls the parameters of the FEC  1801  while the FEFOs within the modules  1935 ,  1903  and  1937  provide the needed buffering to maintain a steady data flow through the FEC. The microcontroller can also control the clocks which process the data in the modules  1935 ,  1903  and  1937  by using a table in memory, which relates the different parameters to the clock rate. Microcontroller  1939  can also be provided with an algorithm that will predict the clock rate based on module parameters, or the microcontroller can be provided with an algorithm or table relating the initial byte clock rate to the parameters and then examine the data flow through the module and adjust the other clocks in line as needed. Examples of values, as may be inserted into a ROM table, are given below. 
     The first three examples are for a input symbol rate of 21.2 megasymbols per second: (1) at a coding rate of 2/3, the byte clock is running at 5.13 megahertz, the FEC clock is running at 42.38 megahertz, and the bit clock is running at 63.57 megahertz; (2) at a 5/6 coding rate, the byte clock is running at 6.39 megahertz, the FEC clock is running at 52.98 megahertz, and the bit clock is running at 63.57 megahertz; (3) at a coding rate of 8/9, the byte clock is running at 6.81 megahertz, the FEC clock is running at 56.51 megahertz, and the bit clock is running at 63.57 megahertz. 
     The fourth through sixth examples describe clock rates versus coding rate for an input stream of 25 megasymbols per second: (4) at a code rate of 2/3, the byte clock is running at 6.05 megahertz, the FEC clock is running at 50 megahertz, and the bit clock is running at 75 megahertz; (5) at a coding rate of 5/6, the byte clock is running at a frequency of 7.54 megahertz, the FEC clock is running at a frequency of 62.50 megahertz, and the bit clock is running at a frequency of 75 megahertz; (6) at a coding rate of 8/9, the byte clock is running at 8.03 megahertz, the FEC clock is running at 66.67 megahertz, and the bit block is running at 75 megahertz. 
     The previous exemplary values illustrate that by knowing the input symbol rate and the coding rate, tables can be developed for nominal byte clock, forward error correcting clock, and bit clock rates. Such computations, as illustrated in the six examples just described, can be used to rapidly switch coding rates and clock frequencies on a frame-by-frame basis. In addition, because of the asynchronous nature of the modules within the FEC  1801 , any temporary change in coding rate can be buffered by the FIFOs which connect the internal processing functions. The overall scheme employed to keep the modules functioning without queue overflow or underflow is that the upstream clocks are responsive to downstream queue sizes. Which upstream clocks are responsive to which downstream queues are design parameters that can vary from implementation to implementation. In the described exemplary embodiment, FIFO queues  1921  and  1925  are chosen to control the byte clock NCO  1811 . Other combinations are possible such that the upstream clock rate is controlled by a downstream FIFO size. For example, if the training sequence and marker insertion module  1937  were eliminated from the FEC  1801 , as might be the case in particular embodiments, the size of FIFO  1921  could then alone be used to control the byte clock NCO  1811 . 
       FIG. 16  is a graphical illustration of a generalized data frame produced by the TSMI  1937 , according to the present exemplary embodiment of the invention. The data frame  2101  is, in general, composed of two different sections. The first section in the header section as exemplified by  2103 . The second section is the data section as exemplified by  2105 ,  2109  and  2113 . The header is, in general, data which identifies the beginning of a frame of data. The header may comprise various data sequences. For example, one of the data sequences within the header, according to an embodiment of the present invention, is a training sequence  2117 . The training sequence  2117  is a known pattern of data which the receiver system may use to lock to the frame. The training sequence can be varied in length or modulation according to parameters of the satellite system. In the present exemplary embodiment, a training sequence size of 64 symbols is used. The remainder of the header  2103  may be used for a variety of purposes, for example, but not limited to, identifying the data within the frame, identifying the modulation scheme of the data in the frame, identifying the modulation of future frames, or identifying positions of items within the data stream, such as submarkers  2107  and  2111 . 
     Submarkers  2107  may be inserted into the data within the frame  2101  in order to provide a method for the receiver system to synchronize or track an incoming signal. In one exemplary embodiment, the submarker size is 16 symbols, and the spacing between submarkers is 10,240 symbols. 
     In one exemplary embodiment, the data  2105  comprises sequential blocks of turbo coded data. Submarkers may be interspersed between blocks of data (See FIG.  17 ). Such submarkers may be particularly useful when used with codes, such as turbo codes, which are designed to be received at low signal levels. At low signal levels receiver systems many need mechanisms to assist in signal acquisition and training. 
     The submarkers may be known sequences such as, for example, copies of the training sequence. Such submarkers can be used to track the incoming signal, by performing correlations on the submarker or may even be used to acquire the signal in a case, for example, when the incoming signal is disrupted by a noise burst. 
     The spacing and arrangement of submarkers is further illustrated by examples illustrated in  FIGS. 17 and 18 . 
       FIG. 17  is a graphical illustration of a frame of data having three submarkers.  FIG. 17  comprises a training sequence  2201  followed by turbo encoded blocks  1  and  2  ( 2203 ). A submarker  2205  follows turbo encoded blocks  1  and  2 . Following the submarker  2205  turbo encoded blocks  3  and  4  are followed by submarker  2209 . Submarker  2209  is followed by turbo encoded blocks  5  and  6 , which are then followed by submarker  2203  which is followed by the final turbo encoded blocks  7  and  8 . Turbo encoded blocks  7  and  8  ( 2215 ) comprise the final elements within the frame. A new frame begins immediately thereafter with training sequence  2217 . In the example in  FIG. 17 , the frame comprises 41,072 symbols. The training sequence is 64 symbols and the submarker size is 16 symbols. The spacing between submarkers is 10,240 symbols. 
       FIG. 18  is a graphical illustration of an exemplary arrangement of data blocks and submarkers within a frame. In  FIG. 18  submarkers are interspersed within the data blocks. The frame begins with a training sequence ( 2301 ). One-half of turboencoded block  1  is then positioned after the training sequence. The half of turbo encoded block  1   2303  is followed by submarker  2305 . Submarker  2305  is then followed by the second half of turbo encoded block  1  and the first half of turbo encoded block  2  ( 2307 ). Submarker  2309  follows next, followed by the second half of turbo encoded block  2  and the first half of turbo encoded block  3  ( 2311 ). This sequence continues until the final half of turbo encoded block  8   2335  is in place. The final half of turbo encoded block  2335  marks the end of the frame. The end of the frame is followed by a training sequence  2337  signaling the start of the next frame. The example illustrated in  FIG. 18  includes a frame of 41,152 symbols. The training sequence size is 64 symbols and the submarker size is 16 symbols. The spacing between submarkers is 5,000 symbols versus 10,400 in the example illustrated in FIG.  17 . The spacing and submarker size may vary according to the characteristics of the receiver that is receiving the signal. For example the submarkers may comprise copies of the training sequence. 
     Because of the flexibility of the system, a frame as illustrated in  FIG. 17  may be followed immediately by a frame as illustrated in FIG.  18 . The frame in  FIG. 18  may be used, for example, when delivering data to a receiver system which is receiving a signal of low signal strength. 
     The following examples are not exhaustive. For example, submarkers may be placed between every block of turbo coded data, or they may be placed between every block of turbo coded data and additionally within the block of turbo coded data. 
     The Set Top Box 
     A generalized block diagram of a set top box  115 , according to an exemplary embodiment of the present invention, is illustrated in FIG.  19 . The set top box  115  receives the signal from the ODU  119 . The signal received from the ODU  119  may include a frequency band of signals, for example 950 to 2150 megahertz. The signal from the ODU  119  is coupled into the exemplary set top box  115 . 
     Within the set top box, the tuner  2901  selects the desired band of frequencies from the signal provided by the ODU  119 . The desired band of signals is then down converted in the tuner from a high frequency signal into a frequency which may be accepted by a signal demodulator  2903 . 
     The demodulator  2903  accepts the down converted signal from the tuner  2901 , and converts the signal into a data stream. The data stream from the demodulator may then be provided to a turbo decoder  2905 . 
     Within the turbo decoder  2905 , the process of recreating the data sent by the transmit system is begun. As described earlier, the data stream from the FEC of the transmit system has been turbo code encoded to help correct errors which occur in the signal. Errors in the signal occur due to factors such as noise within the channel, atmospheric conditions, electrical interference, and a variety of other interference sources. Turbo codes are used, in part, because they provide a robust way of encoding a signal. Turbo codes may enable a signal to be reconstructed at lower signal levels, i.e., lower signal to noise ratios, than may be possible with other codes alone. 
     The turbo decoder  2905  is coupled to and cooperates with an interleaver  2907 . An interleaver is a device that, in the transmit system, changes the order of the data from the order received into another order. A receive interleaver may change the data so that it is no longer in sequential order, but in a random but known sequence. A receive interleaver, e.g.  2907 , is alternatively known as a de-interleaver. The receive interleaver converts the random data sequence back into normal sequential order. Interleavers may also cooperate with encoders in the transmit system in order to encode the data signal, and decoders in the receiver system to decode the data signal. A burst error, which affects an adjacent sequence of bits, can be minimized if the signal has been interleaved. In an interleaved signal, the bits containing errors may be decoded with other bits, which were transmitted at a time when the interfering signal was not present. The interleaver  2907  further produces a data stream which is then provided to a Reed-Solomon decoder  2909 . 
     The Reed-Solomon decoder  2909  further decodes the data stream. By successfully coding a signal with a turbo coder in conjunction with a Reed-Solomon coder the signal can be made more robust, i.e., can be decoded at lower signal to noise levels, than if either type of coding had been used separately. The block including the demodulator  2903 , the turbo decoder  2905 , the interleaver  2907  and the Reed-Solomon decoder  2909  are generally referred to as the receiver  2919 . The output of the Reed-Solomon decoder is provided to a video unit  2911 . The video module accepts the data from the Reed-Soloman Decoder  2909  and converts it into a form which may be used by a user device such as the television. 
     The Demodulator 
       FIG. 20  is a block diagram of an exemplary demodulator in accordance with an embodiment of the present invention. In-phase and quadrature signals are received by analog to digital (A/D) converters  3103  and  3105 . The A/D converters  3103  and  3105  are controlled by a free-running sample clock (not shown). The frequency of the sample clock driving A/D converters  3105  and  3103  is a non-integer multiple of the symbol rate. 
     An automatic gain control (AGC)  3107  examines the signal size at the output of the A/D converters  3103  and  3105 . Maximum resolution can be obtained with a input signal whose maximum size corresponds to the range of the A/D converters. The AGC controls the gain of amplifiers that amplify input signals prior to providing them to the A/D converters. From the A/D converters the data is provided to the multiplier block  3113 . In the multiplier block a digital frequency shift is introduced to cancel any frequency offset which exists between the clocks of the transmitter system and receiver system. 
     The frequency loop  3115  serves to change the multiplication factor and thereby cancel the frequency offset. The data is then coupled into decimation filters  3119  and  3121 . The decimation filters reduce the data rate, thereby filtering the data. From the decimation filters the signal values are coupled into symbol sample (SS) blocks  3127  and  3129 . The SS blocks  3127  and  3129  extract symbol values from the signal values presented to it by the decimation filter. The symbol extraction is synchronized using a symbol timing loop  3131 . The symbols retrieved by the SS blocks are next provided to Nyquist filters  3133  and  3135 , which provide the symbols with Nyquist filtering. From the Nyquist filter the symbols are coupled to the demodulator output  3109  and  3111 . 
     Receiver Synchronization with Frequency Offset 
     As explained earlier, in a receiver system, such as illustrated in  FIG. 20 , two basic problems need to be dealt with. The first problem is that the center frequency of the signal, which the receiver system is attempting to synchronize with, may be offset by a considerable amount (referred to as the delta frequency). The delta frequency arises because the receiver clock is not the same frequency as the transmitter clock. The receiver system must adjust its clock to match the transmission clock. The frequency of the transmission clock, also called the carrier frequency, may also vary over time. A second problem is that transients, such as intermittent noise, changing atmospheric conditions, and interference from other signal sources, may cause relatively short duration deviations or interruptions of the signals input to the system. These deviations may cause transient phase deviations, which need to be accounted for by the system. The process of matching the carrier frequency to the receiver frequency is often referred to as acquisition, and the process of maintaining phase lock to a signal, despite transient noise and interference conditions, is often called tracking. 
     The frequency offset of the incoming signal is adjusted within the multiplier  3113 , illustratively a digital complex multiplier. The basic operation of the multiplier  3113  is to multiply the incoming signal and thereby create a frequency translation of the incoming frequency. The multiplier  3113  will create frequency images equal to the input frequency plus the multiplying frequency and also equal to input frequency minus the multiplying frequency. The multiplying frequency is provided from within a frequency loop block  3115 . 
     The frequency loop  3115  includes a phase detector  3149 , which detects the phase difference of the outputs of the Nyquist filters  3133  and  3135  and the input signal as provided by multiplier  3113 . 
     The symbol timing loop  3131  adjusts the timing of symbol sampling, that is it selects the value which will be decoded as a symbol. 
     When the system is initially turned on, both the frequency loop  3115  and the symbol timing loop  3131  are disabled by the correlator  3101 . The correlator then examines the incoming symbols and estimates, the frequency offset between the receiver clock and the transmitter clock, as previously described in the section on the differential correlator. Once the correlator  3101  has estimated the difference between the carrier frequency and the receiver clock frequency, the value of the offset between the transmitter frequency and the receiver frequency, that value can be inserted, as a starting value, into the frequency loop  3115 . Once the offset frequency has been loaded into the frequency loop by the correlator, the correlator may then enable the loop. The frequency loop can then quickly lock in and track the carrier frequency. The header processor also estimates the phase offset within the symbol timing loop  3131 . Once the correlator-processor  3101  has an estimate of the phase offset in the symbol timing loop, it can be inserted into the symbol timing loop. When the header processor couples the initial value into the symbol timing loop  3131 , the header processor may enable the symbol timing loop  3131 . By providing starting value for the frequency loop  3115 , the acquisition time of the frequency loop  3115  is decreased over a conventional type methodology, in which the phase detector alone causes the synchronization of the loop starting at an arbitrary value. Similarly, by providing an initial value for the symbol timing loop  3131  and then enabling the loop, the tracking function within the symbol timing loop can lock to the symbol stream much faster than if it had started from an arbitrary value. Additionally, because the frequency of the loop  3115  does not traverse a range of values, the false lock problem, in which the system may lock to an incorrect frequency, is minimized. 
     Receiver Synchronization Using Interspersed Data 
     Additionally, in order to aid acquisition and tracking, the correlator-processor can enable the symbol timing loop  3131  in such a manner that only the training sequence and submarkers are used for synchronizing the symbol timing loop  3131 . The same training sequence and submarkers may also be gated into the frequency timing loop  3115  for the purpose of synchronizing the frequency timing loop  3115 . 
     Although a preferred embodiment of the present invention has been described, it should not be construed to limit the scope of the appended claims. Those skilled in the art will understand that various modifications may be made to the described embodiment. Moreover, to those skilled in the various arts, the invention itself herein will suggest solutions to other tasks and adaptations for other applications. It is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than the foregoing description to indicate the scope of the invention.