Abstract:
Methods and apparatus to extract data encoded in media content are disclosed. An example method includes receiving a media content signal, sampling the media content signal to generate digital samples, determining a frequency domain representation of the digital samples, determining a first rank of a first frequency in the frequency domain representation, determining a second rank of a second frequency in the frequency domain representation, combining the first rank and the second rank with a set of ranks to create a combined set of ranks, comparing the combined set of ranks to a set of reference sequences, determining a data represented by the combined set of ranks based on the comparison, and storing the data in a tangible memory.

Description:
RELATED APPLICATIONS 
     This patent is a non-provisional patent application of U.S. Provisional Patent Application Ser. No. 61/108,380, “STACKING METHOD FOR ENHANCED WATERMARK DETECTION,” filed on Oct. 24, 2008, the disclosure of which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure pertains to monitoring media content and, more particularly, to methods and apparatus to extract data encoded in media content. 
     BACKGROUND 
     Identifying media information and, more specifically, audio streams (e.g., audio information) is useful for assessing audience exposure to television, radio, or any other media. For example, in television audience metering applications, a code may be inserted into the audio or video of media, wherein the code is later detected at monitoring sites when the media is presented (e.g., played at monitored households). The information payload of the code/watermark embedded into original signal can consist of unique source identification, time of broadcast information, transactional information or additional content metadata. 
     Monitoring sites typically include locations such as, for example, households where the media consumption of audience members or audience member exposure to the media is monitored. For example, at a monitoring site, codes from the audio and/or video are captured and may be associated with audio or video streams of media associated with a selected channel, radio station, media source, etc. The collected codes may then be sent to a central data collection facility for analysis. However, the collection of data pertinent to media exposure or consumption need not be limited to in-home exposure or consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example system for encoding data in a media content signal to transmit the data to a location where the media content signal is decoded to extract the data. 
         FIG. 2  is a graph of an example frequency spectrum and code indexing. 
         FIG. 3  illustrates an example sequence that may be encoded in an audio signal by the example encoder of  FIG. 1 . 
         FIG. 4  illustrates an example message thread. 
         FIG. 5  is a block diagram of an example apparatus to implement the decoder of  FIG. 1  that includes stack and rank functionality. 
         FIG. 6  is a flowchart of an example process to decode a message in audio. 
         FIG. 7  is a flowchart of an example process to decode a message in audio using stacking. 
         FIG. 8  is a schematic illustration of an example processor platform that may be used and/or programmed to perform any or all of the example machine accessible instructions of  FIGS. 6-7  to implement any or all of the example systems, example apparatus and/or example methods described herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an example system  100  for encoding data in a media content signal to transmit the data to a location where the media content signal is decoded to extract the data. The example system  100  includes an encoder  102  and a decoder  104  with stack and rank functionality. According to the illustrated example, the encoder  102  encodes a received audio signal with a received data by amplifying or attenuating frequencies of interest as described in detail herein. The encoded audio signal is transported to another location where it is received by the decoder  104 . The decoder  104  includes a stack functionality to stack consecutively received portions of the audio signal. In addition, the decoder  104  includes rank functionality to assign ranks to frequencies that may have been amplified or attenuated by the encoder  102 . For example, where frequencies are grouped in neighborhoods of five frequencies, a rank of 0 to 4 may be assigned to each frequency. The decoder  104  then extracts the data from the stacked audio signal as described in detail herein. Stacking the encoded audio signal will, for example, improve the detection reliability of the decoder  104  when stacked portions include redundant or semi-redundant encoded data. While not shown in the illustrated example, the audio signal may also be output by the decoder  104  to be presented on a media presentation device (e.g., a radio, a television, etc.). Alternatively, the encoded audio signal may be transmitted to a media presentation device in parallel with the example decoder  104 . 
     According to the example of  FIG. 1 , the encoder  102  receives as input an audio signal and data. The encoder  102  further divides the audio signal into frames, which are blocks of digital audio samples. As described in detail below, the encoder  102  encodes (embeds) the data into the framed audio signal and the encoded frame of audio is tested by the encoder  102  to determine if the modifications to the framed audio signal are significant enough to cause the encoding to be audibly perceptible by a human when the framed audio signal is presented to a viewer (e.g., using psychoacoustic masking). If the modifications to the framed audio signal are too significant and would result in an audible change in the audio, the framed audio is transmitted (e.g., broadcast, delivered to a broadcaster, etc.) without being encoded. Conversely, if the encoded audio frame has audio characteristics that are imperceptibly different from the un-encoded audio frame, the encoded audio frame is transmitted. 
     The encoder  102  inserts a unique or semi-unique 15-bit pseudorandom number (PN) synchronization sequence at the start of each message packet. To signal to the decoder  104  that a synchronization sequence is to be transmitted, the first code block of the synchronization sequence uses a triple tone. The triple tone is an amplification of three frequencies causing those frequencies to be maxima in their spectral neighborhoods. Thus, by looking for the triple tone, the decoder  104  can detect that a synchronization sequence is about to be sent without the need for decoding the entire synchronization sequence. An example implementation of a triple tone is described in U.S. Pat. No. 6,272,176 (‘176 patent’), which is hereby incorporated by reference in its entirety. The example synchronization sequence is one approach for enabling the decoder  104  to detect the start of a new message packet. However, any other indication, signal, flag, or approach may be used. 
     The example encoder  102  transits as many as ten 15-bit PN sequences of message data following the synchronization. Thus, each message in the illustrated example comprises 11 groups: one 15-bit synchronization sequence followed by ten 15-bit message data sequences. However, any number of message data sequences may be transmitted between synchronization sequences. The example message data is transmitted in 15-bit PN sequences having ten error correction bits and five message data bits. In other words, message data is divided into groups of five bits each (e.g., ten 5-bit groups for a 50-bit message). Alternatively, any combination of message data bits and error correction bits may be included in a message data sequence. Each bit of the 15-bit PN sequence is encoded into a 512-sample block of audio. In the example system  100 , one bit is transmitted at a time. Each 5 bits of payload data that is encoded as a 15-bit sequence uses 15 blocks of 512 samples (i.e., 7680 samples total). The example encoder  102  includes a 16 th  block called the null block after the 15 blocks representing the 15-bit sequence. Thus, each message in the illustrated example uses 176 audio blocks: 16 blocks per sequence and 11 sequences per message. In the illustrated example, each message is followed by 11 unencoded blocks to adjust the total message duration to be approximately two seconds in the example encoding. While example encoding and block sizes are described, any desired encoding and block sizes may be used. 
     To insert a data bit (e.g., one bit of a 15-bit sequence) into an audio frame, the example encoder  102  makes a first selected frequency of the audio frame a local maximum and makes a second selected frequency of the audio frame a local minimum. For example, as shown in  FIG. 2 , the encoder  102  uses two audio frequency bands or neighborhoods  202  and  204 , each including five frequencies or residents. One of the neighborhoods  202  and  204  is encoded to include a resident that is a local maximum and the other neighborhood  202  and  204  is encoded to include a resident that is a local minimum. The residents that are selected to be local maximum and local minimum are based on the coding block on which the example encoder  102  is operating and the value of the data bit to be transmitted. For example, to encode a logical “1” in the fifth encoding block, a resident having index number  50  in the neighborhood  202  is made a local maximum and a resident having index number  60  in the neighborhood  204  is made a local minimum. Conversely, to encode a logical “0” for the same encoding block, the resident having index number  50  in the neighborhood  202  would be made a local minimum and the resident having index number  60  in the neighborhood  204  would be made a local maximum. In other words, the frequencies that are selected do not represent the bit to be sent, the amplitudes at the selected frequencies represent the value of the bit because the same frequencies may be used whether the bit is a logical “1” or a logical “0”. After encoding, the audio signal may be broadcast to a consumer location, may be transmitted to a broadcaster for broadcasting, may be stored to a storage media, etc. 
     The example system  100  may be configured to perform stacking and ranking in a system that is implemented with the Nielsen Audio Encoding System (NAES) described in the &#39;176 patent. While this disclosure makes reference to encoding and decoding techniques of the NAES system described in the &#39;176 patent by way of example, the methods and apparatus described herein are not limited to operation in conjunction with the techniques of the &#39;176 patent. To the contrary, the example methods and apparatus may be implemented in conjunction with any type of encoding or decoding system. For example, the data rates, data grouping, message lengths, parameter lengths, parameter order in messages, number of parameters, etc. may vary based on the implemented encoding system. 
       FIG. 3  illustrates an example sequence  300  that may be encoded in an audio signal by the example encoder  102  of  FIG. 1 . The example sequence  300  includes 15 bits that are encoded in 15 blocks of audio data (e.g., 512 sample blocks). The message bits  302  convey five bits of message data. The message bits  302  are the payload data to be conveyed by the encoding. The error correction bits  304  convey ten bits of error correction data that may be used by the decoder  104  to verify and correct a received message. Each bit of the sequence  300  is encoded in a block of audio data. As described in conjunction with  FIG. 1 , for each block of audio data, a first selected frequency is made a local maximum and a second selected frequency is made a local minimum. 
       FIG. 4  illustrates an example message thread  400 . The message thread  400  of the illustrated example includes a synch sequence  402 , a first sequence  406 , a second sequence  410 , a third sequence  414 , and no mark blocks  404 ,  408 , and  412 . The example synch sequence  402  is a 15 bit sequence that indicates the start of a new message thread. The first sequence  406 , the second sequence  410 , and the third sequence  414  of the illustrated example are 15 bit sequences that each convey five message payload bits and ten error correction bits as described in conjunction with  FIG. 3 . The no mark blocks  404 ,  408 , and  412  are single blocks that include no encoding (e.g., 512 samples of audio data in which no frequencies are amplified or attenuated by the encoder  102 ). While the example message thread  400  is formatted as described, any other formatting may be used. For example, more or fewer sequences may be included in a message thread  400 , sequences  406 ,  410 , and  414  may contain more or fewer data bits and/or error correction bits, the no mark blocks  404 ,  408 , and  412  may include multiple blocks, more or fewer no mark blocks  404 ,  408 , and  412  may be included, etc. 
       FIG. 5  is a block diagram of an example apparatus to implement the decoder  104  of  FIG. 1  that includes stack and rank functionality. The example decoder  104  includes a sampler  502 , a time domain to frequency converter  504 , a ranker  506 , a rank buffer  508 , a stacker  510 , a stacker control  512 , a comparator  514 , and a reference sequence datastore  516 . The example decoder  104  receives an input audio (e.g., an audio portion of a television program) and processes the audio to extract and output data encoded in the audio. 
     The sampler  502  of the illustrated examples samples the incoming audio. The sampler  502  may be implemented using an analog to digital converter (A/D) or any other suitable technology, to which encoded audio is provided in analog format. The sampler  502  samples the encoded audio at, for example, a sampling frequency of 48 KHz. Of course, other sampling frequencies may be selected in order to increase resolution or reduce the computational load at the time of decoding. Alternatively, the sampler  502  may be eliminated if audio is provided in digitized format. 
     The time domain to frequency domain converter  504  of the illustrated example may be implemented using a discrete Fourier transformation (DFT), or any other suitable technique to convert time-based information into frequency-based information. In one example, the time domain to frequency domain converter  504  may be implemented using a sliding DFT in which a spectrum of the code frequencies of interest (e.g., frequencies indexed 1 to N in  FIG. 5 ) is calculated each time four new samples are provided to the example time domain to frequency domain converter  504 . In other words, four new samples are shifted into the analysis windows, four old samples are shifted out of the analysis window, and the DFT of the analysis window is computed. Because the boundaries of blocks are not known when decoding, a sliding DFT may operate by sliding 4 samples at a time to give 128 distinct message threads to analyze per 512 samples of audio that are received. Thus, at the end of 128 slides (of four samples each), all 512 samples (i.e., one block worth of samples) will have been processed and analyzed. The resolution of the spectrum produced by the time domain to frequency domain converter  504  increases as the number of samples (e.g., 512 or more) used to generate the spectrum increases. Thus, the number of samples processed by the time domain to frequency domain converter  504  should match the resolution used to select the residents shown in  FIG. 2 . The finer the frequency spacing between the residents, the more samples that will be used to generate the spectrum for detection of the residents. 
     The spectrum produced by the time domain to frequency domain converter  504  passes to the ranker  506 . The ranker  506  of the illustrated example ranks the amplitude of each frequency of interest (e.g., RANK  1  to RANK N for the 1 to N frequency indices of interest in  FIG. 5 ) in neighborhoods in the received spectrum relative to the amplitude of the other frequencies in the neighborhood. For example, when there are five frequencies in each neighborhood, the amplitude of each frequency may be ranked on a scale of 0 to 4, where 0 is the lowest amplitude and 4 is the greatest amplitude. While the forgoing example describes ranking each spectrum frequency, any subset of frequencies may alternatively be ranked such as, for example, only frequencies of interest that may have been amplified or attenuated to embed information in the audio data. The ranker  506  outputs a set of rank values to the rank buffer  508 . 
     The rank buffer  508  stores the set of rank values in a circular buffer such that once the buffer has been filled, each new set of ranks will replace the oldest set of ranks in the buffer. The rank buffer  508  of the illustrated example stores the 128 sets of ranks (e.g., 128 sets of ranks 1 to N) corresponding to each slide of the time domain to frequency domain converter  504 . In addition, the rank buffer  508  may store multiple messages worth of ranks. For example, as described in detail below, the rank buffer  508  may store five messages worth of ranks so that the blocks of messages may be averaged. While the rank buffer  508  is described as a circular buffer and type of data structure and storage may be used. For example, the rank buffer  508  may comprise one or more registers, one or more files, one or more databases, one or more buffers of any type, etc. 
     An example set of ranks may be: 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 DATA BITS 
                 ERROR CORRECTION BITS 
               
             
          
           
               
                 BLOCK 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
               
               
                   
               
               
                 RANKS 
                 2, 4 
                 1, 4 
                 4, 1 
                 4, 0 
                 0, 4 
                 3, 1 
                 3, 0 
                 4, 1 
                 4, 1 
                 4, 2 
                 2, 3 
                 4, 3 
                 4, 1 
                 0, 4 
                 4, 0 
                 0, 0 
               
               
                   
               
             
          
         
       
     
     The stacker  510  takes advantage of message-to-message redundancy to improve the detection of data encoded in audio signals. In particular, when enabled by the stacker control  512 , the stacker  510  retrieves the ranks of consecutive messages from the rank buffer  508  and adds the ranks of corresponding blocks of the consecutive messages. The stacker  510  then divides the sums by the number of messages added together. Accordingly, the stacker  510  determines an average of the ranks for consecutive blocks. When messages include redundancy, the ranks will average in order to eliminate errors introduced by noise or host audio. For example, an encoded message may be 50 bits including a broadcaster identifier (e.g., a 16-bit station identifier) followed by a timestamp (e.g., a 32-bit timestamp that denotes time elapsed in seconds since, for example, Jan. 1, 1995), followed by a level specification that allows multiple levels of messages to be included (e.g., a 2-bit level specification). In the example 50 bit message, all but the least significant bits of the message will be repeated for several messages in a row. In the example encoding where messages are divided into ten groups and include one synch group (e.g., 11 total groups), it is expected that the first ten groups will repeat from message to message and the last group (e.g., that contains the three least significant bits of the timestamp and two level specification bits) will change from message to message. Because the three least significant bits can represent eight seconds and messages in the example encoding are encoded into approximately two seconds of audio each, the fourth least significant bit of the message will change after four messages. Accordingly, the synchronization group and the first nine data groups are expected to repeat for four messages (approximately eight seconds). 
     The stacking process may be performed according to the following formulas: 
               r     1   ⁢     km   n         =             ∑     p   =   n       p   =     n   -   S         ⁢     r     1   ⁢     km   p           S     ⁢           ⁢   and   ⁢           ⁢     r     2   ⁢     km   n           =         ∑     p   =   n       p   =     n   -   S         ⁢     r     2   ⁢     km   p           S             
where p is a message index (e.g., 0≦p≦5) when five consecutive messages are to be averaged), k is a block index (e.g., 0≦k≦16 when there are 16 blocks per sequence), S is the number of consecutive messages to be averaged (e.g., 5 when five consecutive messages are to be averaged), r 1km     n   , is the average rank of the first frequency of interest in the k th  block of a message m n , and r 2km     n   , is the average rank of the second frequency of interest in the k th  block of message m n . For example, a message may be a station identifier and a timestamp that are encoded every 2 seconds. While the least significant bits of the time stamp (e.g., seconds) may change from message to message, the other bits (e.g., more significant bits of a timestamp) will not change between every message. Accordingly, when the ranks of the current message are added to the ranks of the previous four messages, the average ranking can improve detection by reducing the effect of any noise that may have been present for less than all of the messages. When the stacker  510  is enabled, the stacker  510  outputs the stacked set of ranks (e.g., RANK_S  1  to stacked RANK_S in  FIG. 5 ) to the comparator  514 . When the stacker  512  is not enabled, the stacker  510  outputs the set of ranks (e.g., RANK_S  1  to RANK_S N) retrieved from the rank buffer  508  to the comparator  514 .
 
     In an example, the following ranks may be determined for corresponding packets that are repetitions of the same message: 
                                                                                                                   DATA BITS   ERROR CORRECTION BITS            BLOCK   0   1   2   3   4   5   6   7   8   9   10   11   12   13   14   15               RANK   2, 4   1, 4   4, 1   4, 0   1, 4   3, 1   3, 0   4, 1   4, 1   4, 2   2, 3   4, 3   4, 1   0, 3   4, 0   0, 0       MSG0       RANK   0, 4   1, 4   4, 1   4, 1   0, 2   4, 1   3, 0   3, 1   4, 1   4, 2   2, 3   4, 3   4, 2   0, 4   4, 1   0, 0       MSG1       RANK   0, 4   1, 4   3, 1   4, 2   0, 4   3, 1   3, 0   4, 1   4, 2   4, 2   2, 3   4, 2   4, 1   0, 4   4, 2   0, 0       MSG2       RANK   1, 4   1, 4   4, 2   4, 0   2, 4   3, 2   3, 0   4, 1   4, 1   4, 1   2, 4   4, 3   4, 1   0, 4   4, 0   0, 0       MSG3       RANK   4, 2   1, 4   4, 1   4, 2   0, 3   3, 1   3, 0   4, 1   4, 1   4, 2   2, 3   4, 3   4, 1   0, 4   4, 0   0, 0       MSG4                    
The sum of the ranks is:
 
                                                                                                                   DATA BITS   ERROR CORRECTION BITS            BLOCK   0   1   2   3   4   5   6   7   8   9   10   11   12   13   14   15               RANK   7,   5,   19, 6   20, 5   3,    16, 6   15, 0   19, 5   20, 6   20, 9   10,   20,   20, 6   0,   20, 3   0, 0       SUM   18   20           17                       16   14       19                    
The average of the ranks is:
 
                                                                                                                   DATA BITS   ERROR CORRECTION BITS            BLOCK   0   1   2   3   4   5   6   7   8   9   10   11   12   13   14   15               RANK   1.4,   1,4   3.8,   4,1   0.6,   3.2,   3, 0   3.8, 1   4,   4,   2,   4,   4,   0,   4,   0, 0       AVG   3.6       1.2       3.4   1.2           1.2   1.8   3.2   2.8   1.2   3.8   0.6                    
As shown in the example, even when Block 0 of Message 4 has been ranked in a manner that suggests the opposite data bit as the previous four messages (i.e., 4,2 would suggest a bit value of 1, while the other values suggest a bit value of 0), averaging of the ranking results in an average that suggests a bit value of 0. Accordingly, even when error due to noise is introduced, averaging of the ranks can result in ranking that more closely matches the encoded data.
 
     The stacker control  512  controls when the stacker  510  is enabled or disabled. For example, when the stacker  510  is disabled, messages may be processed one at time without any averaging of the ranks. When the stacker  510  is enabled by the stacker control  512 , stacking of messages is performed as described herein or using any other process. The stacker control  512  may enable stacking based on any criteria. For example, the stacker control  512  may enable provide selective stacking by automatically enabling stacking when noise is detected, when a poor quality audio connection is present (e.g., when a microphone is used rather than a physical connection), when the decoder  104  is at a distance from an audio source (e.g., a mobile device across the room from an audio source), etc. Additionally or alternatively, the stacker control  512  may be manually controlled to enable stacking when requested by a user and/or may be remotely controlled by a message from a central location, the encoder  102 , etc. 
     The comparator  514  of the illustrated example receives the set of ranks or stacked ranks (“set of ranks”) for a sequence from the stacker  510  and determines if a synch sequence has been recognized. If a synch sequence has not been detected, the comparator  514  compares the received set of ranks to a reference synch sequence and sets a synch detected flag if the set of ranks is determined to correspond to a synch sequence. If a synch sequence has previously been detected, the comparator  514  compares the set of ranks to a reference set of sequences stored in the reference sequence data store  516 . The reference set of sequence comprise a listing of possible ranks and associated high or low indications for the frequencies of interest for each block. For example, when each sequence includes 5 data bits, 10 error correction bits, and one blank block, there would be 2 5  possible Bose and Ray-Chaudhuri (BCH) codewords of 15 bits, each bit having an indication of whether each of two frequencies of interest were attenuated or amplified (i.e., 30 indications). To determine the sequence corresponding to the set of ranks, the set of ranks is compared to each of the reference sequences. The reference sequence with the smallest different from the set of ranks is identified as the received sequence. 
     For example, when the received set of ranks provided by the stacker  510  is: 
                                                                                                                   DATA BITS   ERROR CORRECTION BITS            BLOCK   0   1   2   3   4   5   6   7   8   9   10   11   12   13   14   15               RANKS   2, 4   1, 4   4, 1   4, 0   0, 4   3, 1   3, 0   4, 1   4, 1   4, 2   2, 3   4, 3   4, 1   0, 4   4, 0   0, 0                    
The closest reference sequence may be the following set for data bits  0 , 0 , 1 , 1 , 0 :
 
                                                                                                                   DATA BITS   ERROR CORRECTION BITS            BLOCK   0   1   2   3   4   5   6   7   8   9   10   11   12   13   14   15               RANKS   0, 4   0, 4   4, 0   4, 0   0, 4   4, 0   4, 0   4, 0   4, 0   4, 0   4, 0   4, 0   4, 0   0, 4   4, 0   0, 0       Bit Val.   0   0   1   1   0   1   1   1   1   1   1   1   1   0   1   —                    
When compared by determining the distance or absolute value of the difference of the reference ranks and the received set of ranks, the difference is:
 
                                                                                                                   DATA BITS   ERROR CORRECTION BITS            BLOCK   0   1   2   3   4   5   6   7   8   9   10   11   12   13   14   15               DIFF.   2, 0   1, 0   0, 1   0, 0   0, 0   1, 1   1, 0   0, 1   0, 1   0, 2   2, 3   0, 3   0, 1   0, 0   0, 0   0, 0                    
The numerical difference (e.g., hamming distance) is the sum of the difference row, which equals 20. This difference would be compared to the difference for all other possible sequences. If this difference was less than all other distances, then the reference sequence is determined to be the closest match.
 
     In addition to determining the closest sequence from the reference set of sequences, the comparator  514  may also determine if the difference for the closest sequence exceeds a threshold. For example, the comparator  514  may discard the result if the difference is greater than a threshold, meaning that the closest reference sequence was significantly different than the received set of ranks. In other words, the comparator  514  may ensure that the received set of ranks are close enough to the determined reference sequence before outputting the sequence. 
     The example comparator  514  is further configured to reconstruct the least significant bits (LSB) of a detected sequence. The LSB may need to be reconstructed when the stacker is enabled and several messages are averaged. Such averaging will cause the LSB (or other rapidly changing data) that varies among the averaged messages to be recreated. Any method for reconstructed the data may be used. For example, if the data to be reconstructed is the LSB of a timestamp, one message may be detected without the use of stacking and a timer may be used to determine the difference in time between the known LSB and the current message so that the LSB of the timestamp can be recreated and the determined message modified to include the correct LSB. 
     The reference sequence  516  of the illustrated example may be implemented by any type of data storage. For example, the reference sequence datastore  516  may be a file, a database, a table, a list, an array, or any other type of datastore. While the example reference sequence  516  stores the 32 possible BCH sequences, any number of sequences may be stored. For example, a partial set of sequences may be stored. 
     Flowcharts representative of example processes that may be executed to implement some or all of the elements of the system  100  and the decoder  104  are shown in  FIGS. 6-7 . 
     In these examples, the process represented by each flowchart may be implemented by one or more programs comprising machine readable instructions for execution by: (a) a processor, such as the microprocessor  812  shown in the example computer  800  discussed below in connection with  FIG. 8 , (b) a controller, and/or (c) any other suitable device. The one or more programs may be embodied in software stored on a tangible medium such as, for example, a flash memory, a CD-ROM, a floppy disk, a hard drive, a DVD, or a memory associated with the processor  812 , but the entire program or programs and/or portions thereof could alternatively be executed by a device other than the microprocessor  812  and/or embodied in firmware or dedicated hardware (e.g., implemented by an application specific integrated circuit (ASIC), a programmable logic device (PLD), a field programmable logic device (FPLD), discrete logic, etc.). For example, any one, some or all of the example mobile communications system components could be implemented by any combination of software, hardware, and/or firmware. Also, some or all of the processes represented by the flowcharts of  FIGS. 6-7  may be implemented manually. 
     Further, although the example processes are described with reference to the flowcharts illustrated in  FIGS. 6-7 , many other techniques for implementing the example methods and apparatus described herein may alternatively be used. For example, with reference to the flowcharts illustrated in  FIGS. 6-7 , the order of execution of the blocks may be changed, and/or some of the blocks described may be changed, eliminated, combined, and/or subdivided into multiple blocks. While the processes of  FIGS. 6-7  are described in conjunction with the decoder  104 , any apparatus or system may implement the processes of  FIGS. 6-7 . 
       FIG. 6  is a flowchart of an example process to decode a message in audio. The process of  FIG. 6  begins when the sampler  502  updates a current audio block by sampling 4 samples and discarding 4 samples from an analysis window (block  602 ). The example time domain to frequency converter  504  performs a sliding FFT to convert the sampled audio from the time domain to the frequency domain (block  604 ). The ranker  506  ranks the code frequencies in the converted audio (block  606 ). For example, as described above, frequencies of interest may be ranked on a scale of 0 to 4 when there are five frequencies in each neighborhood. The determined ranks are stored in the rank buffer  508  (block  608 ). When the rank buffer  508  is a circular buffer, the addition of the determined ranks will eliminate a previously stored rank. In addition, when the rank buffer  508  is a circular buffer, an index indicating the point at which the next set of ranks should be inserted to the rank buffer  508  is incremented (block  610 ). 
     The comparator  512  then generates a rank distribution array across the number of blocks in a sequence (e.g., 15 blocks) (block  612 ). Next, the comparator  514  determines if a synch sequence has previously been detected (block  614 ). The synch sequence indicates the start of a message. Therefore, when the synch has previously been detected, a message thread has started. When a synch sequence has not previously been detected, control proceeds to block  624 , which is described below. 
     When a synch sequence has previously been detected (block  614 ), the comparator  514  generates match scores against all potential sequences (e.g., 32 possible BCH sequences) (block  616 ). For example, the comparator  514  may determine a distance between the rank distribution and each of the potential sequences. The comparator  514  then selects the potential sequence with the greatest score (e.g., smallest distance) (block  618 ). The comparator  514  determines if the selected score exceeds a threshold (block  620 ). For example, if the score is a distance, the comparator  514  determines if the distance is less than a threshold distance. When the score does not exceed the threshold, control proceeds to block  602  to continue processing. 
     When the score exceeds the threshold (block  620 ), the comparator  514  assigns the value to the sequence (block  622 ). Control then proceeds to block  602  to continue processing. 
     Returning to block  624 , when a match has not been previously detected (block  614 ), the comparator  514  generates a match score for the synch sequence (block  624 ). For example, as described above the comparator  514  may determine a distance between the rank distribution and the reference synch sequence. The comparator  514  determines if the score exceeds a threshold (block  626 ). When the score does not exceed the threshold, control proceeds to block  602  to continue processing. When the score exceeds the threshold, a flag is set indicating that a synch has been detected (block  628 ). Control then proceeds to block  602  to continue processing. While a flag is described above, any indication that a synch has been detected may be used. For example, a variable may be stored, the synch sequence may be stored in a table, etc. In addition, while the example process includes a separate branch for detecting a synch sequence, synch sequences may be detected in the same branch as other 
       FIG. 7  is a flowchart of an example process to decode a message in audio. The process of  FIG. 7  utilizes stacking to improve decoding accuracy. The process of  FIG. 7  begins when the sampler  502  updates a current audio block by sampling 4 samples and discarding 4 samples from an analysis window (block  702 ). The example time domain to frequency converter  504  performs a sliding FFT to convert the sampled audio from the time domain to the frequency domain (block  704 ). The ranker  506  ranks the code frequencies in the converted audio (block  706 ). For example, as described above, frequencies of interest may be ranked on a scale of 0 to 4 when there are five frequencies in each neighborhood. The stacker  510  then adds the determined ranks to the ranks of corresponding blocks of previous messages and divided by the number of messages to determine an average rank (block  707 ). For example, the determined ranks may be added to the corresponding ranks of the previous 4 messages. 
     The average ranks are stored in the rank buffer  508  (block  708 ). When the rank buffer  508  is a circular buffer, the addition of the average ranks will eliminate a previously stored rank. In addition, when the rank buffer  508  is a circular buffer, an index indicating the point at which the next set of ranks should be inserted to the rank buffer  508  is incremented (block  710 ). Alternatively, the ranks may be stored in the rank buffer  508  after block  706  and may retrieved from the rank buffer  508  as part of block  707 . 
     The comparator  514  then generates a rank distribution array across the number of blocks in a sequence (e.g., 15 blocks) (block  712 ). Next, the comparator  514  determines if a synch sequence has previously been detected (block  714 ). The synch sequence indicates the start of a message. Therefore, when the synch has previously been detected, a message thread has started. When a synch sequence has not previously been detected, control proceeds to block  724 , which is described below. 
     When a synch sequence has previously been detected (block  714 ), the comparator  514  generates match scores against all potential sequences (e.g., 32 possible BCH sequences) (block  716 ). For example, the comparator  514  may determine a distance between the rank distribution and each of the potential sequences. The comparator  514  then selects the potential sequence with the greatest score (e.g., smallest distance) (block  718 ). The comparator  514  determines if the selected score exceeds a threshold (block  720 ). For example, if the score is a distance, the comparator  514  determines if the distance is less than a threshold distance. When the score does not exceed the threshold, control proceeds to block  702  to continue processing. 
     When the score exceeds the threshold (block  720 ), the comparator  514  assigns the value to the sequence (block  722 ). The comparator  512  then reconstructs any data that may have been corrupted by the stacking process. For example, that comparator  512  may determine a corrupted portion of a timestamp (e.g., a second indication) by decoding one message and tracking the amount of time that passes between the decoded message and a currently detected message. Control then proceeds to block  702  to continue processing. 
     Returning to block  724 , when a match has not been previously detected (block  714 ), the comparator  514  generates a match score for the synch sequence (block  724 ). For example, as described above the comparator  514  may determine a distance between the rank distribution and the reference synch sequence. The comparator  514  determines if the score exceeds a threshold (block  726 ). When the score does not exceed the threshold, control proceeds to block  702  to continue processing. When the score exceeds the threshold, a flag is set indicating that a synch has been detected (block  728 ). Control then proceeds to block  702  to continue processing. While a flag is described above, any indication that a synch has been detected may be used. For example, a variable may be stored, the synch sequence may be stored in a table, etc. In addition, while the example process includes a separate branch for detecting a synch sequence, synch sequences may be detected in the same branch as other sequences and processing may later be performed to identify a synch sequence that indicates that start of a message thread. Further, while the process of  FIG. 7  is illustrated as a continuous loop, any flow may be utilized. 
       FIG. 8  is a schematic diagram of an example processor platform  800  that may be used and/or programmed to implement any or all of the example system  100  and the decoder  104 , and/or any other component described herein. For example, the processor platform  800  can be implemented by one or more general purpose processors, processor cores, microcontrollers, etc. Additionally, the processor platform  800  may be implemented as a part of a device having other functionality. For example, the processor platform  800  may be implemented using processing power provided in a mobile telephone, or any other handheld device. 
     The processor platform  800  of the example of  FIG. 8  includes at least one general purpose programmable processor  805 . The processor  805  executes coded instructions  810  and/or  812  present in main memory of the processor  805  (e.g., within a RAM  815  and/or a ROM  820 ). The processor  805  may be any type of processing unit, such as a processor core, a processor and/or a microcontroller. The processor  805  may execute, among other things, example machine accessible instructions implementing the processes described herein. The processor  805  is in communication with the main memory (including a ROM  820  and/or the RAM  815 ) via a bus  825 . The RAM  815  may be implemented by DRAM, SDRAM, and/or any other type of RAM device, and ROM may be implemented by flash memory and/or any other desired type of memory device. Access to the memory  815  and  820  may be controlled by a memory controller (not shown). 
     The processor platform  800  also includes an interface circuit  830 . The interface circuit  830  may be implemented by any type of interface standard, such as a USB interface, a Bluetooth interface, an external memory interface, serial port, general purpose input/output, etc. One or more input devices  835  and one or more output devices  840  are connected to the interface circuit  830 . 
     Although certain example apparatus, methods, and articles of manufacture are described herein, other implementations are possible. The scope of coverage of this patent is not limited to the specific examples described herein. On the contrary, this patent covers all apparatus, methods, and articles of manufacture falling within the scope of the invention.