Abstract:
A frame formatting technique for filling a fixed length master frame with a number of variable length frames and adding additional data to synchronize the individual variable length frames and increase the randomness of the fixed length master frame. The technique fills unused portions of the fixed length master frame with random data to maximize the randomness of the fixed length master frame which can be used to produce a well behaved modulated signal for digital broadcasting, thereby increasing the efficiency of digital broadcasting system.

Description:
FIELD OF THE INVENTION 
   The present invention relates to digital broadcasting. More particularly, it relates to a frame formatting technique for filling a fixed length master frame. 
   BACKGROUND OF THE INVENTION 
   Currently, digital broadcasting systems are becoming increasingly popular for delivering high quality audio and video content to individual consumers. In a typical digital broadcasting system, as depicted in  FIG. 1 , an analog signal on an input line  10  is sampled by a digital encoder  120 . The samples are digitized and encoded to form data frames which contain digital representations of the analog signal. The data frames are then combined to form a master data frame defining a string of bits known as a bit stream on line  130 . The master data frame is then modulated by a digital modulator  140  for broadcasting the master data frame over a broadcasting channel  150 . The modulated signal is then received and demodulated by a digital demodulator  160  to derive a bit stream on line  170  which is equivalent to the original bit stream on line  130 . A digital decoder  180  then decodes the bit stream  170  to obtain an analog signal at output line  190  which can be output to an analog device such as a speaker. 
   The digital encoder samples the analog signal, digitizes the samples, and encodes the samples. Generally, the digital encoder collects a number N of digitized samples and generates a frame of encoded data based on the set of N digitized samples.  FIG. 2  depicts a typical digital encoder. An analog signal on input line  210  is sampled by the digital encoder  220  at its input for a specified period of time, x. The digital encoder  220  then produces an output signal on output line  230  which is a digital representation of the analog signal on input line  210 . In a fixed rate encoder, the output for each set of N digitized samples of the analog signal on input line  210  will contain the same number of bits which are defined by individual data frames of equal length. 
     FIG. 3  depicts a master data frame M F  which is completely filled with the individual data frames, m 1 -m N . Each master data frame M F  contains a few bits of master frame information M Fi , such as the length of the master frame and the number of individual data frames within the master frame M F . The remainder of the master data frame M F  is filled with the individual data frames, m 1 -m N . The individual data frames, M 1 -m N , also contain a few bits of individual data frame information, m 1i -m Ni , such as the length of the frame. Because the individual data frames, m 1 -m N , are fixed in length, a system can be designed where master data frames M F  are entirely filled with a specified number of individual data frames, m 1 -m N . 
   Presently, however, many digital encoders are variable rate digital encoders. For example, the digital encoders used to encode audio signals are generally variable rate encoders. In a variable rate encoder, the digital output for each set of N digitized samples of the analog signal may contain a different number of bits which are defined by data frames of unequal length. Variable rate audio encoders are used because they produce better digitized audio signals than fixed rate audio encoders in terms of lowering the number of bits required to accurately encode the audio signal without reducing the quality of the audio signal. The variable rate audio encoders are typically designed around the human perception of audio signals in order to minimize the number of bits required to accurately portray the audio signal digitally. 
   The audio encoders encode the audio samples into individual frames using a variable bit rate, resulting in individual frames which fluctuate in length based on the complexity of the audio segments being sampled. The sampling of an audio signal having segments, S 1 -S N , of varying complexity is depicted in FIG.  4 . Each segment of the analog audio signal is for a specified period of time, t, and produces frames, m 1 -m N , which define a number of bits that may vary in number from segment to segment. 
   Most digital transmission systems transmit digital signals using fixed length master frames, with each fixed length master frames comprising master frame information and a number of smaller frames of encoded data where each frame is preceded by a synchronization pattern. Ideally, the total number of bits defined by all of the digitized samples and synchronization patterns and the master frame information would equal the number of bits defined by the fixed length master frame M F . However, due to the variable bit rate of a variable rate encoder, the total number of bits defined by the encoded audio segments and master frame information may be less than the number of bits defined by the fixed length master frame M F . When the total number of bits is less than the fixed length master frame M F , some of the bits in the fixed length master frame M F  are unused. As depicted in  FIG. 5 , the unused bits in a fixed length master frame M F  are generally filled with a fixed pattern, such as all zeros, to indicate that no data is being transmitted. The unused portion may also be used to send other information such as text messages or other data of a non-random nature. However, filling the unused portion with a non-random pattern such as zeros or text, decreases the randomness of the fixed length master frame M F  which results in system inefficiency. As is known in the art, system efficiency is maximized in a system which transmits modulated signals by maximizing the randomness of the signal to be transmitted. Non-random data patterns result in a modulated signal with higher peak power. This causes an increase in the systems peak-to-average power ratio (PAR). The higher PAR necessitates more amplifier power to transmit the modulated signal with the increased peaks operating in a linear region of the amplifier. Increasing the randomness of the data pattern results in a modulated signal with a lower PAR, thus reducing the need for more amplifier power. Hence, system efficiency increases as randomness increases. 
   SUMMARY OF THE INVENTION 
   The present invention discloses a superior frame formatting technique for optimally filling a fixed length master data frame with a number of variable length frames and additional data to increase the randomness of the fixed length master frame. The frame formatting technique involves introducing random data to the fixed length master frame. Increasing the randomness of the fixed length master frame results in greater broadcast efficiency in a digital broadcasting system. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a prior art digital broadcasting system. 
       FIG. 2  is a block diagram of the prior art digital encoder of FIG.  1 . 
       FIG. 3  is a pictorial representation of a prior art master frame filled with fixed length data packets. 
       FIG. 4  is a graph of a analog signal of varying complexity. 
       FIG. 5  is a pictorial representation of a prior art master frame filled with variable length data packets. 
       FIG. 6  is a block diagram of a digital broadcasting system in accordance with the present invention. 
       FIG. 7  is a pictorial representation of an embodiment of a master frame filled with variable length data packets in accordance with the present invention. 
       FIG. 8  is a pictorial representation of a preferred embodiment of a master frame filled with variable length data packets in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention discloses a technique for maximizing randomness in a fixed length master frame comprising variable length encoded data frames. In accordance with the invention, the fixed length master frame is filled with master frame information, variable length encoded data frames, synchronization patterns, and additional random data to randomize the data defined by the fixed length master frame. The additional random data is added to unused portions of the fixed length master frame. The present invention increases the randomness of the fixed length master frame compared to using a non-random pattern for the unused portion of the fixed length master frame. By maximizing the randomness of the fixed length master frame, it is well known in the art that a more efficient modulated signal can be produced. Non-random data patterns may result in a modulated signal having a higher peak to average power ratio (PAR). The higher PAR necessitates more amplifier power to transmit the modulated signal. Increasing the randomness of the data pattern results in a modulated signal with a lower PAR, thus reducing the need for more amplifier power. The random data may be another data channel or filler. If another data channel is used, it may be randomized by a scrambler at the transmitter and de-randomized by a de-scrambler at the receiver. If it is filler, pseudo-random data generated by a processor may be added at the transmitter and then removed or disregarded at the receiver. 
     FIG. 6  depicts a digital broadcast system in accordance with the present invention. 
   In  FIG. 6 , an analog signal on input line  10  is used as an input for digital encoder  12 . Digital encoder  12  samples the analog signal and produces a bit stream which comprises variable length data frames which are digitally encoded representations of the individual segments of the analog signal. The variable length data frames are then formatted by formatter  14 . Formatter  14  combines the variable length data frames with synchronization patterns and adds random data to the unused portion of the frame to produce a fixed length master frame. By adding random data to the unused portion of the frame, the randomness of the fixed length master frame is increased. It is well known in the art that maximizing the randomness of the fixed length master frame produces a better behaved modulated signal for transmission in terms of the systems peak-to-average ratio (PAR). The PAR translates to a requirement on the power amplifier. This requirement dictates how much power the amplifier must provide linearly, above the average power of the amplifier, to avoid non-linearities caused by the peaks due to the non-randomness of the transmitted data. The fixed length master frame is modulated by digital modulator  16  and transmitted over a digital broadcast channel  18 . 
   The modulated signal is then received and demodulated by digital demodulator  20 . The demodulated signal is then de-formatted by de-formatter  22  which removes the synchronization patterns and the random data to derive the original bit stream which comprises variable length data frames which are digitally encoded representations of the individual segments of the analog signal. The bit stream is then decoded by digital decoder  24  to produce an analog output signal on output line  26 . The analog output signal can be output to an analog device for perception by an end user. 
     FIG. 7  represents an embodiment of a fixed length master frame  32  in accordance with the present invention. The master frame  32  formatting may take place in a formatter, a digital encoder, or a combination of the two. The master frame  32  is formatted by inserting master frame information  33 , variable length data frames m 1 -m N , synchronization patterns  34 , and random data  46 . Each variable length data frame m 1 -m N , such as variable length data frame  36 , is preceded by a synchronization pattern  34 . After the master frame information  33 , variable length data frames m 1 -m N , and synchronization patterns  34  are inserted into the ter frame  32 , any unused portion is filled with random data  46 . It is important to note that the random data portion may be added via a lookup table, thereby requiring very little computational complexity. The lookup table would be large enough to introduce randomness to the application at hand, eg. 32 K bits, and would be used over and over again, as needed, within a master frame period. The addition of random data  46  increases the randomness of the master frame  32 , producing an efficient fixed length master frame  32  suitable for modulated transmission. In this embodiment, the randomness is increased in the master frame  32  by adding random data to the end of the master frame  32 . While adding random data  46  to the end of the master frame  32  increases the randomness of the master frame  32 , the randomness of the master frame  32  could be increased even further if the random data  46  was interspersed throughout the entire length of the master frame  32 . 
     FIG. 8  represents a preferred embodiment of a fixed length master frame  50  in accordance with the present invention. In the embodiment depicted in  FIG. 8 , a frame formatting technique in which the randomness of the master frame  50  is maximized by interspersing random data r 1 -r N  throughout the entire length of master frame  50  is disclosed. This technique offers even more randomness than the technique described in  FIG. 7 , which introduces random data only at the end of the master frame  32 . The fixed length master frame  50  formatting may take place in a formatter, a digital encoder, or a combination of the two. The master frame  50  is formatted by inserting master frame information  53  and a number of fixed length frames M, such as fixed length frame  58 . The fixed length frames M all define the same number of bits. Additional fixed length frames  66  fill fixed length master frame  50 . The total number of bits defined by the master frame information  53  and all of the fixed length master frames M equals the number of bits defined by fixed length master frame  50 . Fixed length frame  58  comprises a synchronization pattern  52 , variable length frame information  55 , variable length data frame  54 , and a sufficient amount of random data  56  to completely fill fixed length frame  58 . The addition of random data  56  maximizes the randomness of the fixed length data frame  58 . 
   Each fixed length frame M comprises a synchronization pattern  52 ; a variable length data frame  54 ,  62 ,  72 , and  82 ; and random data  56 ,  64 ,  74 , and  84 . The amount of random data required to fill a fixed length frame M varies inversely to the length of the variable length frame, m 1 -m N . As the length of a variable length data frame, m 1 -m N , increases, the number of random bits required to fill a fixed length frame M decreases. For example, if variable length frame  62  defines more bits of data than variable length frame  54 , less random data  64  will be required to produce fixed data frame  68  than would be required to produce fixed data frame  58 . The addition of random data, r 1 -r N , to each of the fixed length frames M maximizes the randomness of the fixed length master frame  50 , producing an efficient fixed length master frame  50  suitable for modulated transmission. 
   A further advantage of the preferred embodiment over the embodiment depicted in  FIG. 7  arises from the synchronization patterns being more evenly dispersed throughout the master frame  50 . This results in efficiencies in the de-formatter  22  of  FIG. 6  since, as is well known in the art, a de-formatter  22  functions more efficiently when the sync patterns between data frames are arranged in a more regular fashion. This is especially advantageous during periods of non-complex signals, such as periods of silence, where, in the embodiment depicted in  FIG. 7 , the synchronization patterns would be concentrated toward the beginning of master frame  32  since the non-complex signal would result in short variable length data frames m 1 -m N . 
   Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.