Abstract:
A method of storing a data file, particularly in the MPEG format and including a flow of different frames, comprises a protection system for the data file based on a parameter stored in the data file. Advantageously, the storage method comprises selectively protecting the frames by storing parameters that are associated with corresponding different frames whose values are selected to provide a playing quality level requested by an end user. Also, a method is provided for decoding a data file, particularly of the MPEG type and including a flow of different frames, wherein the data file is stored per above.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional application that claims priority from Italian Patent Application No. MI2001A001309, filed Jun. 21, 2001, and U.S. patent application Ser. No. 10/177,729, filed Jun. 20, 2002 now U.S. Pat. No. 7,295,764, which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates generally to a method of storing a data file, particularly a file in the MPEG format. 
     Specifically, the invention relates to a method of storing a data file, in particular of the MPEG type and comprising a flow of different “frames”, said method comprising a system for protecting said data file, which system takes into account a redundancy factor stored in said data file. 
     The invention further relates to a method of decoding a data file, in particular an MPEG type of file that comprises a flow of different frames, said data file being stored by a selectively protecting storage method. 
     The invention relates, particularly but not exclusively, to a method of storing audio/video files in the MPEG format for application to video recorders, this description making reference to this field of application for convenience of illustration only. 
     2. Prior Art 
     As is well known, PVRs (Personal Video Recorders) are an expanding branch of the HDD (Hard Disk Drive) market. 
     In particular, HDDs for use in PVRs are required to exhibit a high storage capacity for storing files of considerable size, as audio/video (AV) files are, to be cost-efficient. 
     To optimize the storage space taken up, it is common to compress the audio/video (AV) files using different standards. A common compression format is the MPEG (Moving Pictures Experts Group) format. 
     In particular, the MPEG standard of compression provides for each flow of images to be resolved into a succession of frames, advantageously described as basic images and difference images (i.e., images including differences from the basic ones). 
     Taking the instance of a moving object against a stationary background, an explanation of the MPEG Standard can be approximated by the following: an image is processed by first storing a complete accurate starting image (basic image); thereafter, only variations of each following image from the basic image are encoded, and only the difference-image stored. 
     In fact, the images that follow the starting image typically contain an unchanged part (i.e. information that is redundant for the purpose of storing the moving images) of the starting image. The MPEG Standard effectively removes the need to store a large amount of unnecessary data, in particular data concerning the unchanged part of an image, and advantageously reduces the overall size of the compressed audio/video (AV) file, i.e. of the resultant file in the MPEG format. 
     In particular, the MPEG Standard provides a set of special algorithms for compressing a sequence S of digital images, which algorithms are directed to reduce space and time redundancies in the sequence S. 
     Space redundancy is reduced by compressing each image independently with mathematical operations such as quantization, discrete cosine transform, and Huffman encoding. 
     Time redundancy is reduced by utilizing the relation that exists between successive images in the sequence S, each image being expressed as a translation of the preceding and/or the following image in the sequence S. 
     As an overview, the MPEG Standard provides three types of basic image or basic frame: 
     I-frame (Intra coded frame), where the whole basic-image is encoded to provide a reference for storing the frames to follow; 
     P-frame (Predicted frame), where only the difference of information from one other frame, such as the I-frame, is encoded, so a P-frame has a reduced informational contents; and 
     B-frame (Bidirectionally predicted frame), where image variations due to the motion described by the frame sequence are typically encoded with reference to multiple other frames (P or I). 
     Any one sequence S starts with an I-frame and goes on as an alternating succession of P-frames and B-frames, as shown in  FIG. 1 . 
     It should be noted that I-frames are encoded independently, P-frames are encoded relative to either an I- or a P-type previous frame in the sequence S, and B-frames are encoded relative to two frames, of the I- or P-type, being a previous and a following one in the sequence S. 
     A typical succession of images could be: I B B P B B P B B I B . . . , this being in particular the order in which the images are displayed. In actual practice, due to the above-outlined definition of the I-, P- and B-frames, an image decoder in the MPEG format is necessary that receives the P-frames ahead of the B-frames, and the I-frames ahead of the P-frames, as in the sequence illustrated by  FIG. 1 . 
     As shown in  FIG. 2 , a system adapted to carry out the MPEG compression just described, indicated as MPEG system  1 , comprises at least one sequential encoder COD S  effective to receive an initial image IM 1  comprising a series of frames in an initial order, and to encode them in accordance with the algorithms of MPEG Standard, to eventually produce the sequence S shown in  FIG. 1 . 
     The MPEG system  1  further includes a decoder DEC effective to rearrange and decode such frames to enable display of a final image IM 2 . 
     In particular, to encode a B-frame, the encoder COD S  is required to retain, in a special memory called the frame memory, the previously encoded (and already decoded) I- and P-frames to which the B-frame is related. 
     All this requires an amount of memory, specially for encoding and decoding according to the MPEG Standard. 
     In addition, the MPEG system  1  should be capable of detecting errors and protecting private information. For example, important data can be duplicated and lost packets retrieved. 
     It should be noted that, in consequence of the hierarchy enforced on the frames by the MPEG Standard, the same error in different type frames would carry different weights. In particular, if one or more bytes in a B-frame becomes corrupted, the ultimate image may include an error that is not perceivable by an end user, but several corrupted bytes in a I-frame can deteriorate a whole sequence of frames. 
     To give a quantitative idea, take the instance of HDDs as used in a PC, in workstations, or network servers. An error allowance of 1*E −12  to 1*E −14  is specified for such HDDs, depending on the application involved. Such high levels of reliability are achieved by encoding user&#39;s data using chained codes, namely: 
     an internal (or line) code COD-I; and 
     an external code COD-E with error detecting and correcting facilities. 
     The internal code COD-I provides periodic up-dating or refreshing for a loop path of a R/W channel, and especially for a timing recovery loop. The internal code COD-I also provides an amount of SNR gain, i.e. enables good performance, with reference to the encoding error, even at low SNR values. 
     The external code COD-E is more often a Reed-Solomon code (code RS). This code RS has three characterizing parameters: 
     the length m of the symbol employed; 
     the interleaving depth IN; and 
     the number of redundant symbols in each interleave, or redundancy value indicate with 2t. 
     With the redundancy value given as 2t, a number of errors equal to one half the redundancy value, i.e. equal t, can be detected and corrected. 
     Actually, codes are known whereby the locations of wrong symbols can be found, marking them by means of “erasures”: in this case a number 2t of errors corrected. However, most of the commercially available hard disks are not equipped with such codes. 
     At the end of external RS-code processing, an encoded data sector  2  has the structure shown in  FIG. 3 , where the encode parameters are the following values:
 
IN=3 2t=4
 
     This outlines an instance where two errors can be detected and corrected per interleave. 
     In particular, shown schematically in  FIG. 3  are: 
     a sequence  3  of user&#39;s data D 1 , . . . , Dk; 
     a memory array  4  where the user&#39;s data D 1 , . . . , Dk are reordained as appropriate; and 
     encoded data sector  2 . 
     It should be noted that each sector  2  includes essentially three logic sections, containing: 
     (I) user&#39;s data; 
     (II) redundant data; 
     (III) symbols of a CRC (Cycle Redundancy Check) code, i.e. a code that only allows detection of errors, if any; actually, two codes CRC 1  and CRC 2  are appended to each sector  2 . 
     The CRC code is usually employed to check that a sector  2  has been re-read properly. In other words, the CRC code is employed to check if the RS-encoding process has applied wrong corrections. 
     In conventional systems, the protection must be afforded uniformly to the data stored in a hard disk. Thus, the amount of redundant data is set at a maximum tmax, to provide all-round reliability. 
     To determine the performance of a hard disk, it is known to calculate the BER (Byte Error Rate) of the system following RS decoding, i.e. after decoding a file that has been encoded by RS coding, according to a corresponding BER input to a decoder DEC where the RS decoding is effected. 
     It is customary in this field of application to refer to the byte error rate output from the decoder DEC as the “hard” BER, and to the byte error rate input to the decoder DEC as the “soft” BER, as shown schematically in  FIG. 4 . 
     Taking an input soft BER of 10 −5  to the decoder DEC, the plot of the hard BER against time t is shown schematically in  FIG. 5 . For example, a hard BER of 10 −14  ensures that the system will be a reliable one. In particular, worst-case, best-case, and intermediate hard BERs are plotted, with the worst and best cases being dependent on the error events over the interleaves. 
     The admissible values for the hard BERs depend on the RS encode/decode applications. In particular, in the instance of audio/video (AV) files, e.g. motion-image or musical pieces, these values are very high due to the requirements for high-fidelity playing. Furthermore, on account of the large size of audio/video files, the demand for accuracy greatly expands the memory requirements of video/audio applications. 
     Consequently, a need has arisen for a method of storing compressed audio/video files, whereby playing fidelity can be acceptable for an end user, while the storage space requirements of such files can be reduced so allowing the use of hard disk HDD of reduced size, thereby overcoming the limitations of prior art audio/video file storage methods and obviating the need of large inconvenient HDDs. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, MPEG files are stored with selective protection, i.e. with different hard BER values according to the type of audio/video file involved and the level of playing fidelity expected by an end user. 
     According to an embodiment of the invention, a method of storing a data file, particularly in the MPEG format and comprising a flow of different frames, comprises a data-file protection system based on a parameter stored in said data file, and is characterized in that said frames are selectively protected by storing parameters that are associated with corresponding different frames and have values selected to provide a playing quality level specified by an end user. 
     According to another embodiment of the invention, a method of decoding a data file, particularly in the MPEG format and including a flow of different frames, said data file being stored by a storage method according to an embodiment of the invention, characterized in that the method comprises the following steps: 
     receiving a data file; 
     scanning each received frame, and for each frame verifying said values stored in said identification byte of said data file; 
     decoding each frame using redundancy factors that are different and correspond to said values stored in said identification byte. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the storage and the decoding method of this invention will become apparent from the following description of embodiments thereof, given by way of non-limitative examples with reference to the accompanying drawings. 
       In the drawings: 
         FIG. 1  shows a conventional sequence of images according to the MPEG Standard; 
         FIG. 2  shows a conventional MPEG system; 
         FIG. 3  shows the data setup based on a RS encode/decode according to the prior art; 
         FIG. 4  shows a detail of RS encode/decode according to the prior art; 
         FIG. 5  shows a specific error distribution largely employed in conventional systems; 
         FIG. 6  shows an HDD controller employing a storage method according to an embodiment of this invention; 
         FIG. 7  shows a detail of the controller of  FIG. 6 ; 
         FIG. 8  illustrates a storage method according to an embodiment of this invention by means of a write operation flowchart. 
     
    
    
     DETAILED DESCRIPTION 
     The following discussion is presented to enable one skilled in the art to make and use the invention. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention as defined by the appended claims. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     The instance of an HDD for recording audio/video (AV) files will be considered herein for convenience of illustration. 
     Advantageously according to an embodiment of this invention, each audio/video file is associated with a different hard BER tolerance, according to the frame type to be processed and the audio/video quality requested by an end user. 
     Thus, it is conceivable that an end user would expect high-fidelity when playing motion-images and music, and accept a lower quality level when news and sport services are played. 
     Referring to  FIG. 6 , it should be noted that at the audio/video-file recording stage, an HDD or disk controller  11  associated with a storage system such as the MPEG system  10  for processing audio/video files “knows” the type of frame that is to be store, i.e. whether it is an I-, P- or B-frame. 
     The protection for said frames is obtained from a parameter, specifically a redundancy factor RID associated with each frame. 
     This is an inherent feature of new-generation controllers, the so-called new-generation HDDs, for PVR camcorder applications, in view of that the controller  11  is to encode a flow  12  of audio/video (AV) data incoming through an input IN into an MPEG file  13  that is output (OUT) to an HDD  14 , as shown in  FIG. 6 . 
     It can be appreciated from the foregoing that a method of storing data files, particularly in the MPEG format, according to an embodiment of the invention comprises the operations described here below. 
     1) If a user requests high quality, the controller  11  will use, in storing the MPEG file  13 , the same redundancy factor RID MAX  for all types of frame. 
     Thus, this storage method provides the same level of reliability for all the frames, i.e. the same hard BER value as set at 10 −15 , for example. 
     2) If a user does not request high quality, the controller  11  will use, in storing the MPEG file  13 , different redundancy factors RID I , RID P , RID B  to suit the frame types. 
     Thus, this storage method provides different levels of reliability for different type frames, i.e. respective hard BER values for I-, P- or B-frames. 
     In particular, the values shown in Table I below are used to bring out the play quality for the MPEG encoded audio/video data flow  12 . 
     
       
         
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Frame 
                 Hard BER 
               
               
                   
               
             
             
               
                 I 
                 10 −15   
               
               
                 P 
                 10 −10   
               
               
                 B 
                 10 −8   
               
               
                   
               
             
          
         
       
     
     Advantageously, the redundancy factors RID MAX , RIDI, RIDP, RIDB are added to the ECC (Error Correction Code). 
     Let us see now the operation of an HDD system  10 , shown in detail in  FIG. 7 , which implements a storage method according to an embodiment of the invention. 
     From the hard BER values shown in Table I above, a controller  11  for an HDD  14  provides a PR type of protection for application to a sector  15  being recorded, and marks the sector  15  with a specific CSM (Controller Sync Mark) identification byte at the beginning of a data field  16  of the MPEG file  13 . 
     Then, when the sector  15  is read back, the controller  11  reads the CSM identification byte from the data field  16  and sets a decoder  17 , particularly of the RS encode/decode type accordingly. The decoder  17  reconstructs the imaging contents of the original data flow  12  by decoding the MPEG file  13  frame by frame according to the CSM identification byte, and therefore according to the redundancy factor applied to each frame during the compression step. 
     The flowchart of  FIG. 8  illustrates graphically the sequence of operations performed by the system  10  using a storage method according to an embodiment of this invention. 
     In particular,  FIG. 8  is a flowchart for a write operation being carried out in the HDD  14 , comprising the following steps: 
     receiving the data flow  12 ; 
     checking for a quality level Q (Qhigh/Qlow) requested by an end user. 
     The method comprises two different sequences of operations according to the quality level requested, i.e. the level of reliability required for each frame type. 
     When a high quality level (Qhigh) is requested: 
     allotting a first value FFH to the identification CSM byte, i.e. storing such a value FFH into the data field  16  of the corresponding MPEG file  13 , independently of the frame type that is stored; 
     end of the write operation. 
     In this way, the storage method provides a same level of reliability and single hard BER value for all the frames. 
     When a low quality level (Qlow) is requested, the sequential steps become: 
     scanning one frame to check if it is an I-frame; 
     in case of positive result, allotting the first value FFH to the identification CSM byte of such a frame, i.e. storing the first value FFH into data field  16  of the corresponding MPEG file  13 . 
     It should be noted that the first value FFH for the identification CSM byte is the same value as is used for high-quality requirements, because the propagation of errors that lead to degradation of the re-constructed image is controlled by I-frames and the level of reliability of I-frames should be the highest. 
     In case of negative result, scanning the frame to check if it is a P-frame; 
     in case of positive result of this further checking step, allotting a second value F0H to the identification CSM byte of that frame, i.e. storing the second value F0H into data field  16  of the corresponding MPEG file  13 ; 
     in case of negative result, allotting a third value FAH to the identification CSM byte of that frame, i.e. storing the third value FAH into data field  16  of the corresponding MPEG file  13 ; 
     end of the write operation. 
     In this way, the storage method provides different levels of reliability according to whether a frame is an I-, P- or B-type. 
     It should be noted that the different values FFH, F0H and FAH allotted to the identification CSM byte correspond to respective frame redundancies tmax, t 1  and t 2 . 
     In particular, the redundancy t 2  of a B-frame is less than the redundancy t 1  of a P-frame, the redundancy of the latter being less than the maximum redundancy tmax of an I-frame. 
     It can be seen that the flow of read operations has the opposite pattern. In particular, the controller  11  recognizes the frame type from the value stored in the identification CSM byte, and sets consequently the decoder  17  to perform an RS decode according to the known values of the redundancy factors RID MAX , RID I , RID P  and RID B . 
     To assess the extent of the space saving afforded by this storage method, a simulated encode of a MPEG format audio/video file was carried out. 
     In particular, an MPEG file was used which comprised a single sequence of 4,332,554 bytes. In terms of storage space, this means 8,463 standard sectors, each of 512 bytes. 
     Two different starting situations were also simulated which corresponded to a soft BER value of either 10 −5  or 10 −4 , these being specified values currently in use. 
     Shown in Table II are, for each starting situation: 
     a number ECC bytes of redundancy bytes required for storing the MPEG file; and 
     a percentage OVERHEAD of disk area used. 
     Table II is comparative of a conventional storage method, in particular using uniform protection for frames of any types to be stored, and the storage according to an embodiment of this invention, using selective protection according to the frame type to be stored. 
     The percentage OVERHEAD is used as an efficiency parameter of either methods to afford end reliability. 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
             
             
               
                   
                   
               
               
                   
                 Uniform Protection 
                 Selective Protection 
               
             
          
           
               
                   
                 SBER = 
                   
                 SBER = 
                   
               
               
                   
                 10 −4   
                 SBER = 10 −5   
                 10 −4   
                 SBER = 10 −5   
               
               
                   
                   
               
             
          
           
               
                 ECC Bytes 
                 330.057 
                 228.501 
                 221.958 
                 163.338 
               
               
                 OVERHEAD 
                 7.6% 
                 5.3% 
                 5.1% 
                 3.8% 
               
               
                   
               
             
          
         
       
     
     It can be seen from Table II that the gain obtained with the selective protection storage method of this embodiment of the invention may vary between 1.3% and 2.5%, depending on situations. 
     Table III shows the number of corrections per interleave, i.e. the redundancy parameter t, used in the simulations (interleaving depth is always 3). 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE III 
               
             
             
               
                   
                   
               
               
                   
                 Uniform Protection 
                 Selective Protection 
               
             
          
           
               
                   
                 SBER = 
                   
                 SBER = 
                   
               
               
                   
                 10 −4   
                 SBER = 10 −5   
                 10 −4   
                 SBER = 10 −5   
               
               
                   
                   
               
             
          
           
               
                 Number t of 
                 6 
                 4 
                 I-frame: t = 6 
                 I-frame: t = 4 
               
               
                 corrections 
                   
                   
                 P-frame: t = 4 
                 P-frame: t = 3 
               
               
                 per 
                   
                   
                 B-frame: t = 3 
                 B-frame: t = 2 
               
               
                 interleave 
               
               
                   
               
             
          
         
       
     
     A similar simulation, carried out on an MPEG file having a small motion contents, provided exactly the same gain percentages as regards efficiency. 
     Savings of 1 to 2% are regarded substantial for an HDD system. Such savings, while not reflecting on increased capacity and higher formatting level, advantageously increase the throughput, and hence the final cost, of HDDs. 
     Although reading to and writing from a disk is described, this selective coding/decoding technique has other applications. For example, an I-, P-, and/or B-frame can be coded having different redundancy factors, transmitted, then decoded at different quality levels using the different redundancy factors. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention.