Patent Publication Number: US-7710454-B2

Title: Video bit stream test

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/900,845 for “VIDEO BIT STREAM TEST” by Chao-Kung Yang, filed Jul. 27, 2004, which is hereby incorporated by reference herein. 

   TECHNICAL FIELD 
   The present invention relates generally to a method for MPEG compliance testing for a decoder device and more particularly to a video test bit stream for MPEG compliance testing of a set top box, also known as an integrated receiver/decoder. 
   BACKGROUND OF THE INVENTION 
   Television systems that receive digital video data streams that have been encoded and compressed require a decoder, such as an integrated receiver/decoder, to decode and decompress the incoming digital video data streams. To ensure proper operation of the decoder, it is necessary to perform tests to determine if the video decoders are properly decoding and decompressing the digital video data streams. 
   An organization called Moving Pictures Experts Group (MPEG) created standards to establish a standard coding and decoding strategy to try to minimize problems with inter-operability of equipment. Test bit streams are provided by the MPEG committee. The bit streams contain various types of content and are generated by feeding the video through the software MPEG encoder. The test bit streams provide adequate materials for testing the development of the MPEG algorithm. However, they fail to exercise all possible values of the MPEG parameters. 
   For many years, other suppliers have provided standard test bit streams for MPEG-compliance video testing. One current test is based on preserving an error through its testing cycle. An error is usually evidenced by the erroneous final image of the test sequence. There are several drawbacks associated with the current testing method. 
   One such drawback is that the results of the testing conditions are determined by human perception of the video images. When the error conditions are subtle, the testing operators are not always able to discern their existence. 
   Another drawback is that there are certain errors that are not suitable for testing using the current test bit stream method. Examples include integrated receiver/decoder timing error and frame skipping/repeating error. These problems go undetected because the current method does not include adequate timing information. 
   Yet another drawback is the conflict between the typical group of picture (GOP) structure used for testing and the practical GOP structure used for broadcasting. MPEG allows for one long group of picture (GOP) structure. However, this is not the typical structure for practical GOP use. Typically integrated receiver/decoders are designed to handle a limited GOP duration, which is usually one-half to one-second periods due to the requirement of starting video from a random position. Due to the emphasis on the human visual pass/fail conditions and real-time testing by the current “predictive” testing method, the error evidence has to be preserved until the video stops. Therefore, the testing video sequence can only be constructed in one long GOP. 
   SUMMARY OF THE INVENTION 
   The present invention is a method for MPEG compliance testing that designs test bit streams such that every frame of the expected decoded video is known. The test bit stream is generated, decoded and stored a s an expected output decoded video. The test bit stream is fed to the device under test where it is decoded. Actual output video is compared with the expected decoded video on a frame-by-frame basis. The test results are indicated by discrepancies, which can be visually examined by either a human or a computer, through a comparison of the expected video and the actual output video. 
   It is an object of the present invention to improve the accuracy of MPEG compliance testing. It is another object of the present invention to eliminate the drawbacks associated with human error in detecting errors during the testing process. It is still another object of the present invention to detect errors associated with timing error, skipping error, and repeating error. 
   Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention, reference should now be had to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention. In the drawings: 
       FIG. 1  is a block diagram of the bit stream testing system of the present invention; 
       FIG. 2  is a flow chart of the method of bit stream test process according to the present invention; 
       FIG. 3  is a flow chart of the method for generating the bit stream; and 
       FIG. 4  is a block diagram of the bit stream generation process. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention is a method for MPEG compliance testing of set-top boxes (STBs), also known as integrated receiver/decoders (IRDs). A video test bit stream is constructed such that every frame of the expected decoded video is known. The decoded expected video is stored. The constructed test bit stream is run through the STB and the output video is compared with the expected decoded video on a frame-by-frame basis. The test can be done in real time, as the video is output. In the alternative, the output video can be recorded and compared at a later time. In either event, the test results are indicated by discrepancies between the stored known results of the constructed test bit stream and the output video of the set top box after the test bit stream is decoded by the device under test. 
   Referring now to  FIG. 1 , there is shown a set top box testing system  10  of the present invention. A computer  12 , or other processing device, is used to generate the test bit stream and decode the test bit stream. Therefore, every frame of the expected decoded output video is known. The expected decoded output video is stored on the computer. It is possible to select frames from the known decoded output video for later comparison. 
   The constructed test bit stream is formatted into its proper form which is dependent on the device under test. The constructed test bit stream is then fed to the device under test  14 , such as the STB or IRD, where it is decoded. The device under test  14  outputs video as it decodes the constructed test bit stream. This output video is recorded  16  and stored on the computer  12 . The output video from the device under test is compared with the expected decoded output video stored on the computer  12 , or the frames selected from the expected decoded output video. 
   If the device under test  14  has decoded the video without any error, the frames of the differential video will be black. If some of the differential video frames are not black, there are errors present. It is possible to examine where and when the non-black pixels occur to determine the error. 
     FIG. 2  is a flow chart of the testing method  100 . The computer generates  102  an MPEG elementary video test bit stream, which will be discussed in detail hereinafter with reference to  FIGS. 3 and 4 . Still referring to  FIG. 2 , after the bit stream is constructed, it is fed through  104  a debugged software decoder to generate  106  reference decoded video frames from the expected decoded output video. The reference decoded video frames are stored  108  on the computer for later comparison. The constructed test bit stream is formatted  110  into its proper form for the device under test. For example, the test bit stream may be formatted to the form required for satellite receiving. 
   The constructed test bit stream is fed  112  into the device under test, where it is decoded  114  by the device under test. The output video from the device under test is compared  118 , or recorded and stored  116  and compared  118 , with the pre-stored reference decoded video frames to generate differential video frames  120 . The test results will indicate an error  122  by non-black pixels in the differential video. If some of the differential frames are not black, it is possible to determine where and when the non-black pixels occur, and then the error can be identified  124 . If the test is without any error, the frames of differential video will be all black. 
   MPEG video consists of a hierarchy of layers. The first layer is the video sequence layer, the second layer is the group of pictures (GOP), which is composed of one of more groups of intra frames (I-frames) and non-intra frames (P and B frames). The third layer is the picture itself. And the fourth layer is called the slice layer. This information is useful in conjunction with the following explanation of the generation of the test bit stream. 
   Referring now to  FIG. 3 , a flowchart is shown for the method  200  of constructing the test bit stream. A starting video frame, also called an I-frame, is arbitrarily chosen  202 . For a given GOP structure, for example N=15, M=3, the subsequent P and B frames are determined  204  by varying  206  the selected testing MPEG parameter with the whole range of its possible values, while keeping other parameters constant  208 . This is a significant advantage over prior art methods that do not take the whole range of possible parameter values into consideration when generating a test bit stream. 
   According to the present invention, only the starting I-frame is known. After the starting frame, the rest of the frames become unpredictable. Therefore, to recreate the video sequence, the bit stream is fed through a debugged software decoder after its construction to produce the expected decoded output video used in the comparison. All re-created video frames constitute the expected decoded video and are stored as reference frames. 
   In MPEG format, an I frame is a starting frame. It is coded spatially with no reference to any other frame. For a P frame, the encoder predicts a future frame from an I frame or another P frame in a forward time manner. For a B frame, or bi-directional frame, the encoder has the option of using forward/backward interpolated prediction. I frames are coded spatially, P frames are forward predicted based on previous I and P frames, and B frames are coded based on a forward prediction from a previous I or P frame, as well as a backward prediction from a succeeding I or P frame. 
     FIG. 4  is an example of how the constructed test bit stream  300  is generated. A starting I-frame is arbitrarily chosen from the video  302 . The I-frame is coded  304  and the I-frame syntax become part of the test bit stream  300 . Likewise, the P-frame is coded  306  and the P-frame syntax becomes part of the test bit stream  300 . The creation of the P-frame  308  may be accomplished by frequency domain inverse transformation techniques, such as Inverse Discrete Cosine Transform (IDCT). 
   The frequency domain selected MPEG parameter is varied by all of its possible values  310 . The B-frame is then coded  312 , and the B-frame syntax becomes part of the test bit stream  300 . The entire bit stream is fed through a software decoder  314 , and reference video frames are selected to create the reference video  316 . 
   According to the method of the present invention, which was described in detail above, the test bit stream is generated such that the expected output can be calculated analytically using a software decoder. The expected decoded output is then stored and used as a comparison to the output generated by the device under test. The comparison can be accomplished in real time, or recorded, stored and analyzed off line. An additional advantage is that the test bit stream can be designed in a special pattern, with the expected output known ahead of time 
   The invention covers all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the appended claims.