VISUALmpeg PRO MPEG Analyser Software
1. Introduction VISUALmpeg PRO is a powerful tool set intended for detailed off-line analysis of Video Elementary Streams in MPEG-1 or MPEG-2 video format. The analysis results can be visualised in many ways, providing deep insight into all video-coding aspects. The following input formats are supported: - Elementary Stream - System Stream - Program Stream - Transport Stream The first three formats (Elementary-, System- and Program Stream) can be opened and analysed directly starting from the Open File command button. Opening a Transport-Stream file launches the Transport-Stream Analyser, which allows extraction of Elementary Streams. The resulting file can be opened again for detailed analysis. VISUALmpeg PRO allows you to analyse all parts of MPEG video from the top-level header layer down to the block layer. The following coding structures and parameters can be analysed: - VBV Buffer - Header Data - Picture Coding Type - Macroblock Coding Types - MB Quantiser - Motion Vectors - DCT Coefficients - Bit allocation - Slice structure - Picture Frequency analyzing - Display of differential Frames - Display of Motion compensated Frames - Video Quality Analyser The current version of VISUALmpeg PRO is completely software based. The tool has not been designed for speed, but is however arranged in a functional, educational and visually attractive manner. Therefore this software is also an ideal training tool for explaining and teaching MPEG.
2. Main Toolbar When an Elementary Stream has been opened, the main toolbar appears at the top of the VISUALmpeg PRO workspace. The command buttons on this toolbar allow the top-level control of VISUALmpeg PRO and determine which analysis results are shown in the work space. Main toolbar New Clears the entire workspace Open Header Tree Display Thumbnail Display Options Frame Display Control Panel Display Header Table VBV Diagram Open a file. VISUALmpeg PRO auto-detects the type of the file (Elementary-, System-, Program- or Transport Stream), and starts the corresponding parser. Opening a Transport-Stream file will start the Transport-Stream Analyser. Toggle display of the header tree on and off. This tree shows the high-level structure (GOPs, pictures) of the stream, and the position in the stream where headers occur. Analysis can be directed to a certain picture or a certain header by selecting a node in the header tree. Toggle display of the thumbnail strip on and off. The thumbnail strip shows a part of the stream as a sequence of small decoded pictures. The thumbnail strip can also be used to select a picture. Toggle display of the options frame on and off. The options frame can be used to select which result of the analysis process is shown in the main window. 3 Toggle display of the control panel on and off. This panel contains play/pause/stop buttons, as well as a number of slider controls, which can be used to select the decoded picture, or to decode pictures in sequence. Toggle between display of decoded header information and decoded picture. 3.1.3 Toggle display of the control panel on and off. 3.3
Detailed Macroblock Info Full Frame Display Toggle display of detailed macroblock information on and off. Detailed macroblock information is also shown when clicking the left mouse button on the decoded picture. Right-clicking causes the macroblock information to disappear again. 3.4 Stretch the decoded picture so that it covers the entire main window. Centre Frame Centre decoded picture in the main window. Quality Measurement Error Logger Launch the video-quality analyser in a separate window. 4 Launch the error logger in a separate window. 5
3. Analysis Windows 3.1. Options Frame The Options frame allows you to select which specific results from the analysing process are visualised in the decoded picture. Options bar 3.1.1. Picture Display Luma and Chroma When the Picture-Luma check box is checked, the luminance component of the decoded picture is shown. The Picture-Chroma check box controls whether the chrominance component is shown. Turning on both options will show the decoded picture in full colour. 3.1.2. Macroblock Coding Type Depending on the picture coding type, different macroblock coding types are available. When the Macroblock-Coding-Type radio button is checked, VISUALmpeg PRO displays these different coding types by using a different colour for each coding type. Display of macroblock coding types within a frame
The luminance component of the decoded picture can still be toggled on/off. As different colours are used to visualise the macroblock coding types, chrominance cannot be displayed. 3.1.3. Macroblock Quantiser Quantisation is a key part of MPEG, as reducing the number of quantisation steps is one of the most basic mechanisms to reduce the number of bits required to encode a picture. When the Macroblock-Quantiser radio button is checked, VISUALmpeg PRO shows the value of the quantiser across the picture. The higher the intensity of a block, the higher the value of the quantiser. The colour of the image shows the picture type: red for I-pictures, blue for P-pictures and green for B-pictures. The exact value of the macroblock quantiser can be obtained by clicking on a macroblock, and checking the q_scale field in the macroblock details pane in the upper-left corner of the workspace. Display of quantiser values within a picture 3.1.4. Bit Allocation VISUALmpeg PRO is able to display the bit allocation within a picture, both per 16x16 macroblock (check box Bits per MB ) and per 8x8 block (check box Bits per Block ). The number of bits per block is visualised using the intensity of the blocks.
The exact value of the number of bits per block and per macroblock quantiser can be obtained by clicking on a macroblock, and checking the q_scale field in the macroblock details pane in the upper-left corner of the workspace. Visualisation of bits per macroblock within a picture Visualisation of bits per 8x8 block within a picture
3.1.5. Macroblock Grid You can check the Macroblock-Grid check box to see the arrangement of macroblocks in the decoded picture. Picture with macroblock grid turned on 3.1.6. Motion Vectors One of the most important parts of the MPEG coding standard is motion estimation. Low bit rates can only be reached by using motion-compensated pictures (P and B frames). They are very efficient in their coding. The program displays the motion vectors of the P and B frames. The derived motion vectors can be displayed separately or together with other picture- or macroblock information. Display of forward coded motion vectors 3.1.7. Picture Frequencies With this feature it is possible to analyse the frequency content of a picture. The DCT Min and DCT Max controls can be used to select a sub-range of DCT coefficients. Just DCT coefficients inside this range will be included in the inverse DCT that transforms the 8x8 coefficients of every block from the frequency- into the picture domain.
8x8 matrix that allows selection of DCT coefficients to be taken into account If, for example, only the first coefficient is chosen (DC coefficient at the upper left position in the matrix) the picture shows DC values only. It is of course also possible to choose and display only the high frequencies of the picture. With this feature it is very easy to explain and understand the importance of the different coefficients in the frequency domain. DC Information within a frame High picture frequencies
3.2. Header Information VISUALmpeg PRO shows decoded headers when the header display is turned on with the headertable toolbar button ( ). The header tree, located on the left side of the main window, shows the grouping of the different headers. The following headers may occur: GOP Group of Pictures I, B, P Picture Header PCE Picture Coding Extension SEE Sequence Extension SEQ Sequence Header To display the information contained in a certain header, you can click on it. The table shows for each header parameter the binary code, the corresponding value and the meaning of the header value. Display of header tree and table of content 3.3. VBV Diagram The Video Buffer Verifier (VBV) diagram shows the load of a hypothetical VBV Buffer. The encoder has to ensure that the VBV buffer does not overflow, nor underflow. It s a requirement on valid MPEG video bit streams that they satisfy this VBV Buffer model.
The VBV buffer builds up load at a constant rate, which is the elementary-stream rate. Data is removed from the VBV buffer whenever a picture is decoded. In the VBV buffer model, removing of data is instantaneous: all encoded data for the picture being decoded is removed in one go. The vertical jumps in the VBV Diagram reflect this instant decoding. VBV buffer diagram The green line in the VBV diagram shows the load of the VBV buffer at a certain point in time. The red line shows the size of the VBV buffer. If the green line crosses the red line, VBV-buffer overflow has occurred. 3.4. Macroblock and DCT Information Detailed decoding information about macroblock is shown when the Detailed MB Info button has been checked. A specific macroblock can be selected by left-clicking the mouse in the decoded picture area. The macroblock information consists of: - Decoded macroblock header - DCT coefficients per block (Y0-Y3,Cr,Cb) before or after inverse quantisation - Quantiser matrix - Pixel values after IDCT
The radio buttons in the macroblock pane can be used to select which information is shown, and for which block (Y0-Y3,Cr,Cb). Detailed macroblock information
3.5. Differential Frames VISUALmpeg PRO can analyse and display the motion compensated, differential picture information of every MPEG frame, before adding it to the reference frames. With this feature you can demonstrate the amount of picture information per frame (e.g. comparison between I and B frame). Differential frame information 3.6. Motion-Compensated Frames An important part of MPEG encoding is the motion compensation for B- and P-frames. A good motion estimator at the encoder will significantly decrease the picture information that has to be coded, thereby saving a lot of bits. VISUALmpeg PRO allows you to display the motion-compensated frame before the differential frame (the correction) is added. This provides important hints to judge the quality of the encoder s motion estimation. Motion-compensated frame information Together with the Differential frame display Feature, VISUALmpeg PRO shows you how B- and P- frames are constructed at the encoder, and how they are reconstructed by the decoder:
Motion-compensated frame + Differential frame Reconstructed frame
4. Video-Quality Analyser Although video quality is very difficult to measure objectively (without having the original input data), the Quality-Analyser Tool of VISUALmpeg PRO includes four different diagrams showing curves related to picture quality: - Quality Index - Distortion Index - Bit Rate - Bits per Frame 4.1. Quality-Index Diagram By combining different MPEG Parameters of the appropriate stream, the Quality Index Qi is a measure indicating the quality level of the encoded MPEG video. 4.2. Distortion Index Diagram This curve is measuring MPEG artefacts (e.g. blocking) which is a typical effect of bad picture quality and/or bad encoding quality.
4.3. Bit-Rate Viewer One possible reason for bad picture quality is a lack of bits during encoding. This diagram shows the actual bit rate that was used to encode different stream parts. You can measure for example the deviation from the nominal bit rate Bn. 4.4. Number of Bits Per Frame I-, B-, and P-Frames have got extremely different bit amounts due to the compression techniques. This diagram shows the bit amount of every frame, which is an important hint for picture quality measurements.
5. Error Logger The Error-Logging tool in VISUALmpeg PRO detects syntax errors inside an MPEG stream. The tool is started by clicking the button in the main command bar. The Error Logger launches in a separate window and immediately starts checking the MPEG stream frame by frame. For every correctly decoded frame, an OK message is logged. If a frame contains errors, an appropriate message is logged.
6. Transport-Stream Analyser VISUALmpeg PRO is able to open a Transport Stream and perform an elementary analysis on it. The statistical view shows a Table with all Video Packets included in the Transport Stream The main function of this tool is to extract a Video Elementary stream out of the Transport Stream into a file. By clicking on a Video packet stream, you can extract and save that Video stream on the hard disk, in order to analyse the VES with VISUALmpeg PRO.