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1. Introduction

In addition, thanks to extensions to the Wave format, Editable MPEG files can be interleaved and may contain MPEG-compressed Audio data, thus further reducing the file size.

Finally, hardware accelerators will be shortly available to perform real-time compression as well as the transcoding from Editable MPEG to fully compressed MPEG. This will allow users of personal computers to capture video in real time, edit it and store it efficiently on their desktop computer.

The main benefits of the Editable MPEG file format are summarized below:

• AVI encapsulation insures compatibility with existing authoring systems
• Provides significant compression of video data, while maintaining high image quality
• Frame accurate editing
• Multiple compression-decompression iterations
• Supported by hardware accelerators which will allow capture, editing and storage on the desktop
• Support interleaved MPEG Audio

2. Application

On the other hand, almost all multimedia tools and packages today rely on the AVI file format to handle audio and video data. As stated in the introduction, this document describes an extension to the AVI file format that handles a particular type of MPEG files and thus allows editing and conversion to a normal MPEG file format, while maintaining the image quality.

The figure below shows a simplified data flow for an Authoring application. The authoring system interfaces with the MPEG Compression & Decompression Accelerator through the Video Codec. For editing, Editable MPEG files are decompressed through the Video Codec and the Accelerator and returned to the Authoring System as bitmaps, which can then be manipulated, displayed or mixed with bitmaps originating from other types of AVI files. The resulting bitmap is then fed back to the Accelerator for compression either into an Editable MPEG file or into a distributable MPEG format. In practice the Accelerator may be a collection of hardware and/or software devices.

Authoring Application

Once editing is completed, the Editable MPEG file is transcoded by the Accelerator into a regular MPEG file.

Note also that with the recent extension of the Wave format to support the MPEG Audio compression standard [Ref 5], compressed audio streams can be interleaved with the MPEG video stream following the normal AVI interleaved format [Ref 2].

The Editable MPEG files contain only Intra-coded frames (I-frames). A MPEG I-frame has self-contained information to represent a picture. Each frame will include the MPEG sequence header with the quantization matrix information, in order to facilitate editing operations without requiring decompression and recompression. Furthermore, we encapsulate each frame in a Group of Pictures (GOP) and thus provide each frame with a SMPTE time code.

3. Extension to the AVI File Format

Only a few parameters in the Stream Header Chunk need to be assigned specific values:

• The MPEG AVI file does not use the "strd" chunk (This area is typically used by the installable compression/decompression driver)
• The fccType field is set to "vids" which stands for MPEG video stream.
• The fccHandler field is set to "MPGI" which will activate the installable compression/decompression VIDC.MPGI driver in the Microsoft Windows environment.

If index chuncks are being used, each frame should be flagged as a "key frame", using the AVIIF_KEYFRAME flag.

4. MPEG DIB Extension

typedef struct tagEXBMINFOHEADER {
	BITMAPINFOHEADER	bmi;
	MPEGINFOHEADER  	mpegi;
} EXBMINFOHEADER;

typedef struct tagBITMAPINFOHEADER {
	DWORD 			biSize;
	LONG  			biWidth;
	LONG  			biHeight;
	WORD  			biPlanes;
	WORD  			biBitCount;
	DWORD 			biCompression;
	DWORD 			biSizeImage;
	LONG  			biXPelsPerMeter;
	LONG  			biYPelsPerMeter;
	DWORD 			biClrUsed;
	DWORD 			biClrImportant;
} BITMAPINFOHEADER;

typedef struct tagMPEGINFOHEADER {
	BYTE  			bPixApectRatio;
} MPEGINFOHEADER;

The required values for the MPEGINFOHEADER fields are as follows,

• biSize = sizeof(BITMAPINFOHEADER)+sizeof(MPEGINFOHEADER);
• biPlanes = 1;
• biBitcounts = 24;
• biCompression = mmioFOURCC('M','P','G','I');
• bPixApectRatio = [Ref 1]

The MPEGINFOHEADER together with the AVI RIFF headers is meant to provide the application, such as an authoring system, all the information required to process and display the video stream. It has been defined so as to ensure that two streams with identical MPEGINFOHEADER can be merged into a single stream without having to decompress/recompress the video data (assuming that the video editing operation lends itself to it).

See section 6 for the correspondence between AVI-DIB and MPEG parameters.

5. LIST "movi" Chunk

MPEG-I DIB '##dc'

BYTE abBits[];

abBits[] is a fully MPEG-1 compliant I-frame picture, preceded by the sequence header, the GOP header and the Picture header. Keeping extra information like the MPEG GOP header and picture header with the compressed data for each I-frame picture eases the work load of the video codec, since the whole abBits[] can be sent to the decoder right away. Furthermore, sequence headers allow cut/paste operations at any frame by providing the quantization matrix information.

The following shows the mandatory parameters in the Sequence header, Group of Picture header, and the Picture header.
For the detailed syntax of MPEG-1 streams see Reference 1.

(*) Note that the intra_quantizer_matrix is only present if the load_intra_quantizer_matrix bit is set to 1. If load_intra_quantizer_matrix is set to 0, the default quantization matrix specified by the MPEG standard is used.

6. Constraints on MPEG Parameters

7. Relationships between AVI Parameters and MPEG Parameters

Note 1: The relationship between dwSuggestedBufferSize & vbv_buffer_size is only meaningful in the case when the application is performing the decompression itself. Otherwise, these two numbers are independent, and the suggested buffer size would then typically be, for interleaved files, the size of a complete record.

Note 2: dwQuality is computed such that the baseline bit rate of 1.2 Mbit/s corresponds to a quality level of 5,000, and that a bit rate of 12 Mbit/s, where the quality of the compressed video is typically indistinguishable of the original, is equal to 10,000. The scaling factor '3,000' is due to the fact that bit_rate is a number in units of 400 bits/second (bit_rate with a value of 3,000 corresponds to an actual bit rate of 1.2 Mbit/s). Note that dwQuality must be constrained to be between 0 and 10,000.

8. Video Codec

8.1 AVI -> MPEG

• Strip the AVI headers
• Strip the DIB headers from each "movi" chunk
• Concatenate the remaining data. This will result in a fully MPEG compliant data stream, containing I-frames only.

8.2 MPEG -> AVI

8.3 Audio

9. References

[2] Microsoft Video for Windows Development Kit - Programmer's Guide

[3] Microsoft Windows Multimedia Programmers Guide and Microsoft Windows Multimedia Programmers reference.

[4] Video Compression/Decompression drivers technical note from Microsoft.

[5] MPEG-Audio Wave format - Microsoft Corporation.