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MPEG Extension to AVI File Format
"Editable MPEG File Format"
Draft Version 1.1 of 5/13/94

1. Introduction

The objective of this document is to describe an extension to the AVI file format that supports a specific type of MPEG files [Ref 1], called Editable MPEG. Editable MPEG files offer significant compression of video data while preserving an image quality close to the original, and allowing frame accurate editing as well as multiple compression-decompression sequences. These are significant advantages over normal MPEG files, at the expense of a higher compressed data rate: typically 600 KByte/s (vs. 150 KByte/s).

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

The MPEG standard [Ref 1] specifies a technique for significant data compression of audio and video data. It has emerged as the leading compression technique for multimedia applications. While quite efficient for data storage, the MPEG format is not conducive to video editing since some frames (P and B types) require significant amounts of computation to obtain the decompressed image, and since successive decompression-recompression operations usually affect the image quality.

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

A MPEG AVI file is an AVI file. It has the mandatory RIFF structure of the AVI files, but uses a new extension to the DIB format . Following is a diagram describing the RIFF form of the MPEG AVI 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

The stream video format "strf" of a AVI stream is a MPEG DIB extension. The MPEG DIB extension (EXBMINFOHEADER) format contains the standard DIB header (BITMAPINFOHEADER) followed by a structure (MPEGINFOHEADER) defined to describe the characteristic of the MPEG video stream. The definitions are as follows,

    typedef struct tagEXBMINFOHEADER {
    	MPEGINFOHEADER  	mpegi;

    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,

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

bPixApectRatio Height/Width Example
0 forbidden
1 1.0000 VGA etc.
2 0.6735
3 0.7031 16:9 - 625 lines
4 0.7615
5 0.8055
6 0.8437 16:9 - 525 lines
7 0.8935
8 0.9375 CCIR 601 - 625 lines
9 0.9815
10 1.0255
11 1.0695
12 1.1250 CCIR 601 - 525 lines
13 1.1575
14 1.2015
15 reserved

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

Following the AVI stream header is a LIST "movi" chunk that contains the actual data of the stream. As in any RIFF chunk, a four-character code is used to identify the chunk. The MPEG AVI file uses "##dc" sub-chunks where ## is the stream id in the AVI file and 'dc' for "DIB compressed". The data chunk for the compressed DIB has the following form:

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.

Sequence header
MPEG Parameter #bits
sequence_header_code 32
horizontal_size_value 12
vertical_size_value 12
sample_aspect_ratio 4
frame_rate 4
bit_rate 18
marker_bit 1
vbv_buffer_size 10
constrained_parameter_flag 1
load_intra_quantizer_matrix 1
intra_quantizer_matrix 8*64 (*)
load_non_intra_quantizer_matrix 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.

Group Of Pictures header
MPEG Parameter #bits
group_start_code 32
time_code 25
closed_gop 1
broken_link 1

Picture header
MPEG Parameter #bits
picture_start_code 32
temporal_reference 10
picture_coding_type 3
vbv_delay 16

6. Constraints on MPEG Parameters

Parameter Constrained
sequence_header_code 000001B3
sample_aspect_ratio If the code is not 0001, then
the image does not have square
pixels. This may be create
artifacts in the display if not
handled properly
marker_bit 1 Reserved
constrained_parameter_flag 0
intra_quantizer_matrix Optional - Contains 64 bytes of
quantizer table, if
load_intra_quantizer_matrix == 1
load_non_intra_quantizer_matrix 0 All frames are Intra coded
group_start_code 000001B8
closed_gop 1 All GOPs are closed
broken_link 0
picture_start_code 00000100
temporal_reference 0 1 frame per GOP
picture_coding_type 001 All frames are Intra coded

7. Relationships between AVI Parameters and MPEG Parameters

AVI Parameter MPEG Parameter Relationship
dwMicroSecPerFrame frame_rate dwMicroSecPerFrame = 1e6 / frame_rate
dwMaxBytesPerSec bit_rate dwMaxBytesPerSec = 50 * bit_rate
dwSuggestedBufferSize vbv_buffer_size dwSuggestedBufferSize =
vbv_buffer_size * 2048
See Note 1
dwInitialFrames See Section 8.3
dwWidth horizontal_size_value dwWidth = horizontal_size_value
dwHeigth vertical_size_value dwHeigth = vertical_size_value
dwScale See below
dwRate frame_rate frame_rate = dwRate / dwScale
dwQuality 5,000 * [1 + log10(bit_rate/3,000)]
0 dwQuality 10,000
See Note 2
dwSampleSize 0

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

The MPEG-AVI file format was designed to make transformations between AVI-MPEG files and MPEG compliant files as simple as possible. In particular, the AVI to MPEG conversion only requires stripping the AVI-specific header and framing information.

8.1 AVI -> MPEG

To convert from MPEG-AVI to MPEG, all that is required is:
  • 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

To convert an MPEG file containing only I-frames, the sequence header is first extracted to generate the AVI and DIB headers. Then each I-frame, including its sequence header, GOP and Picture headers, is concatenated with a "movi" chunk header and concatenated to the data stream. A Sequence header must be included with the first frame, however, the application may select not to copy consecutive identical sequence headers. During the processing, the length of the file is computed, and the dwTotalFrames, dwStart & dwLength parameters are written in the main and stream AVI Headers.

8.3 Audio

It is to be noted that MPEG decompression system usually expected Video before Audio, and not the opposite as is recommended in the AVI file format [Ref 2 - "AVIStreamHeader"]. Consequently, appropriate buffering must be allocated when converting the file from AVI to MPEG and vice-versa.

9. References

[1] ISO 11172 document. Coded representation of picture, audio and multimedia/hypermedia information.

[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.