MPG File Format
What is an MPG file format?
The file with a .mpg extension belongs to the group of file extensions for MPEG-1 or MPEG-2 audio and video compression. MPEG-1 Part 2 video is not easily available, and this extension (MPG file format) typically points to a MPEG program stream which is defined in MPEG-1 and MPEG-2, or an MPEG transport stream which is defined in MPEG-2. Other extensions such as .m2ts also exist specifying the accurate container, in this case, MPEG-2 TS, but this has little pertinence to MPEG-1 media. The .mp3 is the most common extension for files containing MP3 audio. An MP3 file is a typical stream of raw audio; the traditional way to tag MP3 files is by writing stream data to “garbage” segments of each frame, which save the media information but are discarded by the mpg file player. This is a similar technique used to tag the AAC files, but less supported nowadays.
MPEG Compression
The name MPEG stands for Moving Pictures Experts Group. MPEG is a tool for video compression, which involves the compression of images and sounds, as well as synchronization of the two. There currently are several MPEG standards.
- MPEG-1 is proposed for intermediate data rates, on the order of 1.5 Mbit/sec.
- MPEG-2 is proposed for high data rates of at least 10 Mbit/sec.
- MPEG-3 was proposed for HDTV compression but was found to be redundant and was merged with MPEG-2.
- MPEG-4 is proposed for very low data rates of less than 64 Kbit/sec.
Program stream of MPG file format
The program stream is a container for multiplexing digital audio, video, and more. The Program Stream format is specified in the 1st part of MPEG-1 (ISO/IEC 11172-1) and 1st part of MPEG-2, Systems (ISO/IEC standard 13818-1/ITU-T H.222.0). The MPEG-2 Program Stream is analog-based and similar to ISO/IEC 11172 Systems layer and forward compatible.
Coding details
Here are the coding details of partial MPEG-2 Program Stream pack header format:
| Name | Number of bits | Description |
|---|---|---|
| sync bytes | 32 | 0x000001BA |
| marker bits | 2 | 01b for MPEG-2 version. The marker bits for the MPEG-1 version are 4 bits with value 0010b. |
| System clock [32..30] | 3 | System Clock Reference (SCR) bits 32 to 30 |
| marker bit | 1 | 1 Bit always set. |
| System clock [29..15] | 15 | System clock bits 29 to 15 |
| marker bit | 1 | 1 Bit always set. |
| System clock [14..0] | 15 | System clock bits 14 to 0 |
| marker bit | 1 | 1 Bit always set. |
| SCR extension | 9 | |
| marker bit | 1 | 1 Bit always set. |
| bit rate | 22 | In units of 50 bytes per second. |
| marker bits | 2 | 11 Bits always set. |
| reserved | 5 | reserved for future use |
| stuffing length | 3 | |
| stuffing bytes | 8*stuffing length | |
| system header (optional) | 0 or more | if system header start code follows: 0x000001BB |
The following table shows the partial system header format:
| Name | Number of bytes | Description |
|---|---|---|
| sync bytes | 4 | 0x000001BB |
| header length | 2 | |
| rate bound and marker bits | 3 | |
| audio bound and flags | 1 | |
| flags, marker bit, and video bound | 1 | |
| Packet rate restriction and reserved byte | 1 |