GOP Time Code Error

In MPEG compression, a video sequence is divided into groups of image frames. A typical example is coded pictures of "I B B P B B P B B P B B P B B". This is called a GOP (group of pictures). A header is usually stored before the I-picture, and is called a GOP header, and it’s usually an 8-byte data segment. One of the important information stored in this header is a 25-bit integer, called a SMPTE time code (SMPTE: Society of Motion Picture and Television Engineers). This time code refers to the first picture of the GOP in display order.

This GOP header and its time code provide very convenient points for managing a coded MPEG movie. It allows our MPEG editor to randomly access a lengthy MPEG movie very efficiently. However, this also makes the editor highly dependent on the accuracy of time codes in the GOP header, especially dependent on the continuity of the time codes in a complete MPEG movie. Unfortunately, an MPEG movie may have errors in the time code, or the time code may be disrupted, or the time code may not correctly reflect the coded image sequence. When any of these happens to an MPEG file, we consider it a GOP time code error.

The GOP fixer tool will correct the GOP time code errors based on its calculation from the complete video sequence. It will only modify those errorous 25-bit integers.

Audio PTS Error

Audio Presentation Time Stamps are 33-bit integers inserted in an MPEG stream to allow an MPEG decoder to synchronize the decoded audio sound with the decoded video image display. Those PTS values should match the playback time length of the decoded audio sound. When they are in error, the MPEG movie cannot be played correctly, and the audio and the video will be out of sync.

The GOP fixer tool will correct the audio PTS errors based on its calculation from the complete audio data. It will only modify those errorous 33-bit integers.

DVD GOP Size Violation

The DVD standard has special restrictions on the size of a GOP for a movie to be compressed into MPEG. Specifically, it requires that no GOP should have more than 18 frames for NTSC TV system and 15 frames for PAL TV system, even though the MPEG standards have no such limitations.

To make an MPEG file compliant with the DVD restriction on GOP size, the GOP fixer tool will find the violator and break it into multiple GOP’s of small sizes. For other GOP’s, it will simply copy over.

It may be helpful for you to read other closely related topics:

1. How to save a video clip?
2. How to select video encoder parameters?
3. How to use MPEG MBS Scan?
4. How to use Batch Save Manager?
5. How to use Video Clip List?