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TMPGEnc Settings (GOP Structure)

The Setting dialog box allows you to alter the encoder parameters. It includes the following tabs:



GOP structure

GOP is an MPEG term meaning Group Of Pictures. It is a collection of consecutive frames of video. Usually between 0.5 and 1 second of video will be held in 1 GOP. Each picture within the GOP can be 1 of 3 types:

Type Meaning
I (Intra) A complete picture that can be decoded without the need to decode any other pictures first. It is similar to a JPEG still image.
P (Predicted) P frames are predicted from the previous "reference" (I or P) frame. If the encoder can find correlation between the previous reference and the P frame, macroblocks in the P frame will be derived from the reference with a motion vector and DCT difference information. In the case where a good match cannot be found, the P frame will contain some intra coded macroblocks.
B (Bidirectional) These are predicted from the previous and future reference frames. The encoder can use macroblock information each of these frames to produce the best match for each macroblock in the B frame. If no good matches can be found the macroblock will be intra coded.

A GOP always starts with an I picture.

Number of I, P and B picture(s) in a GOP

These 3 options control how many of each frame type are normally inserted into each GOP. Normal values would be:

Picture Type Normal Range
I (Intra) 1. To increase the I frame rate, it would be better to reduce the GOP size.
P (Predicted) 2 to 10
B (Bidirectional) 2 to 4. For lower motion video sequences this number can be higher.

Output interval of sequence header

Sets how often the video sequence header is sent. Normally you would send this once per GOP to enable decoding to resume at any point. If you are using low bitrates you could increase this value so that the sequence header is only sent say once every 5 GOPs.

MAX number of frames in a GOP

This option allows you to limit the maximum number of frames in a GOP. If the encoder reaches this number of frames a new GOP is started.

Output bitstream for edit (closed GOP)

If you wish to edit the encoded video stream select this option. It removed inter-GOP dependency. Some DVD players are reported to introduce artifacts when fast forwarding video without closed GOPs.

Detect scene change

This option allows the encoder to look for sudden changes in the video, adding a new I-frame when they are detected. This option should usually be selected, however if you have high motion video sequences it can introduce more I-frames than necessary, lowering the overall picture quality.

Force picture setting

This option allows manual setting of many of the encoder options on a per- frame basis. It shows each video frame in a scrolling window. Right click on a frame to change it settings for that frame. As you can imagine, this is time consuming work. If you have short (a few seconds) video sequence and have problems encoding it you could try this. Defiantly not recommended for beginners.

I picture only

Forces the encoder to produce a sequence of I frames only. Since the sequence contains only I frames each frame can be easily edited after encoding since there is no dependency on other frames. Also motion artifacts will be completely eliminated. Since the video sequence will not make use of compression provided by P and B frames a massive bitrate (perhaps 20Mbps) will be required to make the image look acceptable.

Don't use this option unless you know what you are doing.

IP picture

Forces the encoder to produce a sequence containing only I and P frames (no B frames). Since the video sequence will not make use of compression provided by B frames a large bitrate (perhaps 5Mbps) will be required to make the image look acceptable.

Don't use this option unless you know what you are doing.

Standard

The default setting, using I, P and B frames. This obtains maximum compression of the video.


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