This is version 2.18 of the AWS Elemental Server documentation. This is the latest version. For prior versions, see the *Previous Versions* section of [AWS Elemental Conductor File and AWS Elemental Server Documentation](https://docs.aws.amazon.com/elemental-server/).

# Encoding – Group of Pictures (GOP)
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## Description
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The Group of Pictures (GOP) settings define the basic pattern of the video stream itself in terms of how the encoding engine uses I-, P-, and B- frames. A few encoding settings control the GOP structure are as follows: 
+ **GOP Mode**: Select **Fixed**. 
+ **GOP Size**: Defines the interval between I-frames. 
+ **B-Frames**: Defines the maximum run of B-frames. (**Note: ** The encoding engine may make the decision to use a smaller run of B- frames in specific instances within the GOP if it determines this will produce higher quality content.) 
+ **Closed GOP Cadence**: Defines the number of GOPs across which P- or B-frames are allowed to predict for encoding purposes. 
+ **Scene Change Detect**: Enables an algorithm that determines when a scene change occurs and inserts an I-frame. 
+ **Min I-interval**: Specifies a minimum number of frames between GOP Cadence I-frames and Scene Change Detect I-frames. I-frames require more bits than P- or B-frames, so encoding two in quick succession can hurt quality, particularly with small buffer sizes. 
+ **GOP Reference B-Frame **(H.264 and H.265 only): Enables the use of reference B-frames for GOP structures that have B- frames greater than 1. 

## Recommendations
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+  **GOP Mode**: Always choose **Fixed**.The **Follow** mode is obsolete and not recommended. 
+ **GOP Size**: When using the MPEG-2 codec, the recommended **GOP Size** is up to 30 (15 is also very common). For H.264 or VC-1 codecs, our recommendation is to make this as large as possible while still meeting other encoding requirements. 

  For example, for adaptive bitrate delivery in which a segment size of 6 seconds is used for 29.97 fps outputs, the largest GOP size should be 180 frames. 
+ **B Frames and GOP Reference B-Frames**: When using H.264 or H.265, enable **GOP Reference B-Frame** to obtain the best quality and set B-frames to a value from 3 to 5 (3 is recommended). For other codecs, there is no quality benefit to setting the B-frames to more than 2. For high-motion content, use 0 or 1 to produce the best quality. 
+ **Closed GOP Cadence**: For segmented outputs (e.g. for adaptive bitrate content, such as HLS, Smooth, HDS, DASH, etc.), set the** Closed GOP Cadence** to 1. For non-segmented outputs, this can be set to 0 to allow for an open GOP. 
+  **Scene Change Detect**; Always enable this for the best quality. The only scenario where this might be disabled is if a STB or playback device is unable to accommodate an additional I-frame within the normal GOP pattern. (This is rare.) 

  Some service providers, encoding vendors, or manufacturers may state that **Scene Change Detect** should be disabled during encoding of content. This recommendation is typically based on the fact that these providers have systems that require a consistent GOP structure and some encoders "reset" the GOP structure when an I-frame is inserted for a scene change. 

  On AWS Elemental systems, **Scene Change Detect** can be enabled in almost all cases as the default behavior is to simply insert an additional I-frame on a scene change but not disrupt the normal GOP structure defined in the encoding settings. The intent behind this approach is to ensure that adaptive bitrate outputs are always GOP-aligned, and, as a side effect, allow for compatibility with these third-party systems even with **Scene Change Detect** enabled. 
+ **Min I-interval** is enforced by shifting the GOP cadence; so, for segmented outputs that require a fixed cadence of I-frames, it cannot be used and should be set to 0. For non-segmented outputs, a Min I-Interval of 5 is recommended. 

## Location of Fields
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| Location of Field on Web Interface | Location of Tag in XML | 
| --- | --- | 
| Streams – Video > Advanced > GOP Size |  stream\$1assembly/video\$1description/*codec*/gop\$1size where codec is one of the following: [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/elemental-server/latest/ug/vq-gop.html)  | 
| Streams – Video > Advanced > B Frames |  stream\$1assembly/video\$1description/*codec*/  gop\$1num\$1b\$1frames where *codec* is one of the following: [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/elemental-server/latest/ug/vq-gop.html)  | 
| Streams – Video > Advanced > Closed GOP Cadence |  stream\$1assembly/video\$1description/*codec*/  gop\$1closed\$1cadence where *codec* is one of the following: [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/elemental-server/latest/ug/vq-gop.html)  | 
| Streams – Video > Advanced > Scene Change Detect |  stream\$1assembly/video\$1description/*codec*/  transition\$1detection where *codec* is one of the following: [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/elemental-server/latest/ug/vq-gop.html)  | 
| Streams – Video > Advanced > Min I-interval |  stream\$1assembly/video\$1description/*codec*/ min\$1i\$1interval where *codec* is one of the following: [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/elemental-server/latest/ug/vq-gop.html)  | 
| Streams – Video > Advanced > GOP Reference B-Frame |  stream\$1assembly/video\$1description/*codec*/  gop\$1b\$1reference where *codec* is one of the following: [\[See the AWS documentation website for more details\]](http://docs.aws.amazon.com/elemental-server/latest/ug/vq-gop.html)  |