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 – Rate Control Modes
<a name="vq-rate-control-modes"></a>

For **Rate Control Mode**, you will generally get the best video quality per bit by using the Quality Variable Bitrate (**QVBR**) rate control mode. For more information, see [Using QVBR Rate Control Mode](using-qvbr.md).

## Description
<a name="description-vq-rc-modes"></a>

Rate control modes define how the encoding engine uses the buffer model and how bits flow into the engine.
+ The **Rate Control Mode** field has the following options. 
  + **Constant Bitrate** (CBR) mode: Uses the buffer model described above where Max Bitrate equals the defined (Average) Bitrate. In this mode, the system adds fill bits to the stream to ensure a constant bitrate video encoding. 
  + **Variable Bitrate (VBR) **mode: Uses the buffer model described above where Max Bitrate is greater than or equal to the (Average) Bitrate. In this mode, the system adds no fill bits for VBR video encoding. 
  + **Average Bitrate (ABR) **mode: Does not use the buffer model but allows the bitrate to vary as needed while maintaining an overall average bitrate for the entire video encoding. 
  + **Constant Quantizer (CQ) **mode: Disables rate control completely. The system encodes frames using a constant **Quantization Parameter** (**QP**), which you can set using the **Start QP** setting. 
  + **Statmux**: See [Encoding – Statmux Rate Control](vq-statmux.md). 
+  **Start / Min / Max QP**: This field specifies the level of compression to start at when performing rate control and the minimum and maximum levels of compression. 

## Recommendations
<a name="vq-rate-control-modes-recommendations"></a>
+ ** CBR **mode provides the lowest quality at a given bitrate but has the benefit of providing minimal bitrate variation. It is supported by the widest variety of playback devices. 
+ **VBR **mode provides better quality than CBR but has more bitrate variation (though is still limited). VBR is supported by many modern devices but is not as widely supported as CBR, particularly with older playback devices. 
+ **ABR **mode provides the best quality but should only be used if bitrate variation is not critical. 
+ **CQ Mode** is a legacy control mode which does not typically provide any notable benefits for video quality tuning. Use of CQ mode is not recommended.
+ **QP Mode ** has an encoder which adjusts QP values optimally: leave all fields blank. Use of QP is not recommended.

## Location of Fields
<a name="vq-rate-control-modes-api"></a>


| Location of Field on Web Interface | Location of Tag in XML | 
| --- | --- | 
| Streams – Video > Advanced > Rate Control Mode |  stream\$1assembly/video\$1description/*codec*/rate\$1control\$1mode 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-rate-control-modes.html)  | 
| Streams – Video > Advanced > Start QP |  stream\$1assembly/video\$1description/*codec*/qp 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-rate-control-modes.html)  | 
| Streams – Video > Advanced > Min QP |  stream\$1assembly/video\$1description/*codec*/min\$1qp 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-rate-control-modes.html)  | 
| Streams – Video > Advanced > Max QP |  stream\$1assembly/video\$1description/*codec*/max\$1qp 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-rate-control-modes.html)  |