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Understanding Rate Control
This document provides an introduction, to all level of readers, to the process of controlling the bitrate produced by a digital video encoder. It describes the rate control employed by IndigoVision 8000 2.5.0, and earlier, transmitter firmware and provides a set of recommended minimum bitrates to select based on motion and frame rate.
Contents
1 Background
In the default configuration the IndigoVision 8000 transmitters are designed to encode and transmit every frame of video captured by a camera. As such a PAL camera will transmit 25fps and an NTSC camera 30fps regardless of the bitrate or motion. This means that during periods of activity, typically times of interest in CCTV, valuable information is not lost. This is in contrast to other encoders, which will often dynamically drop video frames during motion, as part of their bitrate control, in order to meet some specific target bitrate over very short periods of time. This dropping of frames can lead to jerky video and potentially large sections of critical lost data.
IndigoVision 8000 transmitters do not drop frames, but instead only vary the quality of the video, reducing it as necessary to compensate for higher data rates produced by motion. However, there is a fundamental lowest bitrate that this type of encoder can achieve, and this is dependent on a number of factors including resolution, detail in the scene and motion. This can lead to transmitter configurations whereby a requested target bitrate is simply not achievable, especially during complicated operations, such as PTZ.
IndigoVision offers two methods for reducing the bitrate: a selection of resolutions (SIF, 2SIF, and 4SIF) – the lower the resolution, the lower the bitrate; and a frame rate divisor. This latter method drops frames in a linear and controlled fashion leading to less jerky video than if dropped a nonlinear manner.
This constant frame rate encoding requires careful configuration by the VB8000 administrator. The following sections provide some guidelines to configuring the bitrate with regard to the application.
2 Recommendations
The following tables provide a list of recommended minimum target bitrates for four classes of motion: none, low, smooth and harsh. These motion classes are described below in Table 1.
Type of motion |
Description |
None |
Still scenes on fixed camera, with potentially small background movement. |
Low |
Low amounts of motion, such as people or cars moving in the foreground. |
Smooth |
Slow and smooth PTZ operation. |
Harsh |
Fast and jerky PTZ operation. |
Table 1: Motion type classes
When configuring the video on a VB8000 transmitter you should select a bitrate that is greater than the recommended minimum bitrate, outlined in Table 2 based on the worst class of motion expected in your application.
Type of motion |
||||
Resolution |
None |
Low |
Smooth |
Harsh |
SIF |
32 |
64 |
128 |
384 |
2 SIF |
64 |
128 |
256 |
1024 |
4 SIF |
128 |
256 |
512 |
4096 |
Table 2: Minimum recommended bitrates (kbps) at 25fps (PAL)
If the minimum recommended bitrate for your application is greater than the available bandwidth then you should consider reducing either the resolution or the frame rate, as shown in Table 3.
Type of motion |
||||
Resolution |
None |
Low |
Smooth |
Harsh |
SIF |
32 |
64 |
128 |
256 |
2 SIF |
48 |
128 |
256 |
512 |
4 SIF |
64 |
256 |
512 |
2048 |
Table 3: Minimum recommended bitrates (kbps) at 12.5fps (PAL)
These recommended minimum bitrates provided are a guideline to selecting an appropriate minimum target bitrate for a particular application. However, there are a number of factors that should be considered, as described below:
Important notes:
- These are not hard-and-fast figures and not a guarantee of overshoot not occurring during periods of sustained motion.
- For optimum performance you should tune the target bitrate to your specific application and network configuration.
- Recommendations are based on a medium contrast scene. High contrast, detailed or complicated scenes will require higher bandwidth.
- Reducing frame rate may not lead to a reduced bitrate. This is typically when you have slow moving or panning video. Reducing frame rate works best when there is either very little motion or excessive motion.
- Selecting an I-frame interval greater than 4000ms for 25 or 30fps video may be less efficient on average than when with an I-frame interval of 4000ms.
- For 12.5 or 15fps video, the most efficient I-frame interval is 8000ms, though increased I-frame interval may impact on the accuracy of thumbnails and navigation in Control Center.
- The figures are for a single stream. Other users, camera tours and network packet loss will inject I-frames above the rate dictated by the I-frame interval and can cause a larger than expected bitrate.
- It is not recommended to use a frame rate divisor with 4SIF when transmitting from an interlaced camera to a receiver connected to an interlaced monitor. In order to remove interlace judder only single fields are presented i.e. 4SIF is encoded and transmitted, but only 2SIF is displayed.
- The minimum bitrate selectable through the web administration is 32kbps.
- The maximum bitrate selectable through the web administration is 4096kbps.
- For more information on the effect of frame rate on bitrate, see “Understanding Frame Rate”, IC-COD-REP008, 23/09/2004.
