Balancing quality and bandwidth with MPEG-4
MPEG-4 and H.264 give the system designer the flexibility to balance video quality with the storage and bandwidth consumption. Most commercial network cameras and encoders provide a range of flexible configuration settings to tune the compression to different levels of video quality and data rates. For products using MPEG-4/H.264, this flexibility makes it very difficult to anticipate storage requirements or to compare the data rate performance of one product over another. The end result greatly depends on the degree of motion in the video scene and how the product's compression has been tuned.
Of course every manufacturer will establish default settings for the compression. But a few key values can usually be configured during system installation to customize performance.
• Compression level – As with most compression methods, the level of data rate reduction (sometimes called quantization) can be configured. Usually this setting is determined as a value (0 to 100) or an enumerated setting (high, medium, low).
• I-frame settings – Reducing the frequency of I-frames saves bandwidth but can reduce the quality of the image, particularly when motion is high. Conversely, increasing the number of I-frames will increase the data rate. Another side-effect is that long periods between I-frames can increase the latencies and delays in switching a video monitor from one camera to another. When a PC switches to a new MPEG-4 video source, it must receive an I-frame before it can begin to decode the video stream and render video on the monitor.
• Bit rate control – Adjusting the maximum bit rate and setting it to “variable” or “constant” is a good way of controlling the bandwidth used by the MPEG-4 video stream. Variable Bit Rate (VBR) settings will adjust the bit rate according to the image complexity. When in VBR mode the system will use more bandwidth when the activity in the image increases and less bandwidth when the monitored area is quiet.
Leaving the maximum bit rate as unlimited will provide consistently good image quality, but at the expense of increased bandwidth use whenever there is more activity in the image. Limiting the bit rate to a defined value will prevent excessive bandwidth use but images will be lost when the limit is exceeded.
When using Constant Bit Rate (CBR) you can set a fixed target bit rate which will ensure that the level of bandwidth consumed is predictable and will not change. This mode is desirable when video is accessed over a remote network connection with limited bandwidth. Of course when in CBR mode, the codec will sacrifice quality (frames or image quality) to maintain its target rate.
Ultimately, the advanced compression algorithms in MPEG-4 and H.263 require more processing power and therefore add cost to the DVR or network camera product. The storage savings will usually justify the higher product cost. Dealers and integrators must also be aware of the configuration parameters associated with temporal codecs that can optimize the video quality with the available bandwidth and storage.