H.264, the newest video compression technology, presents a huge step forward for many industries. Without compromising image quality, an H.264 encoder can reduce the size of a digital video file by more than 80 percent compared with Motion JPEG compression and as much as 50 percent more than with the MPEG-4 Part 2 standard. With far less network bandwidth and storage space required for a video file, users can save money or achieve a much higher video quality for a given bit rate.
With such efficient performance, industry experts predict H.264 (also known as MPEG-4 Part 10/AVC for Advanced Video Coding) will become the compression standard of choice in coming years. The technology has already been introduced in the latest mobile phones and digital video players, and is quickly gaining acceptance by end-users. Service providers, such as online video storage and telecommunications companies, are beginning to adopt H.264, and from all indications, the video surveillance industry will be no exception.
H.264 is the joint effort of standards-setting organizations in both the telecommunications and IT fields, which gives the technology the necessary pedigree to become the defacto open, licensed standard for video compression.
The Essence of Video Compression: Encoding and Decoding
Video compression is all about reducing and removing redundant video data so that a digital video file can be effectively transmitted and stored. An algorithm applied to the source video compresses the information. An inverse algorithm applied to the compressed information produces a video that shows almost the same content as the original source video. The algorithm pair is called a video codec (encoder/decoder).
Results from encoders that use the same compression standard may vary because the designer of an encoder can choose to implement different sets of tools defined by a standard. As long as the output of an encoder conforms to a standard’s format and decoder, it is possible to create different implementations. So the performance of a standard cannot be properly compared with other standards, or even other implementations of the same standard, without first defining how it is being implemented.
A decoder, on the other hand, is unlike an encoder because it must implement all the required parts of a standard in order to decode a compliant bit stream. This is because a standard specifies exactly how a decompression algorithm should restore every bit of a compressed video.
The graphic on the right provides a bit rate comparison, given the same level of image quality, among four video standards: Motion JPEG, MPEG-4 Part 2 (no motion compensation), MPEG-4 Part 2 (with motion compensation) and H.264 (baseline profile).
Understanding Video Frame Options
Depending on the H.264 profile — a set of algorithmic features that a standard provides for specific applications — different types of frames such as I-frames, P-frames and B-frames may be used by an encoder.
An I-frame, or intra-frame, is a self-contained frame that can be independently decoded without any reference to other images. I-frames can be used to implement fast-forward, rewind and other random access functions.
A P-frame, which stands for predictive inter-frame, makes references to parts of earlier I and/or P frame(s) to code the frame. P-frames usually require fewer bits than I-frames, but are prone to transmission errors because of their complex dependency on earlier P and I reference frames.
A B-frame, or bi-predictive inter-frame, is a frame that makes references to both an earlier reference frame and a future frame.
Network cameras and video encoders will most likely use the H.264 profile called the baseline profile. The baseline profile uses only I- and P-frames, and, as a result, achieves low latency (the time it takes to compress, send, decompress and display a file), which is critical in surveillance and is particularly important in enabling real-time pan/tilt/zoom (PTZ) control.