The past few years have seen the rise of video surveillance and its widespread adoption throughout the world. This adoption has been driven by a transition from analog to digital systems. This transition is marked by decreasing camera and recording equipment costs, advances in digital sensor and compression technology, and improved IP network infrastructure that reduce the cost of transporting video data over large distances.
While this "cost reduction" approach has been successful in driving rapid growth in the industry, digital surveillance systems have been little more than replacements for their analog counterparts. The true potential of the leading-edge surveillance technology has not yet been realized. This article discusses the technical requirements of future video surveillance systems and the hardware and software changes that are taking place to meet these needs.
Video Surveillance – Not your Father's Video System
Video codecs in a surveillance system have requirements that are very different from those seen in other video applications. For example, latency is relatively unimportant in a broadcast video environment, but it is extremely important in surveillance systems. Surveillance systems often include monitoring personnel who must respond to events in real time. For example, monitoring personnel might track individuals with a Pan Tilt Zoom (PTZ) camera, or they might use an audio link to direct the actions of someone at the scene.
As another example, broadcasters have very little ability to change encoding schemes because any changes they make must be compatible with the receivers already in the field. In contrast, surveillance systems tend to be closed systems. As a result, the operator is free to select standards that best suit his or her needs including frame rate, resolution, and the codecs. Operators may even adjust resolution and frame rate dynamically. This may be done in response to changes in an observed scene or the capabilities of the consuming device. For example, the system might encode several versions of the same video stream. On stream might go to a high-definition monitor in an observation room, while another goes to a PDA carried by on-site security personnel.
Of codecs available to surveillance operators, most are leveraged from other industries, and do not satisfy the requirements of surveillance applications. Table 1 summarizes the high-level features of various common codecs.
Table 1. High Level codec Comparison.
None of the codecs in Table 1 satisfy all the desired characteristics of surveillance systems: low latency, high compression efficiency, resolution and frame rate flexibility, low complexity, and low cost. H.264 Baseline Profile probably offers the best compromise, but lacks the inherent scalability needed in surveillance applications.
Scalable Video Coding
The Scalable Video codec (SVC) is an extension of the current H.264 standard. SVC was developed with a view to using a single encoded stream to satisfy diverse requirements in terms of bit rate, quality and resolution. SVC supports a high degree of scalability. It scales spatially, allowing for varying display resolutions. It scales temporally, allowing for varying frame rates. And it scales in quality, allowing for varying resulting image quality. For example, an H.264 SVC video stream can be decoded by two different devices with different frame rates and resolutions. In conventional video encoding, if the video stream were to be viewed at a reduced resolution on a portable device, the entire stream would have to be decoded and resized. With SVC, only the portion of the stream yielding the desired resolution and frame rate is decoded.
An SVC decoder's flexibility in how it deals with an SVC bitstream results in many benefits to the user. These benefits include ease of adaptation for different displays; resource-conserving video transmission, storage and display; higher transmission robustness; and ease of heterogeneous network support (for example, simultaneously supporting a number of different transmission networks). An additional benefit of SVC is that the compressed stream can be parsed while stored on a disk. The portions of the files that are used to reconstruct high frame rate or high quality images can be progressively removed over time. This is not possible with conventional codecs where the video data is either there or it isn't, and one has to select a date upon which the original resolution file will be totally deleted. With an SVC system, the video can be kept for longer periods as storage requirements are gradually reduced.