Tipping the Megapixel Equation

There has been significant debate in recent years about the surveillance market finally reaching the tipping point into IP technology. In reality, it has only been within the last 12 months that the emergence of megapixel camera technology has made this view possible. This feature uncovers the implications of the technology in terms of storage requirements, and outlines an overall solution design of the (near) future.

A fundamental issue of specifying megapixel cameras — and perhaps IP systems in general — is that many do not fully appreciate the implications of the technology from a storage and bandwidth perspective. Camera manufacturers tend to focus solely on the “sensor” in isolation, rather than the effects of each camera's data output on the overall network and storage. The underlying codec implementation within the camera (fixed or PTZ, static or motion-filled scenes) has significant effects on the overall bandwidth and storage required — much more so than using analog cameras in conjunction with a DVR.

A crude but common approach for many to specify the recording capacity on DVRs has been based on storage and channel count (such as 16 channels on a 2-TB hard-disk drive). An IP system, however, increases the potential margin of error significantly. This makes bandwidth and storage considerations far more critical. For example, the storage needs for 31 days of 24/7 recording of 64 megapixel cameras at 4Mbps would be roughly 96 TBs; dropping the bit rate to 3Mbps brings the overall storage required from 96 TBs down to 72TBs. While many would see the bit rate difference as negligible, the specification of output per camera can make a huge difference.

“The adoption of megapixel technology could be confusing to consultants, integrators and end users alike,” said Craig Howie, Commercial Director for Visimetrics. “First, there are far more camera manufacturers than there used to be, so selecting the right camera(s) for your application has become much more complicated purely by choice. Secondly, you need to consider the individual bandwidth and storage needs for each type of camera relative to its scene. However, every manufacturer implements its own proprietary compression technology; therefore, cameras that appear identical on data sheets can perform completely differently in practice. In many cases, there seems to be a fair bit of ‘trial and error' or field testing to actually determine how much storage capacity you really need under various conditions. When your system includes a lot of cameras with a long retention period, then the technical risk of specifying the storage becomes pretty large”.

CONSTANTLY MOVING
Unfortunately, the influences on storage do not stop there. Storage requirements during the day (under natural light) are vastly different from those of evening or night images for the same camera. Darkness introduces more noise into the scene and hence the images, so digitalizing these with motion-based compression algorithms typically produce much larger files. “Scenes with lots of activity typically use far more storage than calm scenes. It is quite a complex calculation,” Howie said.

The advice is to be very selective. Focus on a small subset of products to allow youself to fully understand particular cameras and how they respond under various conditions. Be very aware that the implementation of compression algorithms, such as MPEG-4 and H.264, is not the same from camera A to camera B. While the underlying tool box of codec may be the same, the ways in which it is implemented by different manufacturers can be wildly different. This can result in significant variances in storage requirements.