VQiPS: Storage

Guide to Defining Video Quality Requirements

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Storage
While video may be used for real-time (e.g., monitoring or tactical) applications, it may also be stored for possible future analysis. If video is not stored properly, it may be rendered unusable, through loss or degradation of the data. For example, proper storage is critical in evidentiary and forensic video applications. Video is often initially stored at a very low bit rate, to save space on the storage media. If it is not simultaneously stored at a higher bit rate, data will be irretrievably lost. For example, some systems may provide a low bit rate stream for wireless monitoring while simultaneously storing a higher quality (higher bit rate) version locally.

You can store video for future analysis. If you are accustomed to the storage of word-processing documents or digital photos, you might surprised at how quickly video consumes storage space, which today ranges from single video cassette recorders or a single hard drive, to systems with multiple cassette recorders or huge arrays. Depending on the frame rate, resolution, and compression of the video, you may need to store terabytes of data. One clear implication of this: the rules for retention, both within your organization and the records-keeping laws in your jurisdiction, will affect the amount of storage needed.

Proper storage is critical in evidentiary and forensic video applications. For example, if video is stored only at a very low bit rate, to save space on the storage media, data will be irretrievably lost.

For the purposes of decreasing the bit rate, storage is also often preceded by some form of processing. This may include altering the file format to fit differing media, such as coding the video in Motion Picture Expert Group video and audio compression specification part 2 (MPEG-2) so that it can be stored on a DVD and played back with a Digital Video Disc (DVD) player. A series of alterations or physical custody changes made to a video file is called the “storage chain.” Almost every change in file format results in a loss of data, so the storage chain should be monitored and documented very closely.

Equipment: NVR or DVR
A network video recorder (NVR) or a digital video recorder (DVR) records digitally onto a hard drive, either locally or at a centralized location. An NVR or a DVR can perform both display and storage functions. Features:

• Hard drive capacity - this is directly linked to the coding/decoding (CODEC) used (see Processing). The output rate of the CODEC, combined with the length of time the video needs to be kept directly determines hard drive storage size.
• Delay - determines how quickly the video can be retrieved.
• Custody and access.

Equipment: VCR
A video cassette recorder (VCR) records to an analog tape. A VCR can also double as decoder (player) equipment. Features:

• Number of tapes needed.
• Physical degrading of storage media over time (e.g., tapes stretching, breaking, or being exposed to magnetic fields).
• Physical custody of the media.

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