DVR Recording Information and Video Clips

"I gave the police the video, but they couldn't identify the burglar's face..."

Welcome to Spy Centre Securities information and demonstration page for DVR/NVR and digital video recording in general. There a several things to consider when putting in a new recording system. However, one of the typically least considered factors is actually the most important: recording usability. Many people have put in system that work just fine until a copy of the video needs to be given over to someone else to review and the recorded video is too dark, not clear, missing the key partial second of footage, or otherwise is just plain too small. The following is an analysis and examples of what characteristics affect your overall recording usability.

Recording Speed

Video recording capacity and its after the fact usability is governed by one main factor among all others: recording speed. Recording speed in short determines how much data is gathered and how quickly it is gathered. The slower a recorder records, the longer a time period can be stored in the same amount of storage space (such as an 80G drive). When you capture less data, there is less data to store. So by slowing down your record rate, you increase a given unit's ability to archive more data, and thus get a longer length of total time from the same amount of hard drive space. However, taken to extremes, slow record rates can be so slow that the desired information is missed in between recording intervals (such as very fast and slight hand movements or cars that speed through entrances). Spy Centre Security recommends security level video recording to be 5 frames per second or better for most situations.

	1 frame per second video is a very slow recording rate usually only used when a system is trying to get the maximum amount of archive possible, or when periodic snapshots is all that is necessary.
	5 frames per second video is standard security recording. It is a good mix of speed and storage capacity.
	15 frames per second video is considered high speed video and is the maximum usually used for most generic recording. It offers extreme fluidity of movement, but does tend to take up lots of storage space.
	30 frames per second video is considered real-time recording. It is usually only used in situations where slight hand movements are being recorded, or when trying to capture license plates. Since cars typically are moving pretty fast, it is necessary to try and capture as much data as possible. Real-time recording requires large amounts of storage capacity.

Image Size

The next main factor that affects the total storage ability of a DVR is the size or resolution at which the video is recorded. The larger the image that is recorded, the more each individual clip takes up on the hard drive. However, small pictures may not contain enough image resolution to clearly display fine details. For example, facial recognition or license plate recognition might be difficult on a video clip recorded at 320 x 240 resolution. Although many DVRs have the ability to digitally zoom an image after the fact from the video playback archive, this effect tends to distort the picture as it gets bigger and bigger, rendering most fine detail blurry and undecipherable. It is always better to record your information at a higher resolution initially, as opposed to trying to digitally zoom in to it after it is already recorded. The last thing to note is that a 640 x 480 resolution picture is actually 4 times the size of a 320 x 240 picture because both the length and the width of the picture are doubled. All things being equal, this means that a 320 x 240 resolution archive will be about 4 times the the archive length (in days) of the same footage that was recorded at 640 x 480 resolution. Generally speaking, Spy Centre Security recommends 640 x 480 resolution recording for most security situations.

	320 x 240 recording resolution is typically used when maximum record length is desired. 320 x 240 mode offers the smallest usable video size which yields the largest usable recording archive, but tends to suffer when it comes to fine detail. This resolution is best used for general coverage shots, wide shots where fine detail isn't necessary, or when trying to truly maximize storage space.
archive length x4 vs. standard archive
	640 x 480 recording resolution is considered standard security video recording. It is a good mix of recording resolution and archive storage capacity. This is possibly the most common DVR/NVR recording size used in the industry today.
standard archive length for security video
	720 x 480 is a newer recording resolution this is slightly "wide screened". It is typically used when your end export media is DVD format or if your monitor is a wide screen monitor. This is a fairly typical size for DVD media. More and more DVRs and NVRs are offering this particular recording size and it is becoming more of an industry standard for recommended recording resolution.
archive length x 0.9 vs. standard archive
	1600 x 1200 recording resolution is 2 mega-pixel recording. This recording resolution is typically used in NVR recordings or very, very high resolution DVR recordings. Since the picture generated at this resolution is so large, most DVRs or NVRs run at comparatively slower recording rates than their 640 x 480 counterparts. High speed recording at this resolution requires extremely large amounts of storage space.Open the picture and then click on it again to see it's full image size
archive length x 0.12 vs. standard archive

Lens field of view

The lens size quite simply determines your field of view. Field of view directly affects the usability of playback video. Most of the time, security camera coverage shots consist of three "typical types" of shots: facial recognition, license plate recognition, and wide general coverage shots. Facial recognition and license plate shots need to have the desired object fill at least 50% of the shot or more to capture the detail necessary to identify the object properly. That generally means that the field of view needs to be about 4 to 8 feet wide to yield usable detail. Wide shots are not really concerned about fine detail so much as viewing width. Typically the widest shot possible is the goal. Taken to extremes, wide-angle shots have next to no recognizable detail and are mostly just monitoring general movement. Trying to pull a face or a license plate out of a wide angle shot at a long distance is all but impossible. After-the-fact digital zooms will distort the picture and destroy any detail their may be. The first step in determining the appropriate lens size is to determining the viewing distance (or focal length) to the desired object. Most standard fixed lens bullets and domes have an approximately 4.0 mm lens by default.

Used for typical general coverage This is a fairly typical distance for wide angle shots of showrooms, small retail sales floors and office coverage. The standard lens on most bullets and domes (3.6 mm - 4.3 mm) is usually sufficient for general coverage shots, providing a usable width as well as usable detail in the shot. License or facial recognition at this distance requires 12+ mm lenses minimum to be appropriate.

Used for typical parking lot coverage This is a fairly typical distance for wide angle shots in parking lots or larger office and retail sales floors. At this distance 6 to 12 mm lenses are required for the general wide angle shots to provide a usable width as well as usable detail in the shot. License or facial recognition at this distance requires 20+ mm lenses minimum to be appropriate.

Used for license plate recognition This is a fairly typical distance for long range license plate recognition for parking lot entrances. Most situations at this distance require 90+mm lenses to see license plates clearly. Wide angle shots of parking lots or warehouses at such a long range distance typically require 25+ mm lenses.

Lighting and camera scene

Scene lighting is the last major component in video usability. In short, if the lighting is too low for a given camera, it cannot "see" the scene properly. This effect results in an all black image that is wasted and unusable. Many different types of cameras exist and each is geared towards one main type of ambient lighting situation. Extreme cases require the ability to see an absolute darkness and require special types of cameras. For example, a small storage closet with a safe inside, or a server room with no overhead lights would require an infrared camera to record much of anything. In general, using a camera with the lowest possible light rating is the most desirable.

There is one main exception to this rule of thumb though: license plate recognition in low light or darkness. License plates are designed to be reflective so they can be seen at night when car headlights make them "light up". Infrared cameras work because they illuminate the scene with a type of light the human eye generally cannot see, however the image sensor of the camera can. Unless the camera in question is a very special type of infrared, this results in the camera projecting light out on to the plate, the light reflecting back at the camera, and thus causing the license plate to appear as a big white blob.