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DVR Recording Knowledge Base


Hard Drive Space Calculation

Many days might pass before someone realizes they need to view a recording. With a hard drive that is too small, important data might already be gone by the time a person realizes they need to go back and view it.

Some important questions to ask when choosing a hard drive for a CCTV system are “How much hard drive can I afford?” and “How much hard drive space is the DVR capable of having?”, but the most important is “How far back do I need to be able to access recordings?”

You can use the calculator below to see how much recording you can expect to get from different hard drive capacities and figure out how much hard drive space you'll need for a desired amount of time.

Most DVRs can be set to only record when there is motion so, for instance, a store that is open 12 hours a day will record video when the store is busy and full of people during the day, but at night when no one is moving in the store, the video is not being recorded to the drive (unless someone or something is there moving around.) In this situation, the recording capacity is effectively doubled. If you estimate what percentage of the time there will be moving objects in the view of the cameras, you’ll be able to estimate how many days, weeks or even months of video the DVR will be able to store. Adjustments in resolution and frame rate can also extend the recording time of a DVR.



Hard Drive Calculator
(Excel XLS format)

Recording Resolution
Standard (Analog) Resolution
Most modern DVRs have the ability to change recording and display resolutions. There are some fairly standard industry terms for these resolutions, although the terminology can change from brand to brand, and the resolution may be slightly higher or lower than the industry standard. The pixels are not square, but instead are slightly wider horizontally than their vertical height. Resolutions are given as “horizontal pixels by vertical pixels”, the vertical pixels are also commonly referred to as “TV lines” or just TVL.

CIF – 352x288 This is the basic resolution for CCTV recording equipment as most analog recording equipment resolutions are based on multiples or fractions of CIF. DVRs that must record many channels at once often record at this resolution, and many apps for smart phones are showing remote video at CIF resolution.

QCIF – 176x144 This stands for “quarter CIF”, and is exactly half as tall and half as wide as CIF resolution. Generally only mobile devices or viewing through slow internet connections will utilize this resolution for streaming, and most DVRs will not allow you to record at this resolution.

2CIF, DCIF or hD1 –720x240 Know as double CIF or half D1, this resolution uses pixels twice as tall as normal CIF pixels to display or record an image that is the shape of standard monitors while using approximately half the data space (or bandwidth, in the case of remote viewing) of D1 resolution.

D1 (4CIF) – 720x480 – Four times the resolution of CIF, D1 is the highest recording resolution of most analog DVRs. Although this resolution takes up the most hard drive space of any standard analog resolution, viewing and recording at D1 will give each camera the maximum level of detail.

Remember that the space a recording file requires is based on its recording resolution and not the camera’s output resolution. In other words, a 420TVL camera and a 700TVL camera will both require the about the same amount of hard drive space for the same recording time. That said, while D1 resolution is only the equivalent of 480 TVL, a 600 or 700 line source can still result in a better recorded image than a 480 TVL source. You might even notice a crisper image from a good-quality, high resolution analog camera actually takes up less space due to how the compression algorithms work.

HD Resolution
HD CCTV cameras are beginning to become popular in the industry because of they have advantages of both analog cameras and IP cameras and avoid a lot of the disadvantages of both. Up to 6 times the resolution of the best analog cameras on the market, HD cameras are plug-and-play and require no networking or IT knowledge to set up, maintain or troubleshoot.

720p/720i – 1280x720The lower resolution of HD video, also sometimes called enhanced definition.

1080p/1080i – 1920x1080The highest resolution of HD video.

P and I stand for progressive and interlaced, respectively. When playing back video, both progressive and interlaced video will be the same size, shape and apparent resolution on the screen, but for progressive scan, the entire image is refreshed, while with interlaced, only half the lines (360 for 720i and 540 for 1080i) are refreshed, alternating between odd and even lines. Progressive scanning will result in smoother motion and more accurate detail when freezing a video frame. Interlaced scanning will result in a smaller video file by approximately half, but still a higher resolution image than analog cameras recording at D1 resolution.

Megapixel IP Resolution
IP cameras are cameras that connect to a network and can send video in the form of digital data to a DVR. Because the signal is digital, it is not limited by attenuation of normal coaxial cable like analog cameras are. Because of this, the cameras are capable of very high resolution images with greater detail at about four times the resolution or more of D1 recordings, with some cameras capable of over 16 times the resolution.

1.3 MP – 1280x1024The basic resolution of Megapixel IP cameras. This is a little more than 4 times the resolution and detail of D1. Some VGA monitors do not even display at resolutions this high, but recording at this resolution means there will be extra detail that can be accessed by digital pan/tilt/zoom, meaning you can virtually zoom in to a section of the live view or recording and search around the video in closer detail.

2.0 MP – 1600x1200This is a medium resolution of Megapixel IP cameras. This is a little more than 6 times the resolution and detail of D1. Many VGA monitors do not even display at resolutions this high, but recording at this resolution means there will be extra detail that can be accessed by digital pan/tilt/zoom, meaning you can virtually zoom in to a section of the live view or recording and search around the video in closer detail.

3.0 MP – 2048x1536This is a high resolution of Megapixel IP cameras. This is about 10 times the resolution and detail of D1. HDMI monitors and most VGA monitors do not even display at resolutions this high, but recording at this resolution means there will be extra detail that can be accessed by digital pan/tilt/zoom, meaning you can virtually zoom in to a section of the live view or recording and search around the video in closer detail.

5.0 MP – 2592x1944This is a very high resolution of Megapixel IP cameras. This is a little more than 16 times the resolution and detail of D1. HDMI monitors and most VGA monitors do not even display at resolutions this high, but recording at this resolution means there will be extra detail that can be accessed by digital pan/tilt/zoom, meaning you can virtually zoom in to a section of the live view or recording and search around the video in closer detail.


Frame Rate
Per Camera Frame Rate
IPS (images per second), FPS (frames per second) or frame rate is how many times in one second the image is changing. 30 frames-per-second is considered “full-motion” or “real-time”. Lowering the frame rate will allow a recording to take up less space on the hard drive. For example, a recording of 15 FPS would take up about half the space of the identical shot at 30 FPS.

With storage dropping in price, drives becoming larger and video compression methods getting better, space is not the issue it was in the days of tape-based storage or even the days hard drives measured in gigabytes. Because of this, many people find no good reason to set their frame rates to anything lower than real time, and they still get all the storage they ever need from a fairly inexpensive hard drive.

The human eye is surprisingly forgiving of lowered frame rates, even as low as 10FPS, so generally only high security applications such as banks, casinos and prisons truly require real time recording. 20 years ago, even banks and ATMs were happy to have frame rates of 1 or 2 FPS.

These odd, animated shapes are demonstrations of frame rate. You'll notice that 15 FPS is almost impossible to distinguish from 30 FPS. Even 10 is hardly noticable. At 5 FPS, clearly a difference can be seen.

Experimenting with lower frame rates is a good way to balance recording quality with hard drive space requirements.

30 FPS

15 FPS

10 FPS

5 FPS

2 FPS



Total Frame Rate
Some recording equipment has limitations on the number of frames that can be recorded at one time. A 4 channel DVR that is advertised as having 120 IPS @ D1, is capable of recording all the channels at once, at the highest resolution, in real time. An 8 channel capture card advertised as having 120 IPS @ D1 will only allow 4 channels to record D1 resolution at the same time at 30 FPS.

Some recording equipment will let you choose whether you’d like to sacrifice frame rate or resolution when the recording needs exceed its limitations, while others automatically do one or the other.

For example, an 8 channel DVR with 120 IPS @ D1 set to record in real time might lower the resolution of the channels being recorded from D1 to CIF when 5 or more channels are recording at once, then go back to recording D1 resolution when 4 or fewer channels are recording. Another system with the same specifications might divide up the available IPS across all the channels, so up to 4 channels can record in real time at D1 resolution, while 5 channels get 24 frames per second, 6 channels get 20 frames per second, etc. down to 15 frames if all channels are recording at the same time.

Consult your DVRs manual for more information on its recording capabilities.

Motion Recording
What is Motion Recording?
Modern DVRs can analyze video for moving objects and can be set to record only when there is motion in the shot. Allowing the DVR to analyze the live video for motion is an excellent way to not only save space, but save time for the person viewing the video in playback mode. If a camera is in an area that doesn’t get much in the way of motion, a 24/7 recording will consist of a lot of nothing happening. If you need to retrieve a video of a vandal that struck last night, sometime between 9PM and 7AM, a 24/7 recording would mean fast forwarding through 10 hours of video. Even at 32X speed, this is about 20 minutes of searching. (Don't blink, or you might have to watch for another 20 minutes.)

If this situation happens with a DVR set to record on motion only, not only will the video instantly jump to the times where something was happening in the video, but the hard drive will allow you to view further back in time since space is not being wasted on videowhere nothing is happening.

Pre-recording and Post-recording
Most DVRs with motion recording capabilities have a recording buffer of 3-10 seconds or so. This function is usually called pre-recording, but check your DVR's manual for exact terminology and specifications. When this feature is enabled, the DVR will sense a motion event and begin recording for as long as there is motion, but the video file will also have the previous video from the buffer tacked on. This can allow you to see from what direction a person entered the shot if they did not immediately set off the motion, or if a small object like a rock broke a window before the motion recording was activated.

Similarly, post-recording is the amount of time the DVR continues recording after motion in the scene stops. Consult your DVR's manual to learn more about this feature.

Motion Sensitivity
The exact settings will vary from DVR to DVR, but most units that are capable of motion recording allow for adjustment of sensitivity. If you notice a system getting too many false motion recordings from small birds or distant objects, or missing out on motion events that should’ve been recorded, it is likely that you’ll need to adjust the sensitivity.

There are many ways that DVRs might allow you to adjust the sensitivity.
- By the size of the object (how large a moving object must be before the unit records)
- By grid squares (how many squares of the motion mask must have motion before the unit records)
- By percentage (what percentage of pixels must change before the unit records)
- By time (how long an object must be moving before the unit records)

Your DVR’s sensitivity settings may vary, and you should refer to your DVRs manual for exact instructions.Sensitivity is always best fine-tuned on-site after the cameras have been installed.

Motion Masking
In many situations, the scene the camera is viewing will have objects that move that aren’t of interest from a surveillance standpoint. A back yard might have a willow tree that blows in the wind. A parking lot view could have a roadway with traffic in the shot. If we just let the DVR record whenever there is motion in these situations, a windy night or a busy street could cause the DVR to constantly record that camera when in reality, nothing of interest is happening. Most DVRs have a built in solution for this known as “masking”.

Masking allows you to tell the DVR not to pay attention to motion in certain areas of the shot, so there will be fewer “false alarms” when it comes to recording motion.

 
A scene with a tree can cause a lot of unwanted motion recording, even in light wind.   By masking out the tree, we conserve hard drive space and have less unimportant video to search through.


When installing a camera, it is important to pay attention to objects in the shot that might cause motion that will be difficult to mask out so you can choose another mounting location with a view of the desired area. Some situations will have no perfect solution, but thinking about the basics of motion-only recording can save a lot of hard drive space and save time for the end-user.

Other Motion Recording Issues
Bad video signals have been known to be interpreted as motion by DVRs, especially ground loop interference in the form of “rain” or wavy lines. In cases like this, troubleshooting the camera and video signal will help save recording space.

A camera mounted to a long, thin conduit pipe or a flimsy mounting bracket can also cause false motion recordings when the wind or vibrations nearby cause the camera to shake. Keep this is mind when choosing mounting hardware.

Spiders seem to love cameras, especially ones with IR illumination, so remember to keep the camera clean and free of spider webs, since this can not only cause motion recording, but affects the quality of the image.

Alarm Input Recording
Many DVRs allow for input-based recording. For instance a situation where a camera is pointed at an office entrance with a magnetic door contact installed can allow the DVR to record for a set amount of time whenever that door is opened.

Perhaps a client wants a few cameras inside the home, but for the sake of privacy only wants them to record when the home security alarm is going off. A DVR with an alarm input can be set to only allow certain cameras to record when the alarm is triggered.

 
In this example, an existing door contact from an alarm system is used to trigger recording of the camera facing the door. The camera will record for as long as the door is open, and then for a set time after. Wired like this, the door contact will still operate as normal with the existing alarm system.  

This is also an excellent way to have more efficient motion recording. In an area with a lot of trees, installing a PIR motion detector will alert the DVR to record only when something that generates heat (such as a person) is moving in the area, no matter how much wind blows the branches.

Existing alarm triggering equipment such as door and window contacts or motion detectors can be wired into the DVR while still allowing them to function for their original purpose with the alarm system. Many DVRs can also accept powered inputs, like from an alarm panel siren output, which means the DVR can be set to record when an alarm signal is sent.

Even a SPST switch wired to the DVR can be used to manually start recording, temporarily (using a momentary switch, like a doorbell button) or constant (using a toggle switch).

Alarm Outputs
If a DVR has alarm inputs, it is likely that it has at least one alarm output. In secure areas where there is not supposed to be any motion, the DVR can send a signal to the alarm panel when it senses something moving. The alarm panel could also be alerted when there is video loss to one or more cameras due to vandalism, power loss or camera failure. The alarm output can also be used as a kind of electronic switch to control lighting or power some sort of audible alert if the DVR senses motion or video loss.

Scheduled Recording
Some DVRs will allow for some degree of scheduling when the DVR is actively recording. The simplest application of this would be always-on recording during a certain time and then no recording for the rest of the time. Other examples include always-on recording between certain hours and motion-based recording for other hours. The methods of setting times and the capabilities of the DVR scheduling will vary from unit to unit, so consult your DVR manual for more information on this feature.

Circular vs. Linear Recording
Most DVRs will give the user the option of how it handles a full hard drive. They may call the feature something else, so refer to the manual of the DVR for exact terminology and instructions, but circular recording means that the DVR will record over the oldest data when the drive becomes full, and linear recording means the drive will stop recording once it is full and wait until data is deleted before recording again.

More than 99% of applications will use circular recording, and if a DVR stops recording because of a full hard drive, changing the record mode to circular will generally solve the problem.

There are only a handful of practical applications for linear recording, for instance some companies might want to pull the hard drive out of the system whenever it is full in order to store it for archival purposes rather than backing up all the data to DVD some other storage device. Your DVR may or may not be capable of hard drive swapping, so consult your manual or test this functionality before trying it in a real-world application.

Data Expiration
Some people might have a reason to have an expiration date on recorded video files. Many DVRs allow for a set amount of time for old data to automatically be deleted, regardless of space on the drive. For instance, if set to 30 days, any data older than 30 days will be removed from the DVR. Most DVR users will not need to use this feature, but on most DVRs the feature is there if it needed to comply with company policy, local laws or personal preference.


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