About DVRs and NVRs

About DVRs and NVRs

Introduction

The following also applies to IP (NVR) recorders.

The function of a Digital Video Recorder (DVR) is to record 1 or more channels of video to a storage medium such as an internal hard drive disk. The recorded video can then be played back directly from the disk via the DVR. The quantity of video stored is dependent on the following factors: 

  • Hard drive size
  • No of cameras
  • Recording frame rate
  • Recording quality
  • Recording compression
  • Recording resolution
  • Continuous, scheduled or motion detection recording 

Modern DVR’s have advanced features for managing the storage of the recorded video. Frame rates and recording quality can be reduced to give smaller recorded file sizes but at the cost of the image quality. DVRs can be set to record only at scheduled times, on motion detect, or when triggered by an external source such as an alarm input.  

Hard drives 

The higher the capacity of the hard drive, the more recorded footage can potentially be stored on it. Most DVRs will accept hard drive sizes up to 4Tb and higher spec DVRs can accept multiple drives. 

Historically, hard drives used the IDE interface, with a maximum capacity of 750 Gb. IDE drives have now been phased out in favour of SATA drives.

SATA drives have faster data transmission and are less likely to suffer data loss. In addition SATA drives have a higher maximum capacity of up to 6Tb but most DVRs are limited to 4 Tb. Adapters are available to allow SATA drives to be used in IDE machines.

It should also be noted that not all hard drives are the same. A hard drive for use in a PC for example is designed to have a read/write ratio of 50/50 whereas a surveillance quality hard drive is designed to have a read/write ratio of 10/90. The hard drives we supply are Western Digital Purple models which are slightly more expensive than standard drives but are fully fit for fitting in surveillance systems. 

Recording Frame Rate 

Most DVRs can display and record at up to 25 frames per second (fps). At this frame rate, even high speed motion such as cars will appear to move smoothly. For action at say normal walking speed then any frame rate above 10fps will appear smooth. In a system with a lot of cameras recording simultaneously, the fps rate can be reduced, but with the availability of high capacity, fast SATA drives this is less of a problem.

Video Quality 

Modern DVR’s can normally reduce the recording resolution if required. Obviously the resolution of the playback cannot be better than the original recording. Again, with the advent of SATA drives, reducing the quality is not always necessary.

The recording resolution of a DVR is best defined by the maximum number of pixels that can be used on recorded footage. The tables below show the number of pixels captured by various resolutions. The D1 and 960h are shown for comparison purposes only – we do not supply standard D1/960H systems as we consider them effectively dead.

 

Pixels Captured

Notes

D1

345,600

960H

460,800

960H is simply the wide screen version of D1.

720P

921,600

1080P

2073,600

1080P is known as full HD and has more than 4x the resolution of D1/960H.

And for IP systems

Term

Pixels (W x H)

Total Pixels

Notes

1.3Mp

1280 x 1024

1,310,720

2Mp

1600 x 1200

1,920,000

3Mp

2048 x 1536

3,145,728

5Mp

2592 x 1944

5,038,848

 

Video Compression

It is not feasible to store video without compressing it. There is a small loss of resolution in the compression techniques.

The two main classes of compression are

  • JPEG compression
  • MPEG compression 

JPEG compression results in better image quality than MPEG but produces larger files and hence needs more disk space.

The basic difference is that JPEG records everything in the picture seen by the camera, whereas MPEG just records everything that moves and overlays it on the still image in the background.

Both compression methods have been modified, but the technique now (2015) widely used is H.264 compression which is based on MPEG compression

 

Recording Schemas 

In the majority of surveillance situations it is not necessary to record everything continuously – it simply uses up disk space. It is usually preferable to use triggers or a schedule to limit the recording to events or times.

Video Motion Detection (VMD)

VMD is a feature of the DVR which monitors the image for changes in for example brightness or colour temperature. When it detects this change it will trigger recording for a user-set length of time. The DVR can be set-up to pre-record for a user-set time before the event occurred. VMD is set-up on an individual channel basis, and the area of the picture which is monitored for motion can be selected.

The sensitivity of motion detection can be set to suit the situation. Typically, when the sensitivity is set to high the DVR will recorder even very small movements. This may be fine for internal use but externally it will record all sorts of things such as moving trees

Record on Alarm 

Some DVR’s have inputs that will allow alarm equipment such as PIR’s or door contact be connected. Providing the correct detection equipment is used and set up properly, alarm activation recording is the most accurate type of recording trigger.

A good example is in a shop where a magnetic contact is fitted to the shop door so that every time the door is opened the recording starts and continues for a user-set time. 

Scheduled recording

DVRs usually have the facility to set a recording schedule where the DVR will record between certain times on certain days.  

Operation Modes

Modern DVR’s are known as ‘Pentaplex’ and can perform a multitude of operations simultaneously. Pentaplex mode allows, record, playback, remote view, menu view and backup at the same time.

DVR Video Inputs and Outputs

Analogue DVRs such as standard D1 and HD-CCTV have BNC inputs for connecting analogue cameras – 1 BNC input per channel. IP NVRs can have multiple RJ45 inputs - 1 per channel, or a single RJ45 which is connected to an external port switch. The RJ45 inputs can also have PoE (Power over Ethernet).

Historically, DVRs had BNC outputs for connecting to monitors. Modern DVRs are fitted with VGA and HDMI outputs and can connect to digital monitors and modern TVs.

Audio Facility 

Higher spec DVRs usually have audio inputs as well as video inputs. The audio inputs allow the connection of a microphone, which may be part of the camera or a separate unit, to record sound simultaneously to the DVR. 

The amount of audio inputs varies from DVR to DVR; some have only 1 input whereas others have 1 input for each channel.

Video Output

 DVR’s can display a single camera on screen or a combination of multiple cameras on screen at any 1 time (a multiplexed image). Most DVR’s also have the ability to sequence through the connected cameras 1 at a time. 

Some DVR’s can also display picture in picture (or PIP) whereby 1 camera will take up the whole screen and a smaller second camera will be will be overlaid on top of the first camera.

DVR Operation

Modern DVRs are controlled by a sophisticated password-controlled on-screen-display (OSD) user menu. The OSD can be accessed using either a USB mouse or an Infra-red Remote control unit (both are usually supplied).

The OSD will have all the required functions governing the setup and record/playback functions. 

Backing up Recordings

There are various methods of backing up recordings from a DVR so that it can be played back on other media. These include: 

CD Backup

This method is not really used on modern DVRs – the capacity of 750Mb is not usually enough to be an efficient back-up method.

DVD backup

Again, this method is not used much on modern DVRs, although the capacity is around 4.7Gb.

Note: most modern DVRs are not equipped with CD/DVR read write units. Also note that recordings to a CD/DVD are not normally playable on a TV connected player. They have to be played on a PC.

USB backup

All modern DVRs have the facility to backup video onto USB sticks with capacities up to 64Gb. Some DVR’s may have a limit on the size of sticks they can accept.

SD card backup

SD card backup is similar to USB. Most DVRs prefer to fit the USB port. 

External hard drive

Some high-spec DVRs have the facility to fit an external Hard Drive.

Disk mirroring

Disk mirroring is more a matter of preventing data corruption than for backup. Higher-end DVRs and NVRs are usually capable of having two or more hard drives fitted. The hard drives can usually be set up to mirror each other if required such that if one of the disks gets corrupted, there will be a backup on the mirrored 

Network backup

When the DVR is connected to a Local Area Network (LAN) or Wide Area Network (WAN) the video can be stored off-site if required. 

In most cases the stored video can only be played back on a PC. DVR manufacturers use various encodings and as such some backed-up video may need a specific program (usually supplied with the DVR) to play back the video whereas others may use something like Windows Media Player. In many cases saving the file as a different format will allow the video to be played in any PC.

PTZ (Pan, Tilt & Zoom) Camera Control

Modern DVR’s have RS-485 outputs for telemetry control of PTZ cameras and they may also have the facility to send the signals on the coaxial cable (Up-The-Coax).

PTZ cameras have an ID number which the DVR uses to send the PTZ commands to the correct camera. Most PTZ cameras use standard industry protocols to communicate with the DVR. The commonly used protocol is Pelco-D and all PTZ cameras will be compatible with all DVRs.  

PTZ cameras can be controlled from the DVR, a dedicated keyboard controller or remotely by a PC or mobile App.

Network Connections 

All DVR’s on the market today have the ability to connect into a TCP/IP (computer) network giving the user the ability to log in and view or take control of their CCTV system from a remote location or a PC on site.

 

Tags: DVR NVR

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