differences between dvr

7/27/2019 Differences Between DVR

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Differences Between DVR / NVR Resolutions

Digital Video Recorders (DVR) and Network Video Recorders (NVR) are the heart of everysecurity camera system. It critiques the final outcome of your captured footage, which is the

entire complexity of its existence. Naturally a consumer wants to know how clear the video

resolution will be from a system before making a purchase decision. This segment focuses

on resolution qualities and what they consist of.

A CCTV cameras resolution is measured in vertical and horizontal TVL (television lines)

dimensions and often limited by the capabilities of both the camera and the recorder. CCTV

systems use an analog video signal which derives from television specifications. NTSC

(National Television System Committee) and PAL (Phase Alternating Line) resolutions are

two respected analog video standards. They are the relevant formats to network video since

video encoders provide like-wise resolutions when they digitize signals from thetransmissions received from analog cameras. In the NTSC world, a true classification of "D1"

entails 720x480 pixels. In the digital video recorder (DVR) world you get a type of, if you will,

a cropped D1; which is a pixel ratio 704x480, also referred to as 4CIF. For me to accurately

describe the concept of resolution of an analog video in a simplistic manner is difficult. Lets

begin with first understanding the difference between a TVL line and a pixel. Resolution in

an analog or digital picture is similar, but there are some vital differences in how it is

defined. As I stated before, an analog video is an image consisting of TV-lines since analog

video technology is derived from the television industry. In a digital system, an image is

made up of square pixels. This picture is composed over a 480 active horizontal lines. When

the picture is sent to the DVR the first field of 240 even-numbered lines is sent followed bythe second field of 240 odd-numbered lines. This occurs at a rate of 60 fie lds (or i frames)

per second. The odd and even fields are offset one line apart, so when viewed in rapid

succession (60 fields per second) they interfix into one another to contrive a complete

picture, commonly referred to as a frame, which is made up of 2 x 240 (480) lines. This is

called interlaced video.


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In the picture above, youll notice it has more width than height. This is called a 3x4 aspectratio, 3 units high by 4 units wide. Horizontal resolution size is defined as the amount of

vertical black and white lines you can see in 3 of the 4 units. E.G., how much detail you can

measure in 75 percent (3/4) of the pictures width. Horizontal Resolution is contributed as

the number of alternating black and white vertical lines that can be resolved per a pictures

height, referred to as Television Lines (TVL). When analog video is digitized, the maximum

amount of pixels that can be created is based on the number of TV lines available to be

digitized. The maximum size of a digitized image is typically 704x480 (D1/ 4CIF).


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Now based on the capabilities of your DVR you may choose to record in a certain picture

resolution based on certain requirements unbeknownst to anyone but you. Some instances

of why some choose to is based on the criteria of the surveillance, HDD space saving

management, optional 24/7 record in one resolution and snapshot capture in another

resolution, or the DVR itself is limited in differential recording sizes. Digitized NTSC video at

CIF resolution is an image that is 352x240. It intentionally matches the NTSC 240 horizontalscan lines in a field so that there is a 1-to-1 conversion from analog state to digital CIF

resolution. However, the 352 format renders the picture to a theoretical 264 TVL (Subtract

75% percent of 352 due to the fact CIF is only a quarter of D1), which does not match the

TVL vertical resolution provided in 420-700 TVL from the cameras. So do you know what

happens to the unused TVL resolution lines? They are forfeited. Using the CIF resolution

ratio any camera capable of delivering over 264 TVL will not appear as a superior. No matter

if you are using a 420TVL camera or a 700TVL camera, the images will be identical. In other

terms: A CIF system throws away half the video information from the camera. Due to our

eyes are not keen enough on averaging this science, the following is not that noticeable, but

this CIF size cuts the vertical resolution in half, and makes the resulting video look jagged

and/or jerky and not as smooth as D1. The chart to the left is a quick reference guide to

the names and sizes used in analog video compression. Remember as well any sufficient

DVR will allow for the highest recording at a frame rate of 30FPS (frames per second)

amongst all your channels simultaneously.


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What you (hopefully) understand of analogIn the analog DVR world, the only adequate resolution is the "horizontal lines of resolution

size". Be careful when you explain about the vertical pixel resolution of a picture. When you

speak of horizontal lines of resolution means that you can display up to 480 alternating

black and white horizontal lines and see them all distinctly. Similarly, the horizontal

resolution is defined in terms of vertical lines. Horizontal resolution is tricky to prove and

test. In an analog system, there is no other way to tell how many vertical lines of resolution

the system can show without measuring/testing it, either with physics or by running a test

signal (or chart) through a system and viewing the result.


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For the most part, nowadays analog cameras provide a line resolution range between a

420TVL to 700TVL. For close-up & short distances, a lower TVL resolution camera can save

you some money and provide an adequate image for your criteria but a 700TVL and above

have superior images. The pictures are sharper and the contrast is great. If you want great

clarity the 630-700TVL range of resolution is the way to go. There are other factors for a

good clear picture like lenses, chipsets, lux, WDR etc.

Once the camera captures the images that you want, it sends them through coaxial cable, or

Cat 5 with the use Baluns, to the DVR. At the DVR, the images are converted from an analog

signal into a digital format to be stored on a hard drive. This is the most important part of

the chain. The greatest most powerful camera will not be effective if the image is

compressed down to nothing by the DVR. The two most popular DVR resolution types are

CIF and D1. CIF is defined as 320240 pixels and D1 is 704480. As you can tell, D1 is 4 times

larger than CIF. This entails that the raw analog signal of the camera will be compressed

less, giving you better detail.

The Relationship Between The Camera TVL And The Resolution Of The DVR

Here is a very common question. How many TVL should the camera have to best use the

resolution of my DVR? This is one of the most misunderstood relationships in CCTV. The

resolution that the DVR records in refers to the SIZE of the image being recorded. So CIF

resolution (360240) and 4CIF resolution (704480) refer to how large the image actually is.

4CIF is an image that is 4 times larger than CIF.


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TVL refers to the Horizontal lines of resolution the camera can display in. This will determine

the amount of detail you may see within a certain size image (CIF, 2CIF, 4CIF). When

selecting the right camera, keep in mind that the resolution being recorded and the

resolution being displayed are usually different. In most cases a DVR will display live video at

4CIF (704480) or D1 (720480) resolution, but be recording in a lower resolution like CIF

(360240). Though a live 4CIF display of video will benefit from a 540TVL camera, the

recorded image in CIF will not see the difference between a 540TVL camera and a 420TVL

camera. So in essence you will only benefit from the live view, not the recorded image.

You will notice that the maximum benefit for a cameras horizontal resolution when

recording in 4CIF is actually around 520TVL. Also, notice when recording in CIF resolution a

420TVL camera and a 540TVL camera will look exactly the same in the recorded image ifrecorded in CIF, but the 540TVL camera will display an enhanced image when recording at

4CIF. This also applies to the live video since it is usually displayed at 4 CIF.

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