Entendiendo Las Resoluciones En Videocamaras

White Paper: Understanding Image Resolution
by Dr Bob Banerjee
This brief white paper demystifies analog video resolutions and their relationship to their digital counterparts. By understanding the relationship, we can optimize the use of our original camera video, without wasting effort compressing, transmitting, and storing parts of the image that can never be seen.

NTSC Analog Video AnNTSC analog video picture is composed of 480 active horizontal lines. When the picture is sent to a monitor or another receiving device, the first field of 240 even-numbered lines is sent, and then the second field of 240 odd-numbered lines is transmitted. This occurs at a rate of 60 fields per second. The odd and even fields are offset one line apart, so when they are viewed in rapid succession (60fields per second), they slot into one another to give the illusion of the complete picture – also called a frame, which is made up of 2 x 240 (or 480) lines. This is called interlaced video.

Resolution (TVL – or TV Lines) is a monochrome specification that states how many black and white lines can be seen in a given area. With 480 active scan lines, we can see how the theoretical maximumvertical TVL resolution is directly affected by the number of scan lines. (Note: TVL is not equal to scan lines, but vertical TVL is affected by the number of scan lines.) Now that we understand the vertical resolution, we need to understand the horizontal TVL resolution (the number of vertical sampling points). This number varies by camera manufacturer, but they all follow the same definition.Bosch Security Systems • 130 Perinton Parkway • Fairport, NY 14450 • 585-223-4060 • www.boschsecurity.us

As you can see in the drawing above, the picture is wider than it is high. This is called a 3x4 aspect ratio. (3 units high by 4 units wide). Horizontal resolution is defined as the amount of vertical black and white lines you can see in 3 of the 4 units. In other words, how much detail youcan measure in 75 percent (3/4) of the picture’s width. This resolution number varies by camera type and camera manufacturer – many “standard” CCTV cameras have 380 TVL, but high-resolution cameras have more, typically 540 TVL. Assuming all things are equal, if you were to connect an analog CCTV camera via coax to an analog monitor, you would see everything. However, when you digitize the analogvideo, compress it, transmit it across an IP network, and then decompress it again, the video changes. To understand how it changes, we need to understand CIF – resolution in the digital world.

Analog to Digital In the analog world, 480 active scan lines and 75 percent of the horizontal area identify resolution. With digital, we still have a 3x4 aspect ratio, but pixels or sensing points now takethe place of scan lines. Like cameras, these digital “picture grabbers” (digitizers) come in different levels of resolution (e.g., 176x144, 352x240, or 704x480). Like a steel chain, the picture quality of your system will only be as good as your weakest link.

What is CIF? CIF and derivatives of this word are terms used to identify the level of resolution that the “picture grabber” can produce.CIF (pronounced “SIF”) stands for “Common Interchange Format” or “Common Intermediate Format.” It ranges from: CIF 352x240, 2CIF 704x240, or 4CIF 704x480.

Cameras and TVL (Horizontal) Resolution
CIF 1/2 D1 330 TVL 2/3 D1 380 TVL 480 TVL 2CIF 4CIF 540 TVL 570 TVL 0 100 200 300 400 500 600

Bosch Security Systems • 130 Perinton Parkway • Fairport, NY 14450 • 585-223-4060 • www.boschsecurity.usCamera 330 TVL 380 TVL 480 TVL 540 TVL 570 TVL

CIF
Good Good Acceptable Acceptable Acceptable

1/2 D1
Better Good Acceptable Acceptable Acceptable

2/3 D1
Best Better Good Good Good

2CIF
Unnecessary Better Better Better Better

4CIF
Unnecessary Best Best Best Best

The table above shows cameras and their CIF recommendations. Note that statements such as ‘Good, Better,...