Alright, let's move on at the same rate as the industry. Let's discuss digital or IP resolution. The first step to IP is to realize that we no longer create two fields and combine them to make a single frame of video image. We create a single image at a time; it's a full scan, so to speak. The second part to be aware of is that we do not work with horizontal and vertical scan lines, creating dots on the screen. Instead, we create digital pixels or squares of colors. Imagine one digital pixel or square of color for each four analog pixels dots. The analog pixels could be thought of as the corner points of the digital square. This is why it takes so much more bandwidth to create a single image of high resolution, digital video as opposed to a single field of analog video. A square carries a lot more space; space equals bandwidth or frequency; frequency equals resolution or detail.
The field of view for IP or digital cameras works in much the same manner except that the image is no longer fixed at 4x3 and we don't count horizontal or vertical sweep lines. With digital, we reference the Common Intermediate Format (CIF); which is measured by squares of color, a.k.a. digital pixels. The CIF resolution standard and definitions were driven by the digital video recorder (DVR) industry and align with the analog (NTSC and PAL) standards. The resolution ratings of all digital or IP cameras are multiples or divisions of CIF. Consequently, we are speaking about true grid resolution -- the number of squares of color that we have as based upon rows and columns. Our most common digital resolutions (to date) are as follows:
- CIF (CIF) 352 x 240 pixels - 84,480 pixels Quarter of CIF (QCIF) 176 x 120 pixels, 21,120 pixels
- 4 times CIF (4CIF) 704 x 480 pixels, 337,920 pixels
- 16 times CIF (16CIF) 1280 x 1024 pixels, 1.31 megapixels
- 25 times CIF (25CIF) 1700 x 1200 pixels, 2.04 megapixels
- 36 times CIF (36CIF) 2112 x 1440 pixels, 3.04 megapixels
- [Information is presented as Ratio, Label, Horizontal/Vertical Pixel Array, and Grid]
To keep perspective, some of you have been listening to pixel resolution versus grid or CIF, so let's do the math in a different format. Let's talk about pixel resolution. CIF = 352 x 240 = 84,480 pixels = 84.5 kpr (kilo-pixel resolution). 4CIF = 337.92 kpr. 16CIF (also referred to as HDTV) = 1280 x 1024 = 1,310,720 pixels or 1.3 mpr (mega-pixel resolution). To understand what this means to image size, see Image 2 above.
Continuing forward with perspective, those of you that have photographic digital cameras will be happy to know (or perhaps not so happy to know) that a 4 megapixel image (approximately 2,000 x 2,000) is equal to 400 ASA film resolution and a 6 megapixel digital image is equal to 100 ASA film resolution. Our original digital standards were developed for the VGA monitors (Video Graphics Association) at 640 x 480. The field of view for each camera will need to be calculated in advance of system installation or design. This allows you to choose the proper focal length (FL) lens with very accurate and expected results, prior to spending a dime.
As with analog, the quality of the image or the detail that we are able to obtain, transmit or record is limited to our ability to first the image, carry the image, and store the image. Because, just as analog resolution equals bandwidth, so does IP resolution equal bandwidth or frequency. However, with IP we have a lot more latitude as to what we can do to preserve our detail. We can limit the number of images that are produced and transmitted per second. Do we really need 30 images per second? Not in most cases. The second thing that we can do is to compress the image. This is done via several different methods and will be the topic of our next column.
The biggest downfall of IP resolution is that enlarging the image is a matter of making small squares of color (or shades of gray) larger. Imagine a round ball made of square blocks. Imagine next that this ball is the size of a house and each block is one foot wide by one foot tall. When we view the ball from two blocks away, it appears round and smooth. But as we walk closer and closer to the ball, we see that it is not smooth. It is jagged, as only squares being placed into a curve can be. The key is to use smaller squares in the same space. However, that takes more bandwidth and the catch-22 grows.
See you next time as we discuss how to choose camera lenses.