As promised in the last installment of this column, we're taking this column to discuss resolution. Both analog and digital need to be discussed so that you can understand the differences and the consequences of those differences. You need to understand the language of resolution in order to competently make decisions about how much is enough, too little or more than enough. So let's start with a simple definition of what resolution really is. In both analog and IP applications, resolution is a number that we assign to describe the detail of the projected, transmitted or viewed video image. The higher the number, the better or finer the detail of the image will be. It is important to understand what numbers we are speaking about and what exactly they represent.
With analog, our resolution is created when a horizontal scan line crosses a vertical scan line. At each point of crossing, a dot or pixel of energy is created (See image 1). More scan lines means more pixels. The more pixels an image has, the higher the resolution rating the image has, and with higher resolution, greater detail can be seen in the picture. Remembering previous lessons, we know two things. First, we are restricted by the National Television Standard Committee (NTSC) to 525 vertical lines of resolution. This was to conserve bandwidth and prevent crowding of our airwaves back in the early days of television. Second, we create an analog image in two steps. We first create the odd horizontal scan lines and then the even horizontal scan lines.
This creates two fields or half pictures. The two fields combined make for one frame. We have 30 fields or 30 frames per second. This is called 2:1 interface and was created to allow recording of the video signal onto a one half inch wide magnetic tape. The whole frame takes up too much bandwidth to be recorded on such a small tape. Therefore, resolution equals bandwidth or a lot of frequencies. It also means that we only see one field (the odd one) during the playback of a video tape. This is why the playback never seems to look as good as the live image. We have 30 fields per second being played back as opposed to watching 30 frames.
When we discuss analog resolution, we always refer to the "horizontal" resolution. This is because we can monkey around with the amount of it where as the vertical is fixed. We are still limited, however, to how much detail we can produce because of the capability of our cameras to produce, the cable to carry, and the various pieces of equipment in between to emulate the bandwidth of the signal. The net result of analog resolution however is always four-fold:
1. The analog image can be enlarged several hundred times before becoming greatly distorted. This is because the pixels and the spacing between them enlarge exponentially.
2. When we discuss the resolution of an analog image, we are speaking about the "frame" resolution. That is, if a camera is rated at 400 horizontal lines, each field produced is only 200 lines. Most equipment that converts the analog to digital and then back (for the purpose of enhancements or control) cannot handle the bandwidth of an analog signal. Therefore, the output resolution is usually much less than the input.
3. Although both fields are recorded on a tape, we only see one field played back. The even field is ignored unless you are working with a frame recorder in which case every other odd field and every other even field are ignored. Remember, the video recorder in our industry can only play back 30 fields, regardless of how many are presented. A forensic person, using the proper tools can play back either or both fields for study and exposition.
4. Our analog image will always be four parts wide to three parts high. That is to say, if we have an image that is 12 feet wide, it will be 9 feet tall. Therefore, increasing the resolution is a matter of cramming more dots into the same size mold. The detail improves, the bandwidth increases, but the image stays the same size.