Video image and signal

Consider a video image and the corresponding signal. Figure 6 shows a piece of a video raster displaying the letter 'T'. In addition, a single line of that piece of raster, displaying the horizontal line of the 'T', and the line of the video signal that scans the horizontal line of the 'T', onto the screen are shown. For the sake of simplicity we will consider the display to be monochrome, though the example could also be applied to a color display.

Figure 6

The video signal is scanned across the raster of the display. Just as with the sine wave, the voltage of the video signal rises and falls, but not necessarily so smoothly. The job of the video signal is to excite pixels on the screen of the display. The higher the voltage that excites the pixel, the brighter the pixel and the brighter that piece of the image will be. This is the image and the defining signal that will have to be reproduced at the other end of a piece of video equipment.

Relevant frequencies

Consider what frequency ranges will be transmitted in a video signal. A video signal has many elements. As shown in figure 7, vertical sync operates in the range of Hz, horizontal sync operates in the kHz range, and the visible video information is in the MHz range. All of these frequencies carry information necessary to display an image, so the lower critical frequency for video is always considered to be zero Hz. The high frequency information alone will define how much bandwidth is necessary in a video system.

Figure 7

The high frequency information in a video signal is carried in the visible video. This is fortunate, for the patterns that result from these high frequency signals can be seen. They occur in fine detail, such as thin vertical lines or small text. The designer of a video system must know the highest frequency the video information will contain in order to design adequate bandwidth into the system. While this information is readily available for some standardized video signals such as NTSC, manufacturers of high-resolution video sources such as computer graphics cards do not commonly publish it. Fortunately, it can be calculated from video graphics parameters that are commonly published. For more information on how this is done, see Calculating Video Signal Frequency at the end of this article.

Bandwidth loss

Consider what will happen to the signal in figure 6 after it passes through any piece of video equipment with insufficient bandwidth. Bandwidth loss in electronic circuits shows that the high frequency portions of the signal will be attenuated, or will lose amplitude. Where are those high frequency portions of the signal? They are in the sharp transitions at the rising and falling edges of the video signal, as is shown in figure 8, where the sharp edges of the video signal are shown to be rounded, or rolled off. On the video display this signal loss is seen as smearing of the character on the right edge, lack of sharpness on the left, and general loss of brightness near both edges. On a page of text this kind of loss could result in characters smeared together and a total loss of legibility.

Figure 8

The effects of bandwidth loss on video signals can be quite devastating to the resulting video image. But the question remains how much bandwidth is adequate for a video signal? Is the bandwidth specified at the -3dB point sufficient for video signals of that same frequency? Consider figure 9a, and the amplitude loss to a 100 MHz signal in a system with a 100 MHz bandwidth. The important question for a video image is will the loss of 30% of the brightness in the high frequency elements of the image be viewable on a video display?

Figure 9a, 9b

The answer is, of course, yes. Even those elements of the display that lose 15 to 10% of their brightness will not go unnoticed. To account for the loss before the -3dB point, Extron has established the bandwidth rule of thumb. The bandwidth rule of thumb states that a piece of video gear must have a bandwidth specification of 2 to 3 times the frequency of the original video signal. This will result in moving the bandwidth curve out, as shown in figure 9b, so that the visible video information falls in the flat part of the curve, the part of the curve with less than 1dB of loss. Losses of less than 1dB will not be apparent on a video display.

Video system bandwidth loss

By using the bandwidth rule of thumb, it should be relatively easy to specify a piece of equipment with adequate bandwidth for a particular signal. But how many video systems consist of only a single piece of gear? Just as is true inside a piece of electronic equipment, each element in a video system adds to the bandwidth loss of the system. Switchers, distribution amplifiers, cable, everything through which a video signal passes contributes to bandwidth loss, and it all adds up at the destination.

For instance, sweep testing of two daisy-chained distribution amplifiers, each with a bandwidth of 100MHz, has resulted in a -3dB point of 60 MHz. The amount of bandwidth loss accumulated through multiple pieces of equipment will vary with the bandwidth curves of the individual pieces, but this example is typical of what can be expected. It is a good idea to select gear with extra bandwidth when designing a system where a signal will pass through multiple pieces of electronic equipment.

Cable also contributes to bandwidth loss. Types of cable do not have a predetermined bandwidth like interfaces or switchers, for cable loss depends on the length of the cable, which varies from cable to cable. Therefore cable attenuation specifications should be given at a particular length and frequency, and for a certain amount of loss. For instance, Extron SHR cable specifies a loss of 2.8dB, at 100 feet, at 200 MHz. This kind of loss is linear with respect to the length of cable, so that the overall cable loss can be calculated for various lengths of cable. So that this same cable at 200 feet would have a loss of 5.6dB at 200 MHz.

In systems with long cable runs it may be necessary to design level and peaking features into a video system with devices such as an Extron PA 250, or a computer video interface, in order to compensate for bandwidth loss before it happens. These features enhance a signal before it enters the bulk of the video system, by increasing the signal level, or peaking the sharp edges of the video, as is shown in figure 10. These signal enhancements are then attenuated through bandwidth loss, and the signal arrives at the destination as it was generated at the origin.

Figure 10

Calculating video signal frequency

Since the low frequency elements of the video signal are assumed to be zero, the high frequency part of the signal is all that needs to be calculated. Consider figure 11, which shows an 'H' pattern on a portion of a display and the signal that generates the uppermost portion of the "H". This is the highest frequency signal that can be generated on a display, where one pixel is turned on fully bright, the next is shut completely off, and so on and so forth until the end of the line of video.

Figure 11

In order to calculate a signal frequency, you must convert this pixel information into frequency information, in cycles per second (Hertz). The examples below and on the next page show how these calculations are performed.

SF = [(TP x Vt)/2]3

Where:

SF = Signal frequency

TP = The total number of displayable pixels.

NOTE: When resolution is listed (640 x 480), multiply these two numbers.

Vt = The vertical scanning frequency, or refresh rate.

Let's calculate CGA:

We know that:

CGA resolution is 320 x 200,

the horizontal scanning rate is 15.75 kHz

the vertical scanning rate is 60 Hz.

Thus:

TP = (320 x 200) or 64000

Vt = 60 Hz

Therefore:

SF = [(64000 x 60)/2]3

SF = 5.76 MHz

Let's calculate VGA:

We know that:

VGA resolution is 640 x 480,

the horizontal scanning rate is 31.5 kHz, and

the vertical scanning rate is 60 Hz.

Thus:

TP = (640 x 480) or 307,200

Vt = 60 Hz

Therefore:

SF = [(307200 x 60)/2]3

SF = 27.6 MHz

Let's calculate a workstation (Sun):

We know that:

Sun resolution is 1280 x 1024,

the horizontal scanning rate is 81 kHz,

and the vertical scanning rate is 76 Hz.

Thus:

TP = (1280 x 1024) or 1,310,720

Vt = 76 Hz

Therefore:

SF = [(1310720 x 7 6)/2]3

SF = 149.42 MHz

Reference — prefixes

Mega(M) = 10 ⁶ Micro(µ) = 10 ^-6

Kilo (k) = 10 ³ Nano (n) = 10 ^-9

Mili (m) = 10 ^-3 Pico (p) = 10 ^-12

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