Page:Gametronics Proceedings.djvu/135

 : Video is a bandwidth limited system. The relationship between risetime RT and bandwidth BW can be expressed as:

where BW is the -3 db point in Mhz and RT in microseconds 10%-90%. A typical value for k is o.35 which gives the following values: Figure 2a shows a maximum resolution pattern of 48 adjacent pixels, alternating black to white. One pair of pixels produces one complete H cycle of video. The 4/3 aspect ratio requires that there be 8 pixels wide for 6 pixels high. In the maximum interlace mode the 480 scan lines imply 640 horizontal pixels, or 320 TV H cycles of video, during the active line time.

Figure 2b depicts the scan spot encountering 1 ideal pixel. The resultant video pulse is the impulse response of the low pass filter, h (t), fig. 2c. The corresponding spectrum of this pulse is shown in 2d. Notice that the major energy is concentrated between 0 and +/- 1/T where T is the half amplitude duration ( had ) of the h(t) pulse. The pulse is known as the (sin T)/T or sine2 impulse.

Since digital video generators produce trapezoidal pulses risetime should be limited to 88 nsec. With a had T of 250 nsec the total pulse rate is 1/338 nsec or about 2.95 Mhz. This results in 155 pixels in 52.5 usee active line time: About 26-5x7 characters can be displayed across the screen at this bit rate. While it is a seeming low value it is compatible with MOS processes such as NMOS. However, in terms of applying, chroma to pixels it is already too small. Because chroma bandwidth is limited to about 1.6 MHz the risetime of a chroma pixel is 220 nsec. Thus not too short a pixel width is desired for true bandwidth limited color video modulation.