Abstract: This article describes an applications circuit for the MAX4455 Arbitrary Graphics On-Screen Display Video Generator to reduce the effects of finite switching time on the insertion of graphics onto a video signal. This application note describes a circuit modification to the MAX4455 EV board that will improve the performance of the MAX4455/MAX4358 chip set by reducing on-screen display (OSD) artifacts caused by finite switch time effects. This modification can be included in any similar MAX4455/MAX4358 application circuit yielding the same performance improvements. The MAX4455 is an 8-channel arbitrary graphics OSD generator and the MAX4358 is a 32 x 16 crosspoint switch with integrated fast MUX for OSD insertion. Explanation of Finite Switch Time Effect In the MAX4455 there are two outputs involved in the generation of the OSD graphics: the first output, the OSDFILL, is the analog representation of the video fill signal, and the second output, the OSDKEY, is a digital signal that controls the turn on and turn off of the fast MUX switch. They are synchronized to within a nanosecond or so at the output of the MAX4455. Due to the finite switching time of the fast MUX in the MAX4358, the OSDFILL signal emerges slightly delayed from the OSDKEY signal. This results in a slightly delayed OSDFILL graphic with a trailing black level (0 IRE) equal in width to the switch delay time (as shown in Figure 1). This effect may be objectionable in applications that display all white characters, due to the high contrast between the white character and the trailing black edge. This is even more obvious on a light background due to the light dark light pattern. Figure 1. Finite switch time effect. Circuit Solution With the circuit modification shown in Figure 2, the trailing black level will be replaced with a trailing white level. If this white level equals the OSDFILL level then the trail effectively disappears, eliminating the finite switch time effect. This is shown in Figure 3. The resistor divider formed by R14 and R sets the brightness of the trailing white level, which can be set to equal the brightness of OSDFILL. When R equals 453Ω, then the trailing white level is nominally 100 IRE. Figure 2 shows a modification to only one of eight channels. Four MAX4616s can be used to improve all eight channels of the MAX4455. Figure 2. Modification to the MAX4455 EV circuit board on channel 0. Figure 3. Fix for the finite switch time effect. 相关型号   APP 1943: Mar 21, 2003 MAX4358 32 x 16、无阻塞视频交叉点开关，带有屏幕显示及I/O缓冲器 完整的数据资料 (PDF, 1.4MB) 免费样品 MAX4455 任意图形随屏显示视频发生器 完整的数据资料 (PDF, 672kB) MAX4616 低电压、高速、四路、SPST、CMOS模拟开关 完整的数据资料 (PDF, 288kB) 免费样品 自动更新 需要自动接收最新发布的应用笔记吗？请订阅EE-Mail™ (English only)。 我们期待您的反馈！ 喜欢？不喜欢？有待改善？或为我们提供建议？请与我们联系 — 我们将根据您的意见或建议改善我们的工作。 网页评价或提供建议   下载，PDF格式 (49kB)  AN1943, AN 1943, APP1943, Appnote1943, Appnote 1943