Out in Front: April 11, 1996
Video preamps feature switched external input
A pair of wideband preamps from National Semiconductor eases the implementation of on-screen displays (OSDs). OSDs override the video image in consumer-TV monitors when you set up a VCR or change channels and when closed-caption subtitles appear at the bottom of the screen, for example. The LM1281 and LM1281 preamps feature a TTL-switched external trio of inputs for the OSD red, green, and blue (RGB) signals in addition to the conventional RGB-signal paths. This single control line lets you switch between the normal video-input signals and the OSD signals.
The 12V LM1281 has three matched preamp channels with 85-MHz bandwidth for 1024×768-pixel resolution; the LM1282's channels have 110-MHz bandwidth supporting 1280×1024-pixel displays. The OSD inputs have 50-MHz bandwidth. The preamps process the TTL-compatible OSD inputs internally to match the OSDs' logic-low level to the video-black level. Independent contrast adjustments for RGB signals let you adjust the contrast of the OSD window without affecting the conventional video portion of the display. To protect the CRT phosphor during power-down, a "spot-killer" circuit blanks the preamp outputs when the supply is lower than 10.6V.
The LM1281 and the LM1282 come in 28-pin DIPs and cost $2.50 and $3.25, respectively (1000).
—by Bill Schweber
National Semiconductor Corp, Santa Clara, CA. (800) 272-9959.
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