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analog passthrough
Analog passthrough is a feature found on some digital-to-analog television converter boxes. Boxes without analog passthrough only allow digital TV (ATSC standard) to be viewed on older, analog-only (NTSC standard) TVs. Those with analog passthrough allow both digital and analog television to be viewed on older TVs.
Before digital television, passthrough originally existed for VCRs (and later PVRs and DVDRs), allowing the TV antenna or cable TV signal to pass through the VCR (with a slight insertion loss) to the TV set automatically when the VCR was turned off, or manually with a button on the remote control. Passthrough was turned off when the RF modulator (typically on TV channel 3 or 4 in North America) was on, as this F connector was originally the only way to send the VCR output to older TVs, until unmodulated composite video and RCA connectors became common.
==The problem==
All digital-to-analog converter boxes have both an antenna input (which accepts the coaxial cable that formerly went directly to the TV) and an RF output (which now goes directly to the TV). They may also have additional outputs. Any converter box converts the digital signal for the current digital sub-channel to an analog signal (at the reduced screen resolution of the analog standard), outputs that signal onto analog channel 3 or 4 (set by the user to avoid any conflict with local over-the-air channels) and sends that signal to the analog tuner in the TV. With a box that lacks analog passthrough, no other signals are sent to the output, so all analog stations are lost.
In the US, this primarily affects low-power and broadcast translator stations, as these are exempt from the FCC mandate to switch to digital broadcasting in 2009, as well as foreign signals that will remain in analog form. A small number of TV receivers were also manufactured with built-in broadcast radio receivers; these included some small portable devices or (more rarely) sets marketed for hotel/motel use. If used with DTV converter boxes, these will need analog passthrough in order not to block incoming FM radio signals.〔(DTV.gov FAQ )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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