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Analog passthrough

From Wikipedia, the free encyclopedia

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. European VCRs did not turn off the passthrough when the unit was on – instead, the VCR added its output as an extra channel (usually 38; it was possible to change the output channel, depending on the model, by turning a screw in the back of the VCR or by selecting an appropriate option in the OSD menu).

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.[1]

The solution

Some converter boxes offer analog passthrough, meaning that the same output cable which carries the converted digital signal (on analog channel 3 or 4) while the converter is operating also retains all remaining analog signals upon turning the converter box off.

While this typically works to some extent with the converter box on, there is often significant signal strength reduction and/or interference on the original analog channels.

Note that analog passthrough signals are only passed to the coax output, not to any other outputs provided by the unit. While other converter outputs may still be connected and used for digital TV reception, the coaxial RF output must remain connected to pass analog TV signals.

For some VCRs (and, less often, TVs) this arrangement may cause a problem if the receiver uses the presence of active composite video or S-video connections to indicate that the signal should be accepted from those sources only. In such a case the two options would be to use the coax output exclusively, either when viewing analog TV only (which requires plugging and unplugging cables) or full-time (with the reduced digital TV quality produced using the coax output).

The majority of TVs have a button on the unit and/or remote labeled something like "TV/VIDEO", "INPUT" or "AUX" which manually switches between coax and other sources, ensuring no problems when using both sets of connections simultaneously.

United States implementation

In the US, conversion to digital TV from analog was complete by June 12, 2009. There are exceptions, however:

  1. Class A stations, low power stations (LPTV), and translator stations (Primarily local-only Television Stations, as well as TV Station Translator/Repeaters in rural communities) may continue to send analog signals until Tuesday, September 1, 2015.[2]
  2. Cross-border stations, many of which are significantly viewed in the US,[3] will continue to broadcast in analog format until 2011 from Canada,[4] and 2021 from Mexico.[5] Portions of Florida are also adjacent to Cuban stations or to ZNS-TV from The Bahamas.

Many will want to continue to receive analog stations both before and beyond the February, 2009 transition date. In setting its requirements for coupon-eligible converter boxes (CECBs), the NTIA permits, but does not require, analog passthrough as an available feature.

Unfortunately, only one of the initially available converter boxes included this feature,[citation needed] and a year before the original transition date of Feb. 17, 2009, Community Broadcasters Association president Ron Bruno said four of 32 NTIA-certified boxes had the feature.[6] In late March 2008, the CBA filed a lawsuit in the U.S. Court of Appeals for the District of Columbia, seeking an injunction to halt the sale and distribution of DTV converter boxes,[7] charging that their failure to include analog tuners or analog passthrough violates the All-Channel Receiver Act.[8] CBA maintained that the lack of analog support would seriously harm the LPTV and class A television stations the group represents, as it is cost-prohibitive for many or most of them to convert to digital transmissions,[9] and the new boxes would prevent viewers from being able to watch (or even being aware of the existence of) their analog-only stations.[10][11]

Responding to CBA's actions, the FCC and NTIA urged manufacturers to include the feature voluntarily in all converter boxes, and manufacturers responded by releasing a new generation of models with the feature. Some new DVD recorders and personal video recorders also provide both analog and digital tuners, and therefore could perform the basic functions of a set-top box in both modes.

In early May 2008, the D.C. district court denied the CBA petition without comment,[12] effectively telling the association that it had not exhausted all its efforts, and that there was not enough merit to take the case to the courts. The CBA responded by concentrating its lobbying efforts on the FCC, and by urging more funding for low power and Class A broadcasters to transition to digital, asking Congress to increase the number of such stations eligible for funds.

See also


  1. ^ " FAQ". Archived from the original on 2008-04-24. Retrieved 2008-07-07.
  2. ^[bare URL PDF]
  3. ^ FCC list of significantly-viewed stations, including foreign signals Archived 2008-05-14 at the Wayback Machine
  4. ^ Industry Canada DTV transition (in English) Archived 2008-09-16 at the Wayback Machine
  5. ^ Cofetel (México) digital transition calendar (in Spanish) Archived July 17, 2011, at the Wayback Machine
  6. ^ Eggerton, John (2008-02-18). "Ready or Not, Here Comes DTV". Broadcasting & Cable. Retrieved 2009-09-10.
  7. ^ "LPTV Industry Calls DTV Converter Box Program Into Question". Archived from the original on 2009-01-01. Retrieved 2008-08-25.
  8. ^ "Community Broadcasters Association petitions court to order DTV converter halt". Archived from the original on December 4, 2008.
  9. ^ "Web Page Under Construction".
  10. ^ "Small stations claim digital TV switch will bankrupt them". USA Today. 2008-03-26. Retrieved 2010-04-23.
  11. ^ " Is for Sale".
  12. ^ "T# - TV Tech". TVTechnology. Archived from the original on July 24, 2008.
This page was last edited on 29 July 2022, at 03:01
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