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.
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.
There are several approaches used with converter boxes that lack analog passthrough:
- Accept the missing analog channel reception.
- Unplug the antenna from the converter box and plug it directly into the TV when watching analog TV.
- Install an A-B switch box to allow selection of whether the signal goes to the converter box (when watching digital TV) or directly to the TV (when watching analog TV). A-B boxes are seldom remote controlled, however.
- Use two different TVs, one with a converter box for digital TV and one without for analog TV.
- Use a splitter before the converter and a frequency-selective combiner (a VHF/UHF splitter/combiner where analog stations are UHF or a channel 3/4 "signal inserter" otherwise) to place converted signals on an unused channel.
- Use a conventional all-female splitter before the converter and a reversed splitter before the television.For pass through, which allows analog TV and fm radio (if the TV also has a radio tuner), plug a standard coaxial patch cable between the two splitters. For dtv, unplug one end of the patch between the splitters, otherwise there will be a hideous signal loss.
- For TVs with two coax antenna inputs, or one coax input and another type of composite video input which matches an output on the converter box, you can use two antennas, one going to the converter box and one going to the additional coax input on the TV, or to not require another antenna, a splitter to direct the antenna to both inputs.
None of these solutions are ideal, however, requiring additional hardware (A-B boxes, splitters, or additional antennae or TVs) or manual intervention to change between digital and analog TV.
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:
- 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.
- Cross-border stations, many of which are significantly viewed in the US, will continue to broadcast in analog format until 2011 from Canada, and 2021 from Mexico. 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, 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. 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, charging that their failure to include analog tuners or analog passthrough violates the All-Channel Receiver Act. 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, and the new boxes would prevent viewers from being able to watch (or even being aware of the existence of) their analog-only stations.