Analog passthrough is a feature found on some
digital-to-analog television converter boxes. Boxes without the analog passthrough feature only allow older, analog-only TVs to view digital TV. Those with analog pass-through allow both digital and analog television to be viewed on older TVs.
Before digital television, passthrough originally existed in
VCRs (and later
PVRs and
DVDRs) that connected to a TV set using
RF connector
An RF connector (radio frequency connector) is an electrical connector designed to work at radio frequencies in the multi-megahertz range.
RF connectors are typically used with coaxial cables and are designed to maintain the shielding that t ...
, allowing the
TV antenna
A television antenna, also called a television aerial (in British English), is an antenna specifically designed for use with a television receiver (TV) to receive terrestrial over-the-air (OTA) broadcast television signals from a television st ...
or
cable TV
Cable television is a system of delivering television broadcast programming, programming to consumers via radio frequency (RF) signals transmitted through coaxial cables, or in more recent systems, light pulses through fibre-optic cables. This ...
signal to be switched to pass through the VCR, or have VCR output added as an extra channel.
The problem
All digital-to-analog converter boxes have both an antenna input (which accepts the
coaxial cable
Coaxial cable, or coax (pronounced ), is a type of electrical cable consisting of an inner Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...
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 on 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
A broadcast relay station, also known as a satellite station, relay transmitter, broadcast translator (U.S.), re-broadcaster (Canada), repeater (two-way radio) or complementary station (Mexico), is a broadcast transmitter which repeats (or tran ...
stations, as these are exempt from the
FCC mandate to
switch
In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...
to
digital broadcasting
Digital broadcasting is the practice of using digital signals rather than analogue signals for broadcasting over radio frequency bands (radio broadcasting). Digital television broadcasting (especially satellite television) is widespread. Digital ...
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
Radio broadcasting is the broadcasting of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based ra ...
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
FM broadcasting is a method of radio broadcasting that uses frequency modulation (FM) of the radio broadcast carrier wave. Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to transmit high fidelity, high-f ...
signals.
The solution
Some
converter boxes offer analog pass-through, meaning that the same output
cable
Cable may refer to:
Mechanical
* Nautical cable, an assembly of three or more ropes woven against the weave of the ropes, rendering it virtually waterproof
* Wire rope, a type of rope that consists of several strands of metal wire laid into a hel ...
which carries the converted
digital
Digital usually refers to something using discrete digits, often binary digits.
Businesses
*Digital bank, a form of financial institution
*Digital Equipment Corporation (DEC) or Digital, a computer company
*Digital Research (DR or DRI), a software ...
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.
Analog pass-through signals are passed only 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.
Some VCRs and TVs have receivers that detect active composite video or S-video connections. When they detect these connections, they are designed to accept signals only from those sources, which can cause issues with the described arrangement. 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. This eliminates 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.
Unfortunately, only one of the initially available converter boxes included this feature, and a year before the original transition date of February 17, 2009,
Community Broadcasters Association president Ron Bruno said four of the 32 NTIA-certified boxes had the feature.
In late March 2008, the CBA filed a
lawsuit
A lawsuit is a proceeding by one or more parties (the plaintiff or claimant) against one or more parties (the defendant) in a civil court of law. The archaic term "suit in law" is found in only a small number of laws still in effect today ...
in the
U.S. Court of Appeals for the District of Columbia
The United States Court of Appeals for the District of Columbia Circuit (in case citations, D.C. Cir.) is one of the thirteen United States Courts of Appeals. It has the smallest geographical jurisdiction of any of the U.S. courts of appeals, ...
, seeking an
injunction
An injunction is an equitable remedy in the form of a special court order compelling a party to do or refrain from doing certain acts. It was developed by the English courts of equity but its origins go back to Roman law and the equitable rem ...
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
The All-Channel Receiver Act of 1962 (ACRA) (), commonly known as the All-Channels Act, was passed by the United States Congress in 1961, to allow the Federal Communications Commission to require that all television set manufacturers must incl ...
. CBA maintained that the lack of analog support would seriously harm the
LPTV
Low-power broadcasting is broadcasting by a broadcast station at a low transmitter power output to a smaller service area than "full power" stations within the same region. It is often distinguished from "micropower broadcasting" (more commonl ...
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.
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 recorder
A DVD recorder is an optical disc recorder that uses optical disc recording technologies to digital recording, digitally record analog signal, analog or Digital signal (signal processing), digital signals onto blank writable DVD recordable, D ...
s and
personal video recorder
A digital video recorder (DVR), also referred to as a personal video recorder (PVR) particularly in Canadian English, Canadian and British English, is an electronic device that records video in a digital format to a disk drive, USB flash drive, S ...
s also provide both analog and digital tuners, and could therefore 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.
The court's decision implied that the association had not exhausted all its efforts. Furthermore, the court believed there wasn't enough merit to bring the case forward. 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
*
All-Channel Receiver Act
The All-Channel Receiver Act of 1962 (ACRA) (), commonly known as the All-Channels Act, was passed by the United States Congress in 1961, to allow the Federal Communications Commission to require that all television set manufacturers must incl ...
*
Composite video
Composite video, also known as CVBS (composite video baseband signal or color, video, blanking and sync), is an analog video format that combines image information—such as brightness (luminance), color (chrominance), and synchronization, int ...
and
S-video
S-Video (also known as separate video, Y/C, and erroneously Super-Video) is an analog video signal format that carries standard-definition video, typically at 525 lines or 625 lines. It encodes video luma and chrominance on two separate chann ...
*
Set-top box
A set-top box (STB), also known as a cable converter box, cable box, receiver, or simply box, and historically television decoder or a converter, is an information appliance device that generally contains a Tuner (radio)#Television, TV tuner inpu ...
and
coupon-eligible converter box
A coupon-eligible converter box (CECB) was a digital television adapter that met eligibility specifications for subsidy "coupons" from the United States government. The subsidy program was enacted to provide terrestrial television viewers with ...
References
{{DEFAULTSORT:Analog Passthrough
Analog circuits
Television technology