Sometimes you want the signal from a source component to travel through an intermediary component without alteration. Thus, when a home theater or handheld electronics product is described as having pass-through capability, it means that the audio or video can exit exactly as it entered.
Why would you want pass-through ability? With it you can choose to process a signal—or not. The latest example is receivers with 3D pass-through capability. You can watch 3D movies from your Blu-ray Disc player on your 3D HDTV set with surround sound because the receiver passes the 3D signal through from source player to target TV through HDMI ports that support HDMI 1.4 for 3D. Older receivers may block the 3D constituent in the video data from passing through. In the latter scenario, the viewer’s choice is 3D without surround sound or 2D with surround sound. Or maybe it’s time for a new receiver.
Another type of pass-through on some receivers is standby pass-through. With it you don’t have to power on the receiver and its attached speakers and subwoofer when all you want to do is watch the news using the plasma TV’s own speakers. The feature is also useful for watching a movie when someone is trying to sleep in the next room. While the traditional way of bypassing the receiver is to simply use an extra set of cables that connect directly from, say, the cable box or DVR to your LED or LCD TV, that method eats up an additional set of ports on the TV and forces you to pick up the remote to switch the TV’s inputs.
Some projectors with a built-in speaker offer an audio pass-through feature that lets you take advantage of the speaker for music or talking through a microphone when the projector is in standby mode.
Pass-through capability isn’t limited to home theater components. Some USB hubs that let you attach multiple USB devices offer audio pass-through for headphones, microphones and speakers. They may also offer a couple of USB ports with pass-through power for charging a cell phone or music player even when the connected computer is turned off.
Some devices that let you capture and digitize an analog signal may also be able to pass through the original signal at the same time. That way, for instance, you can digitize a vinyl record or audio cassette while still being able to listen to the music. A place shifter like Slingbox, which typically intercepts the analog video (component video for best quality) and stereo audio from your DVR for retransmission over your home network or the Internet will pass through the program so it can be simultaneously enjoyed on the attached TV.
Similarly, a wireless presentation gateway that lets you simultaneously project PowerPoint slides from a computer to a wall screen while encoding a video stream for the benefit of Wi-Fi-enabled notebook users in the room does so through a pass-through VGA input and output.
The “pass” in “pass-through” is also found in certain noise-cancellation headphones that offer a “passive” mode. What that means is that if the device’s battery dies or falls through a sewer grate, the headphones can still be used, albeit without noise cancellation.
Pass-through has also come to mean something totally pedestrian when used to describe pass-through holes on cases, protectors or external batteries that let you plug in a USB cable or earphones or press buttons when the portable device isn’t bare. The same stretch-logic applies to home theater furniture and A/V racks with pass-through conduits or rear holes that let you pass through wires and cables.