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Here's the scenario: You bought a new TV and you're planning to use it as an AV switcher for a sound bar. It's a smart move, as most sound bars lack HDMI inputs, so you can simply connect all your gear to your TV via HDMI, and connect your TV's optical output to the sound bar.

The only problem is that most TVs dumb down incoming surround-sound audio signals to a stereo PCM signal. If you're using a sound bar that can take advantage of a true surround sound signal -- such as the Sonos Playbar and Vizio S4251w-B4 -- your TV might be preventing you from getting the best sonic performance.

Thankfully some TVs do preserve Dolby Digital surround sound signals, and we set out to get a better idea of how different models perform. Here's what we found.

The results: Sony, Toshiba and Vizio are 5.1-friendly
In all, we tested 20 televisions -- mostly from 2013 -- from a variety of brands, including LG, Samsung, Sharp, Sony, Panasonic, Toshiba, and TCL. Interestingly, we found that a TV's ability to throughput 5.1 audio depended on the manufacturer, rather than the individual model or price.

If you're looking for a TV that can pass a true 5.1 signal, then based on our results you'd be smart to stick to Sony, Toshiba, and Vizio models. Regardless of cost, all of the televisions from these three manufacturers that we examined were able to pass-through Dolby Digital 5.1 signals via their optical output. And conversely, don't expect a high-end TV from other manufacturers to pass a true 5.1 signal just because it's expensive. Samsung's $2,3000 UN55F8000 fared no better than the company's $400 UN32EH4000F when it came to this test.

How we tested and caveats
To test each TV's ability to pass-through 5.1 signals, we used an Oppo BDP-105 player set to output audio in bitstream format. We hooked the Oppo player up to each television via the HDMI port, and then connected the optical-out from the TV to a Denon AVR-1912 receiver. The receiver is capable of displaying a detailed description of which audio format it was receiving via its onscreen menus.

We tested with a DVD ('Ride, Rise and Roar') and two Blu-rays ('The Golden Compass' and 'Pride and Glory'), which allowed us to check every major format: Dolby Digital stereo, DTS Stereo, Dolby Digital 5.1, Dolby TrueHD, and DTS-HD MA 5.1.

A few things stood our from our testing. One is that DTS signals were downconverted to stereo PCM by every TV. So even in the best case scenario, TVs will downconvert surround sound to stereo for movies that only include a DTS soundtrack.

Biggie smalls total eclipse. We also found that regardless of the type of Dolby soundtrack, pass-through was always limited to Dolby 5.1/48kHz. That means you that you won't be able to get true 'high-res' TrueHD when using your TV as a switcher, although the difference between those Dolby 5.1 and Dolby TrueHD would typically be tough to discern on a sound bar.

We also found that some models weren't clear on how they handled pass-through audio. For example, while the TCL purports to be able to transmit Dolby signals (via a menu option), it always output a stereo PCM 48kHz signal, no matter what signal we fed it. Similarly, the LG 8600 offers the option for Auto or PCM for Digital sound output, but both options revert to PCM. Lastly, it's worth mentioning that in order to enable 5.1 output on a Vizio, you have to change the 'Digital Audio output' option to Dolby Digital; it doesn't pass-through 5.1 signals properly by default.

To be clear, we were only testing these TVs to see how they handled incoming Dolby Digital signals. Although we didn't explicitly test it, most TVs are capable of outputting true Dolby Digital via their optical output when using the TV's internal over-the-air tuner.

Takeaways: 5.1 pass-through support is spotty for now
The main takeaway from our testing is that support for 5.1 audio pass-through on TVs is far from widespread. And even if you have a TV from Sony, Vizio or Toshiba, most of the TVs we tested were from this year, so there's no guarantee that, for example, a Vizio TV you bought two years ago will pass the test.

On the other hand, the ability to pass a true Dolby Digital surround-sound signal only matters with a handful of sound bars that either offer true surround sound -- like the Vizio S4251w-B4 -- or use the surround signal to create virtual surround sound, like the Sonos Playbar and Yamaha's YSP sound bars. We've found most stereo sound bars don't sound dramatically different with Dolby vs. PCM signals, once you make sure they're playing at the same volume level.

Hopefully as more sound bars start to offer true surround sound, more TV manufacturers will support true 5.1 pass-through audio. We'll be keeping an eye on this issue when we get a look at the new slate of TV offerings at CES 2014.

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