Call it the "cascading upgrade effect" (CUE). Every time some newfangled home theater format comes along, like high-definition, 3D, 4K or high dynamic range, you have to upgrade everything associated with it. Not only do you need a new TV, but you also need a new source device, sound bar or AV receiver.

The Roku Ultra provides relief from CUE. It's a source device that serves up the latest 4K and HDR video streams from Netflix, Amazon and more, yet it lets you keep your current not-quite-brand-new audio system intact. The secret? Its optical digital audio output.

Most of today's 4K streaming devices, including the less expensive Roku Premiere and Premiere+, the Google Chromecast Ultra, the Nvidia Shield and the Xiaomi Mi Box, lack that optical output. Instead they pass audio via HDMI. That's fine if you're connecting them directly to the TV, but many people use an AV receiver or sound bar for switching. If you'd rather not upgrade your receiver or bar to a current model that passes 4K and HDR, the Ultra's digital audio output is a great thing to have.

You may already own other 4K streaming options, such as a 4K game console, 4K Blu-ray player or the smart TV system built into your TV (many of which also have optical outputs that help you avoid CUE). If you're fine using those for now, feel free to skip the Ultra. On the other hand, no other single device can match Roku's selection of 4K and HDR apps and services, and no smart TV system is as simple to use or updated as regularly.

Long story short? For people with the right combination of gear, the Ultra is our top choice in 4K streamers.

The ultimate Ultra FAQ

So what's 4K HDR streaming anyway? New here, eh? No problem. Many internet video services, including Netflix, Amazon Video, Vudu and YouTube, stream some of their TV shows and movies in 4K resolution, which promises higher video quality than their other streams. A few of those services offer an even smaller amount of content in high dynamic range (HDR), promising even better quality -- higher contrast, more realistic colors and other improvements. We say "promise" for a reason: often the differences are tough to discern, even for trained eyes like ours.

The Ultra can also serve up lower-quality streams too, and it can access all of the thousands of apps that any other Roku can. Most apps, including heavy hitters like Hulu, HBO Now/Go, Watch ESPN and Sling TV don't offer 4K or HDR yet, or restrict it to certain devices. Historically Roku gets 4K and HDR streams before many other devices, but there are always exceptions. Hulu's 4K, for example, is currently restricted to the newest game consoles.

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Should I get it if I don't have a 4K HDR TV? No. Unless you anticipate buying a new 4K HDR TV very soon, my advice is to get the Roku Streaming Stick or another non-4K device and save the money. If your TV has 4K but not HDR, the Roku Premiere (without the "+") might be worth getting instead.

Why shouldn't I just stick with my smart TV system? You can, but it might be annoying. Every 4K HDR TV we've seen has apps that support 4K and/or HDR. Depending on the TV you have, and what services you enjoy, you might be perfectly fine streaming without an external box. On the other hand, Roku in particular has more streaming apps that offer 4K, HDR and standard video streams, and makes those apps and streams easier to find and use. It's also updated more often than most smart TVs, and provides a single, convenient source for all your internet video.

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Why do you like Ultra better than the competition? The main reason is the selection of 4K HDR apps. As of January 2017, Roku devices like the Ultra offer 4K from 17 apps: Netflix, Amazon, YouTube, Vudu, Plex, Roku Media Player, Fandango Now, UltraFlix4K, 4K Universe, Curiosity Stream, Toon Goggles, Tastemade, Smithsonian Earth, ifood.TV, Picasa, Flickr and 500px. That's more than any other streaming device. Roku also streams HDR from Netflix and Amazon, two of the three biggest services that offer it today. (The third, Vudu, doesn't yet deliver HDR streams to Roku, only to smart TV systems with Dolby Vision.)

How does it compare to the Roku Premiere or Roku Premiere+? All offer 4K video and have the same apps and processor (so they all operate equally fast) but there are some major features differences.

The $130 Ultra has all of the following features:

Compatibility with both 4K and HDR streams*

Remote: Point-anywhere instead of having to aim, allowing you to stash the box out of sight*

Remote: Connect headphones for private listening*

Remote: Finder function to locate the clicker if it goes missing

Remote: Search via voice.

Connectivity: microSD card slot that (when you insert said card) can help apps load faster*

Connectivity: Ethernet port for wired connections, which can be more stable than Wi-Fi in some situations*

Connectivity: USB port for playback of video files from connected drives

Connectivity: Optical digital audio output

*Feature also available on Premiere+ ($100) but not on Premiere ($80).

Between the three our favorite is the Premiere+, and we recommend it over the Ultra for most people. But if you really want the Ultra's extras, particularly the optical digital output, it's worth the extra $30.

We only recommend the $80 Premiere to people who own a 4K TV that lacks HDR compatibility, and even for those people the Premiere+'s extras might be worth it.

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Will it work with my AV receiver or sound bar? Yes. The Ultra is the only current 4K Roku device to have an optical output, so it can pass surround soundtracks to any device with a matching input. With the Premiere+ and Premiere, your receiver or sound bar has to pass 4K and/or HDR signals via HDMI, and most such devices sold before 2016 do not. See below for more details.

Does it support both HDR10 and Dolby Vision formats? No, it only supports HDR10. Of course many HDR TVs today, namely those from Samsung and Sony, don't support Dolby Vision either, so if you own one of those, you're not missing anything with the Ultra or other HDR10-only devices.