AAC 5.1 being down converted to PCM 2.0 by Roku Ultra



I have a Plex server on my Synology NAS and when I try to play back movies the AAC 5.1 audio (from Handbrake) is being down converted to stereo. Netflix plays Dolby Digital just fine so I know the audio set up is fine.

I thought the Ultra was supposed to transcode AAC 5.1 to AC3 5.1. I believe all my settings are correct on the Roku (which is attached to Yamaha AVR by HDMI).

Also, if I encode the movies as AC3 they AVR plays the Dolby Digital 5.1 just fine. Do I need to redo all my movies or is there hope?


AAC 5.1 is not a standard Roku compatible format.
In actuality, it’s not really a standard for many hardware items. AC3(DD), EAC3(DD+), DTS 5.1, sure, pretty standard.

Reference Roku SDK
Each “A” below is a direct quote of particular line from website in response to your questions

Q: I thought the Ultra was supposed to transcode AAC 5.1 to AC3 5.1

A - 1 Multichannel AAC is not supported on all Roku models. Roku TVs, Roku 4, and Roku Ultra set-top-boxes support multichannel decode to PCM stereo.

Q - Do I need to redo all my movies or is there hope?

A - With the Roku Ultra, Dolby Audio is natively integrated. Roku recommends encoding in Dolby Digital Plus instead of Dolby Digital.

That being said, keep your shared users and other devices in mind.
Though Handbrake has default settings for Roku’s now, I still use settings from www.rokoding.com
Personally for my own audio, and because I share with friends/family with older devices, I use two audio streams in my videos. Sure, takes up more space per file, but, cpu horsepower is more expensive than storage space for offering direct play compatible audio streams.
Yes, can take up some space, but, ensures compatibility and direct play.
Audio 1 - AAC - 192kbps (could go higher, but most people I know are using 2 channel on a crappy tv speaker or mobile headphones) - Dolby ProLogic II
Audio 2 - AC3 - 640kbps (unless source is lower (aka many DVDs use 448) - 5.1 channels


Thanks for clearing that up. I went down the AAC path because I read the following. They should probably delete that second sentence.

From the Roku support site I read (https://support.roku.com/article/208754908-how-to-use-roku-media-player-to-play-your-videos-music-and-photos)
“Roku TVs and some Roku players decode 5.1 AAC to PCM stereo for headphone, internal speaker, and HDMI/ S/PDIF (optical) output. However, Roku devices that support Dolby Audio decode connected via HDMI or S/PDIF (optical) to an A/V receiver that also supports Dolby Audio, will convert the audio to Dolby Digital 5.1.”

Does anyone know if there are any plans to support transcoding to Dolby in a future firmware release?


@pdxfisher - Roku is as clear about it’s roadmap as Plex is
(ie, no clue, and no one here would know)

That being said, the quote you find is intriguing, and indeed goes against the SDK page.
Definitely odd
Not only that, I 90% sure SPDIF optical is not bidirectional. Sender=>Receiver - how would it know the receiver is capable of Dolby, allowing it to transcode. Most (all?) versions of HDMI are capable of bi-directional - which allows it to do some great stuff (like auto capability checking)
It’s possible it might decode if you were using HDMI.
Considering testing by buying a new HD receiver capable of HDMI would be expensive, have a friend/neighbor with good wi-fi you could take your Roku to, log into your Plex server remotely, and test? :slight_smile:


I am connected via HDMI and the Roku does DD+ for Netflix so it knows the capabilities of the AVR. I think your original comment saying that the Roku just does not do it (at this time) is the correct prognosis. I plan on using ffmpeg to convert the audio of my library. I read it only takes about 5 minutes per file - I will test that tonight. Thanks again for your comments and insight.


There are two versions of the Roku Ultra. The first version has an optical out port and includes the Dolby decode capability. The second version does not have the optical port, nor does it have the Dolby decode capability.
The Ultra with Dolby will transcode AAC multichannel to Dolby 5.1 before sending it the optical and hdmi port if the AVR or TV supports Dolby.
The Ultra WITHOUT Dolby will decode multichannel AAC and DOWNMIX to stereo pcm and then send that out the hdmi port. This is the same as what the Roku-4 does.


Thanks dluck! I am sans optical but fortunately converting from AAC to AC3 was quick and easy with ffmpeg. Probably lost a small amount of quality but probably not an issue for my old ears.


Good to know info @dluck - 5-6 years ago, Roku wasn’t as bad as some companies were about releasing different models bearing the same ‘group’ name. The models were reasonably delineated. From what you describe, and the small bit of research I did attempting to answer @pdxfisher, it seems they’re getting worse.


Hey all. We have identified the issue and added a workaround to correct it until we can get confirmation that this will be fixed in the firmware.

The Roku firmware should be able to downmix AC3 5.1ch audio to 2ch in stereo environments, and convert AAC 5.1ch to AC3 5.1ch to preserve surround sound in surround sound environments. This however is not the case, so in the interim we will now use the Plex Media Server to transcode the incompatible audio.

The update is available via our Plex Pass channel. We will be submitting this to Roku to get our official channel updated, but that will take some time as it has to go through the review certification process.


I’ve been having the same issue. I bought an Ultra–the second version (4660X running 8.0.1 build 4047-46)–back in December. I had upgraded my home theater, and my old Roku 3 was not parsing the AAC 5.1, and also could not do 4K. After researching on the internet, I was lead to believe the Ultra would pass AAC 5.1 to my receiver–whether it be the first or second version of the Ultra. And, for about two months, it WAS passing AAC 5.1 to my receiver (Marantz NR1608), to be decoded as DD 5.1. All was good. About 3 weeks ago, it started only passing 2.0 PCM. As far as I could tell, there had not been any updates to the Roku firmware, receiver firmware, or the Roku App, but something definitely changed. After 3 weeks of frustrating nothingness from Roku support, I actually bought another Roku Ultra today, thinking it was simply a bad Roku–nope–same issue with the new one. So, I went back on-line, again, to search for answers, and came across this thread. Anyway, I just downloaded the updated Plex channel linked above by ljunkie, and it worked immediately–my AAC 5.1 content once again resolves as 5.1 DD on my receiver–yea!

Big props to Plex for acknowledging and fixing this issue so quickly. This is why I recommend Plex to everyone I know, and also why I upgraded to Plex Pass, even though I don’t use any of the advanced features–I just felt it was necessary to support Plex for their unique and robust product. I used to also recommend Roku to everyone, but now I would think twice about that. I mean, really, my 3-year old Chromecast that I pulled out of my junk drawer was able to resolve the AAC audio to 5.1 (or at least pass it to my receiver to resolve), and my brand new Roku Ultra could not–ridiculous!

Edit: In case it helps either Roku or Plex figure out when this changed, I called Roku support on 2/9, and my system was working 1 or 2 days before that, so it stopped working about 2/7-2/9.