Plex on Shield downmixing multi-channel audio to stereo

Plex Players Streaming Devices
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Server Version#: 1.18.4.2171
Player Version#: Latest

I’m in the UK, recording free to air digital broadcast TV (Freeview) using Plex DVR and a HDHomerun Connect Duo. I have a Samsung UE50MU6120 TV connected to a Marantz NR1506 AVR (using the HDMI ARC port on the TV). My Shield is connected to the Game HDMI port on the AVR.

HD programmes have either ProLogic II audio or Dolby 5.1 audio over AAC_LATM. These playback just fine when using the TV’s Plex client, even switching audio codec mid- session as the recording goes from ads to the show itself.

But when playing back on Plex on the Shield, it always downmixes to stereo. PLII shows as just Stereo on the AVR, and 5.1 as Multi-channel Stereo.

Other sources are fine - DVD and Blu-ray rips play as Dolby Digital. So it only seems to affect anything encoded in AAC_LATM.

Why is this? Why is the TV’s Plex client able to get this right, but the Shield Plex client can’t?

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ProLogic II is not an audio codec. It’s a technology that upconverts stereo to surround.

Dolby 5.1 audio over AAC_LATM

That makes no sense. Those are both different types of codecs.

That also doesn’t make sense. If the DVD or BR has a Dolby audio track, it will play as Dolby. If something is encoded as AAC, the Shield will output it as AAC.

It might be better if you provide a specific example and screenshots showing what you see in Plex on the details page before you start playing and what you see on the AVR while playing.

4

OK, more info …

What I’m seeing is when PMS lists the file as having audio encoded as AC3 5.1, my AVR shows “Dolby D” on the display, whether I play it using the Shield Android Plex client or the TV’s Plex client.

When a PMS is listing the file as having audio encoded as “AAC_LATM Stereo” (which is because it’s been recorded by Plex DVR), the AVR display differs between the Shield Plex client and the TV’s Plex client:

TV:

  • In ads or trailers, before the main programme, AVR shows “PLII”

  • In main programme, either “PLII” or “Dolby D” depending on the programme

Shield:

  • In ads or trailers, before the main programme, AVR shows “Stereo”

  • In main programme, either “Stereo” or “Multi In” depending on the programme

The following link points to an excerpt from the AVR manual, titled “Types of input signals, and corresponding sound modes”: http://manuals.marantz.com/NR1506/NA/EN/GFNFSYwdnzswtk.php

It seems to suggest to me that the AVR is showing “PLII” or “Dolby D” when the input signal is a Bitstream (ie not decoded to PCM), and that it’s showing “Stereo” or “Multi In” when the input signal is PCM.

The Shield Plex client is set to audio passthrough.

It seems to me that what’s happeing is that when decoding the “AAC_LATM Stereo” codec, the Shield Plex client is always doing the decode, ignoring the audio passthrough setting.

File info XML below:

This XML file does not appear to have any style information associated with it. The document tree is shown below.

<

MediaContainer size="1" allowSync="1" identifier="com.plexapp.plugins.library" librarySectionID="6" librarySectionTitle="TV Shows" librarySectionUUID="0c821520-f810-4fd1-a85e-4f28fd227235" mediaTagPrefix="/system/bundle/media/flags/" mediaTagVersion="1577615567">

<Video ratingKey="3586" key="/library/metadata/3586" skipParent="1" grandparentRatingKey="3518" guid="com.plexapp.agents.thetvdb://365172/1/2?lang=en" parentGuid="com.plexapp.agents.thetvdb://365172/1?lang=en" grandparentGuid="com.plexapp.agents.thetvdb://365172?lang=en" type="episode" title="Episode 2" grandparentKey="/library/metadata/3518" librarySectionTitle="TV Shows" librarySectionID="6" librarySectionKey="/library/sections/6" grandparentTitle="Deadwater Fell" parentTitle="Season 1" summary="The residents of Kirkdarroch come out in force for Kate and the girls' funerals. Jess meets an old friend of Kate's and is deeply shocked by what she discovers." index="2" parentIndex="1" viewCount="1" lastViewedAt="1579559355" year="2020" thumb="/library/metadata/3586/thumb/1579298553" art="/library/metadata/3518/art/1579298553" grandparentThumb="/library/metadata/3518/thumb/1579298553" grandparentArt="/library/metadata/3518/art/1579298553" duration="3832645" originallyAvailableAt="2020-01-17" addedAt="1579298526" updatedAt="1579298553">

<Media id="6190" duration="3832645" bitrate="3457" width="1920" height="1080" aspectRatio="1.78" audioChannels="2" audioCodec="aac_latm" videoCodec="h264" videoResolution="1080" container="mpegts" videoFrameRate="PAL" audioProfile="lc" channelIdentifier="104" mediaGrabBeginsAt="1579294800" mediaGrabDevice="device://tv.plex.grabbers.hdhomerun/12401F25" mediaGrabStatus="complete" origin="dvr" videoProfile="high">

<Part accessible="1" exists="1" id="6192" key="/library/parts/6192/1579298524/file.ts" duration="3832645" file="/storage/emulated/0/TV Shows/Deadwater Fell (2020)/Season 01/Deadwater Fell (2020) - S01E02 - Deadwater Fell.ts" size="1803353716" audioProfile="lc" container="mpegts" deepAnalysisVersion="4" packetLength="188" requiredBandwidths="7667,6175,4958,4288,4288,4288,4288,4288" videoProfile="high">

<Stream id="13849" streamType="1" codec="h264" index="0" bitrate="3181" bitDepth="8" chromaLocation="left" chromaSubsampling="4:2:0" codedHeight="1088" codedWidth="1920" colorPrimaries="bt709" colorRange="tv" colorSpace="bt709" colorTrc="bt709" frameRate="25.000" hasScalingMatrix="1" height="1080" level="40" profile="high" refFrames="4" requiredBandwidths="7525,6035,4826,4234,4234,4234,4234,4234" scanType="progressive" streamIdentifier="256" width="1920" displayTitle="1080p (H.264)"/>

<Stream id="13850" streamType="2" selected="1" codec="aac_latm" index="1" channels="2" bitrate="270" language="English" languageCode="eng" audioChannelLayout="stereo" profile="lc" requiredBandwidths="285,282,282,282,282,282,282,282" samplingRate="48000" streamIdentifier="257" displayTitle="English (AAC_LATM Stereo)"/>

<Stream id="13851" streamType="3" codec="dvb_subtitle" index="2" bitrate="7" language="English" languageCode="eng" requiredBandwidths="15,15,15,15,15,15,15,15" streamIdentifier="258" displayTitle="English (DVB_SUBTITLE)"/>

</Part>

</Media>

<Genre id="23286" filter="genre=23286" tag="Other"/>

<Genre id="56" filter="genre=56" tag="Drama"/>

<Extras size="0"> </Extras>

</Video>

</MediaContainer>
5

What you are seeing is normal for AAC audio.

The Shield and the receiver perform a handshake over HDMI, letting each other know what formats they support.

One or the other does not support AAC over HDMI, so the Shield decodes it to PCM.

Decoding AAC to PCM is not a problem. AAC is a lossy codec. PCM is lossless. There will be no loss in quality when decoding AAC to PCM.

When the Shield sends PCM 2.0 you will see “Stereo” on the receiver front panel. When the Shield sends PCM 5.1 you will see “Multi Ch In.”.

I have a Denon receiver, which has basically the same features & functionality as Marantz. I see the same thing with my Shield, Denon receiver, and AAC audio.

Edit / Additional Info:

You can take the Plex client out of the loop.

Load VLC on your Shield and enable audio passthrough in VLC (off by default on mine).

Enable the DLNA server in PMS (you may have to restart PMS after enabling DLNA in the settings).

Play a video with AC3 5.1 / dts 5.1 audio via VLC/DLNA. The Shield sends AC3/dts 5.1 to the receiver.

Play a video with AAC 2.0 audio via VLC/DLNA. The Shield sends PCM 2.0 to the receiver.

Play a video with AAC 5.1/7.1 audio via VLC/DLNA. The Shield sends PCM 5.1/7.1 to the receiver.

6

So I guess the difference between the behaviour of the TV Plex client and the Shield client is that the TV can decode AAC_LATM whereas the AVR can’t.

For a multi channel signal I guess this isn’t a problem - but when a two channel AAC_LATM track is sent as PCM to the AVR, it seems to me that it will be losing the PLII information.

Should (or could) Plex on the Shield decode the PLII signal and send it as a multi channel signal to the AVR?

7

The TV decodes AAC to Dolby Digital because HDMI-ARC does not support multi-channel PCM audio. It is converting from one lossy codec, AAC, to another, AC3. Theoretically information is lost, but you’ll never hear it because of the low bit rates and source quality (TV audio isn’t exactly audiophile level audio).

The Shield decodes AAC to PCM because it has the full HDMI bandwidth and converting to a lossless audio format is preferable to a lossy format.

In theory Yes, but the PLII information may not really be present.

Using the Plex client on my LG TV, playing a video with AAC 2.0 Dolby Pro Logic II audio, the Denon is receiving AC3 2.0 and the front display says “Dolby Surround.”

Playing a video with AAC 2.0 Stereo (no DPLII), the Denon receives AC3 2.0 and the display still says “Dolby Surround.”

The above is with the Sound Mode on the Denon set to Auto. If I change it to Direct, both play as straight stereo, with all audio from the left & right speakers (with “Auto,” the receiver sends dialog to the center channel).

At least with my equipment, there is no way to visibly see if the AAC audio stream is 2.0 stereo or 2.0 with Dolby Pro Logic II matrix information.

It seems like your system is working as intended. You receive 5.1 or 2.0 audio, depending on how the shows are transmitted. The Plex client sends the audio as an AC3 stream, due to limitations of HDMI-ARC. The Shield sends the audio as PCM, since HDMI supports multi-channel PCM and conversion to lossless is preferred to conversion to lossy.

Short of asking the broadcaster, I’m not sure how to verify if the 2.0 audio stream has Dolby Pro Logic II information.

8

dolby pro logic II is a decoding matrix, not an encoding matrix.

DPL2 decodes any dolby (stereo) surround into an expanded surround.

ie DPL2 decodes 2.0 to 5.1 (or however many speakers your receiver is configured for).

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