[morgish]

Hi,

I hope this is in the correct section.

I have a Raspberry Pi 2 running OpenELEC and a Yamaha YAS-203B that is connected to my Toshiba TV via SPDIF.

The sound-bar does Dolby and DTS decoding and has previously been working using audio passthrough. In the most recent update of OpenELEC it has broken this.

This has made me question the importance of the sound-bar decoding the audio when it is only 2.1 channels.

So ultimately my question is, is DTS/Dolby decoding or audio passthrough ONLY worthwhile when using surround sound or will you get a improvement in audio quality by letting the sound bar do the decoding?

If there is no benefit to it, why does the sound-bar have the ability to do it?

Cheers,
Kris

[Gothicawakening]

Passthrough seems to be a hotly debated issue in various media player forums and there isn't much consensus.

Here are my thoughts on this, in terms of general multi-channel audio (which also applies to 2 channel):

A Digital audio stream is digital data, unless you decode and process it, its always the same.

Passthrough

Player sends Encoded audio stream to Soundbar
Soundbar decodes that into multiple (1 per channel) digital audio streams.
Soundbar converts the digital streams into analog for output to speakers.
Decoded By: Soundbar
Convert to Analog: Soundbar

No Passthrough

Player decodes Encoded audio stream into multiple (1 per channel) digital audio streams.
Payer sends multiple digital audio streams to Soundbar
Soundbar converts the digital streams into analog for output to speakers.
Decoded By: Player
Convert to Analog: Soundbar

In theory there is no difference between the 2 methods as the only place a difference in sound could be caused is in the DAC (Converts Digital audio into Analog) and both methods are using the DAC in the Soundbar.
I am making the assumption here that both DTS to Digital Multi Channel decoders are producing bit-identical streams.

One thing that could cause some difference is any additional processing the soundbar does with specific audio formats. That means it might switch to some specific mode internally upon detecting a DTS Encoded Stream of audio, but would not do the same when receiving a multi-channel un-encoded audio streams. That could result in some difference in the way the surround is processed. Without knowing the internal workings of the soundbar it's not possible to know if that is actually the case,

If there is no benefit to it, why does the sound-bar have the ability to do it?

Because not all players can decode DTS and Dolby. While KODI does, others may not be able to and therefor passing the encoded stream directly to the Soundbar to decode is the only way for them to play that audio.

[morgish]

Thanks Gothicawakening,

That's excellent and has helped clear things up.

I think I had in my head, that the decoding would also process it.

If I understand what you're saying correctly, decoding is like unboxing a box of 1's and 0's - It doesn't matter if it is unboxed in location A or location B because there is no changing of the 1's and 0's (processing/transcoding).



Makes me a bit confused as to why it's required for surround sound though. Is it because the player doesn't know which channel to send to to where the receiver does?

[jpcolin]

Hi,

What is your advice with a high end AV receiver?
I have always enabled passtrough, never had problems but if i can improve things..
Does the decoding by the Rpi use some processing power?

thanks

[Gothicawakening]

If I understand what you're saying correctly, decoding is like unboxing a box of 1's and 0's

Not quite. It's converting it from an encoded format to an un-encoded one. Think of the encoded one like a JPEG, and the unencoded one like a plain bitmap. The JPEG is much smaller than the bitmap because of the encoding (or compression if you will), same for Audio. It would be impracticable to have 8 uncompressed audio streams stored on Blurays for each language.

In this instance, you could use Decoding / Uncompressing to mean the same thing although in reality there is more too it.

It doesn't matter if it is unboxed in location A or location B because there is no changing of the 1's and 0's (processing/transcoding).

In theory and for what we are talking about that is correct. As mentioned, some hardware may do additional processing beyond the decoding of the audio which could introduce some differences.

The encoded streams actually contain more than just audio, they also include control data / flags that do things like specify format information etc. This metadata is lost after decoding, so for example if you were going to an AV Receiver the DTS icon might not come on if the player is doing the decoding because the receiver is getting plain uncompressed audio instead of DTS.

Makes me a bit confused as to why it's required for surround sound though. Is it because the player doesn't know which channel to send to to where the receiver does?

Because formats like DTS, TrueHD etc. are all different and require complex code to decode them into plain, multi-channel audio. My old Yamaha amp can't decode DTS-HD, but KODi can and then sends it to the amp as uncompressed audio (which is does understand).

What is your advice with a high end AV receiver?
Does the decoding by the Rpi use some processing power?

Yes, the RPi would need processing power to decode the audio, in some cases this could be significant. With passthrough the RPi doesn't need to do that.
For that reason, if passthrough is working it's best to use it.

One added (visual) benefit of passthrough is that the little DTS / Dolby indicator on your AVR will light up!

Sound quality should be exactly the same (again, making the assumption no actual processing is being done by either the player or AVR when decoding).

[morgish]

Thanks Gothicawakening,

Appreciate you taking the time.

I must be missing something though...

Say I have a full 7.1 surround system. If Kodi can do the decoding; apart from the processing power being taken away from the CPU and having the DTS logo/light on your receiver, what is the benefit to using passthrough?

[Gothicawakening]

Thanks Gothicawakening,

Appreciate you taking the time.

I must be missing something though...

Say I have a full 7.1 surround system. If Kodi can do the decoding; apart from the processing power being taken away from the CPU and having the DTS logo/light on your receiver, what is the benefit to using passthrough?

None.

.. in most cases. KODI can't decode Dolby Atmos or DTS:X yet where the amp might be able to if it's new.

[maxricardo]

Great explanantion Gothicawakening, it has been clarified most of my doubts.

One thing that is odd that I noticed when tested the audio passthrough option in my laptop versus a cheap Rp:
(1) My Dell laptop is running Windows and it is a i7 - 8Gb RAM - 150 GB SSD
(2) My cheap Rpi (because it is a chinese TV box running Android) but it is a dual-core with 2Gb RAM and 4Gb disk

Believe or not (!!):
- If a run in my (1) using audio passthrough (thus, the audio processing occurs in laptop) it works as expected an nicely.
- BUT, if a run in my (2) using audio passthrough (thus, the audio processing does NOT occurs in Rpi) the sound chokes all the time (every 1 sec) and terribly disturb the entire movie watching. Incredibly, if I turn OFF the audio passthrough (to start processing inside Rpi instead of A/V) then it works like a charm.

My A/V Receiver is an Onkyo HST5600 with 7.1 and has a strong sound system which is perfect in my (1), SmartTV, Bluetooth and others.

I am even considering to buy a pretty good Rpi (4Gb-8Gb RAM and 500 SSD) but I am not sure if the same problem may occur. Perhaps, it is an issue with Android OS ?

Any ideas to share ?

Thanks in advance !

[ebr9999]

This has made me question the importance of the sound-bar decoding the audio when it is only 2.1 channels.

I don't know about digital coding being 2.1 .... I expect the sound-bar receiving 2 channel digital stream and then sending the low frequency part of the sound to its sub (and eventually adding some customizable processing). The only difference you can have if you use the analog input of your sound-bar (double DAC conversion quite unlikely improving sound quality)

Anyhow, if your point is that sound comes to sound-bar in 2 channel stream, due to some conversion of the native 5.1/7.1 stream, that is negative. The sound-bar needs to receive 5.1/7.1 to apply its processing to add the spatial sounding typical of the rear and surround speakers.

PS: In case of 5.1/7.1 the x.1 indicated the LFE, which is usually send the sub mixed with bass part (the last with some processing typical of the sound-bar).

[ebr9999]

Great explanantion Gothicawakening, it has been clarified most of my doubts.

One thing that is odd that I noticed when tested the audio passthrough option in my laptop versus a cheap Rp:
(1) My Dell laptop is running Windows and it is a i7 - 8Gb RAM - 150 GB SSD
(2) My cheap Rpi (because it is a chinese TV box running Android) but it is a dual-core with 2Gb RAM and 4Gb disk

Believe or not (!!):
- If a run in my (1) using audio passthrough (thus, the audio processing occurs in laptop) it works as expected an nicely.
- BUT, if a run in my (2) using audio passthrough (thus, the audio processing does NOT occurs in Rpi) the sound chokes all the time (every 1 sec) and terribly disturb the entire movie watching. Incredibly, if I turn OFF the audio passthrough (to start processing inside Rpi instead of A/V) then it works like a charm.

My A/V Receiver is an Onkyo HST5600 with 7.1 and has a strong sound system which is perfect in my (1), SmartTV, Bluetooth and others.

I am even considering to buy a pretty good Rpi (4Gb-8Gb RAM and 500 SSD) but I am not sure if the same problem may occur. Perhaps, it is an issue with Android OS ?

Any ideas to share ?

Thanks in advance !

Passthrough may require more bit rate on the digital interface in comparison to PCM encoding (48KHz/16) usually used in conversion. In case of (2) what physical interface are you using? If HDMI, also cable may explain your problem.

[maxricardo]

Great explanantion Gothicawakening, it has been clarified most of my doubts.

One thing that is odd that I noticed when tested the audio passthrough option in my laptop versus a cheap Rp:
(1) My Dell laptop is running Windows and it is a i7 - 8Gb RAM - 150 GB SSD
(2) My cheap Rpi (because it is a chinese TV box running Android) but it is a dual-core with 2Gb RAM and 4Gb disk

Believe or not (!!):
- If a run in my (1) using audio passthrough (thus, the audio processing occurs in laptop) it works as expected an nicely.
- BUT, if a run in my (2) using audio passthrough (thus, the audio processing does NOT occurs in Rpi) the sound chokes all the time (every 1 sec) and terribly disturb the entire movie watching. Incredibly, if I turn OFF the audio passthrough (to start processing inside Rpi instead of A/V) then it works like a charm.

My A/V Receiver is an Onkyo HST5600 with 7.1 and has a strong sound system which is perfect in my (1), SmartTV, Bluetooth and others.

I am even considering to buy a pretty good Rpi (4Gb-8Gb RAM and 500 SSD) but I am not sure if the same problem may occur. Perhaps, it is an issue with Android OS ?

Any ideas to share ?

Thanks in advance !

Passthrough may require more bit rate on the digital interface in comparison to PCM encoding (48KHz/16) usually used in conversion. In case of (2) what physical interface are you using? If HDMI, also cable may explain your problem.

I am using HDMI interface and I used the same cable. in both devices the difference is pretty audible.