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Bluetooth Audio AAC codec quesitons

I'm not sure what the appropriate forum is for this question, so I will leave it to the mods to move to appropriate forum.

It is a well known fact that Bluetooth audio is unable to transmit music data in a lossless format (By lossless I mean 16/44.1.  16/44.1 is obviously lossy when compared to a 24/96 studio master, even though the two files are transparent).  Because of this, lossy audio codecs are used to compress the data.  SBC is the required codec by the Bluetooth spec, but other optional codecs such as AAC and AptX are supported.

In the world of computer hardware Apple has standardized on AAC as it's preferred codec for Bluetooth Audio.  They do not support AptX or LDAC.  And Android devices also support AAC and SBC in the default Android install and vendors have added AptX and LDAC support to some hardware.

Proponents of AptX claim it has much lower latency, and is therefore superior to AAC, because the use of AAC can cause audio and video to go out of sync.

So, here are my two questions.

If I have a Bluetooth 4.0 headphone connected to a phone or other Bluetooth transmitter, and I know for 100% certianty that both devices have negotiated AAC as their codec of choice, and then I choose to play a file that is already encoded in AAC, will the AAC file "pass through" to the destination devices or will Bluetooth re-encode the AAC stream?

I know when Apple created the iTunes store, they originally released all tracks in AAC, and from what I read they have a license that allows them to use AAC is all their products.  So, going with AAC over another codec was probably a pretty good cost savings for Apple.  Since Apple sells tracks in AAC format, if Bluetooth over AAC just passes through the AAC encoded audio, I tend to think that  this would save on CPU and/or battery.  Is this a possibility?

As for latency, I have NEVER had a video on my iPhone using YouTube, Hulu or Netflix ever go out of sync.  Is Apple somehow compensating for the latency in software, or is the latency somewhat diminished by AAC audio pass-through.

And last question...  If latency is an issue with Bluetooth, why is no one using Opus over Bluetooth?  Isn't one of Opus' goals to be a low latency codec?

Re: Bluetooth Audio AAC codec quesitons

Reply #1
It would depend on the specific program you used to play the audio, but I suspect transcoding is most likely.

Compensating for codec latency is straight forward, so I would not necessarily expect sync problems.

Re: Bluetooth Audio AAC codec quesitons

Reply #2
I tend to think Apple Music on an iPhone talking to an AAC device might pass through.  Especially if you're using a device with Apple's W2 chip in it.

But that' s just a guess on my part.  This kind of information is rather difficult to find.

Re: Bluetooth Audio AAC codec quesitons

Reply #3
Use a Bluetooth client emulator and see if the data sent through the air matches the source file.

Re: Bluetooth Audio AAC codec quesitons

Reply #4
Also, don't these modern devices all need mixers? If you can hear a sound like a notification while music is playing, it's likely not passing through.

Android has supported LDAC and aptX(HD) natively since Oreo, btw, though not the aptX low-latency version.

Re: Bluetooth Audio AAC codec quesitons

Reply #5
Also, don't these modern devices all need mixers? If you can hear a sound like a notification while music is playing, it's likely not passing through.

Android has supported LDAC and aptX(HD) natively since Oreo, btw, though not the aptX low-latency version.

I would assume a phone manufacturer still needs to license AptX or LDAC in order to use the codec.  I have a Google Pixel C tablet, and it always makes an AAC connection to my FiiO uBTR, even though it does AptX.


Re: Bluetooth Audio AAC codec quesitons

Reply #7
Yeah, both LDAC and aptX are in the AOSP, so I don't think licensing is needed for phone manufacturers. It's a pretty smart move, especially for Sony who contributed the code voluntarily IIRC, cause they are the only ones that sell the compatible audio equipment.

Re: Bluetooth Audio AAC codec quesitons

Reply #8
What if a manufacturer doesn't use Sony's hardware?

Re: Bluetooth Audio AAC codec quesitons

Reply #9
Ldac is Sony, aptx is Qualcomm.

Sony makes ldac free-ish to use on Android, but you have to license to use on Bluetooth headphones.

 
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