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iPhone AAC Bluetooth bottleneck and transcoding

Discussion in 'Portable Source Gear' started by neil74, May 8, 2018.
  1. neil74
    The one thing that really grates for me with iOS is that if I want use Bluetooth headphones on my iPhone the quality is compromised vs. other devices. I think that BT is pretty much there now quality wise (for most uses) but I’m not sure that this is the case on iOS as it is still constrained by AAC which is capped at around 250 kbps, whereas for example android now has aptx-hd or LDAC

    Spotify aside BT audio on iOS does sound just fine but it is that sense of compromise I am making that nags at me, but the interweb is not really clear as to exactly what happens. My assumption is that any music that you play will be transcoded to circa 250 kbps and aac before being passed to your headphones. So for example Spotify will be transcoded from vorbis and 320 kbps to AAC and 250 kbps, whereas say tidal premium (which is already AAC) will just have it's bitrate shaved down from 320 to 256 kbps. Either way unless you use apple music (or maybe Amazon too?) Your files will be degraded. Some may argue that you are unlikely to notice but I think for Spotify is a good example in that it imo sounds noticeably worse on iOS than android (imo)

    Does this all sound about right or am I missing something?
  2. joamlt22
    I think Spotify song files has it's own propriety codec and its decoded by spotify app in your iphone to PCM format.
    Then this PCM data is sent via bluetooth over AAC protocol to an AAC capable bluetooth receiver.
    If your headphones is not AAC capable, your iphone automatically switches to SBC protocol then sends the data to your bluetooth receiver.
    So, you are missing a lot actually.
  3. dcfac73
    I heard Spotify uses Ogg Vorbis.

    I think the only thing holding back bt at this stage is the quality of the receiving dac/amp, but only If the source files are high bit lossy or better that is. If they can squeeze so much quality and driving power into a Dragonfly Red, then surely we're not far off a truly audiophile bt headphone.
  4. neil74
    It may be placebo but I am sure that Bluetooth audio on my iPhone X is not as good as on my S9+. I did some non-scientific tests at the weekend both using Spotify and also local files (FLAC converted to 320 kbps AAC and also 500 kbps OPUS) and for all cases the S9 sounded better. FYI the headphones tested are Sony WH10000xmk2, Wi1000x, WF1000x.

    I do not think this is the case for all android phones, the V30 for example imo sounds lack lustre over Bluetooth even if it does support the same codecs.

    I have been leaning towards switching back to iOS as my daily driver but knowing that the audio is just not as good is just stopping me right now
  5. dcfac73
    All sources will sound the same over bt. Arguably codecs may be discernable, but the only variable soundwise will be on the receiving end, ie the headphones.
  6. neil74
    I am prepared to be corrected but I did not think that was the case and I was of the view that the way BT AAC and APTX HD or LDAC work was different. My understanding is as follows:

    AAC – Is re-encoded before being passed to headphones and then will of course be processed by the DAC in the headphones

    LDAC/APTX-HD – Passed as-is to the headphones and the processed by the DAC

    So for LDAC/APTX-HD a 328kbps Spotify file will be passed at it’s full bitrate as will the 500 kbps Opus file. FLAC is slightly more complex so not sure what would happen there?

    So if the above is correct then on an iPhone anything above 256 kbps is a waste over BT whereas some android devices have way more headroom. On an iPhone though it does seem that Spotify or any other non- AAC service is compromised, maybe why a lot of reviews say that Apple music has the best audio quality??
  7. dcfac73
    Whether any of this is perceptable to human hearing is unlikely unless we're talking about low bitrate lossy source files. Stick with 256 kb and above and you should be fine.
    Koolpep likes this.
  8. Koolpep
    I found AAC from my iPhone to be on par with AptX HD from my Android when playing the same song. One benefit is when your files are already AAC that they will be transparently coded - meaning they are not loosing any quality from the re-encoding. As far as I understand AptX HD always requires re-encoding. Anyhow - maybe check out this thread:
  9. neil74
    With my headphones and S9 it defaults to LDAC rather than aptx-hd.
  10. Koolpep
    Which is what you would want - it's the best wireless codec in existence currently.
  11. neil74
    I think I have mentioned this in other threads. If using a 256 kbps AAC source file then the quality should be as good on an iPhone, my issue is if you want to use a non-AAC source file on an iPhone or a AAC source file with a higher bitrate than 256kbps (320 kbps or 512 for AAC FDK is possible)

    So as an example Spotify and it's vorbis files or a high bitrate local Opus will I guess sound better over LDAC on an android than iOS?
  12. Koolpep
    I think that is a fair assumption.

  13. perky2
    I believe the 256 Kbps AAC source will also sound worse on the iPhone as the AAC file has to be recompressed to allow overlay of system sounds etc. so it undergoes double compression. Happy to be proved wrong but that’s the only way it can overlay key clicks etc.
  14. Koolpep
    AAC can be transcoded to AAC without quality loss (with the same Bitrate) In tests it took around 100 reencodes of the file’s copy file etc. to finally start deteriorating - it’s one of the benefits of aac.... have to check to find the source where I read this. Anyhow even if aac is going through: aac to pcm to aac at the same aac Bitrate the information that aac wants to remove is already removed. AAC is quite clever that way.

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