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Is bluetooth aptx perfect for 320kbps mp3?

Discussion in 'Sound Science' started by Harold999, Dec 26, 2019.
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  1. Harold999
    Bluetooth Aptx is 352kbps, which seems perfect/sufficient for streaming 320kbps mp3.
    Is this correct or will Aptx-HD or Ldac offer more benefits in case of mp3 sound?
    Last edited: Dec 26, 2019
  2. castleofargh Contributor
    The idea you maybe have, that the higher bitrate means a 320kbps mp3 file could be sent untouched, is wrong. Not because of a lack of bitrate but because aptx has its own audio codec. The mp3 is decoded to pcm and encoded back to aptx.
    Aptx might not be as transparent as mp3 or some other codecs at the same bitrate because it's not based on auditory masking(which is IMO a strange choice). So there may be some audible benefits to higher fidelity codecs, even to play mp3 sources.
    Then again, 350kbps is already a lot for a lossy codec, even a bad one.
    I have no personal opinion on this, as I use BT headphones only in situations where sound quality is really the least of my problems. So I never bothered testing anything seriously. Others might have more confidence in their subjective experiences.
  3. Harold999
    I said mp3 but that wasn't correct, Tidal and Spotify use AAC and Vorbis. But this doesn't change the question i understand.
    All these will be decoded to pcm and then to aptx?
    I think it's best then to use Tidal HIFI as a source which is pcm already. One conversion less.
    Last edited: Dec 26, 2019
  4. bigshot
    Castle, I agree. The whole purpose of bluetooth is convenience on the go. If you're going to sit down in a quiet room and really listen to something, a wire isn't a problem. But considering the massive convenience factor bluetooth offers, it still sounds pretty darn fine. I can play my Amazon music library in my car without taking my phone out of my pocket. I got a pair of Cambridge Soundworks Oontz Ultra portable speakers for Christmas and they can fill a room with music. No, they don't have low bass and they aren't perfectly balanced, but they sound good enough to set out on the patio to entertain me while I'm BBQing. They connect to my phone with bluetooth, and I can even answer a call with them. The same goes for the AirPod Pros I got a month or so ago. They are smaller than a Altoids box and they stay charged for a full day.

    I really don't understand the photos I see on Head-Fi of people's portable rigs that include DAPs, DACs, a tangle of wires, laptops, hard drives full of lossless files and big portable batteries.... a backpack full of stuff... all in service of sound they can't even hear! In a paper lunch bag, I can carry a TB of music on microSD, my iPhone, a tiny little AirPod case and the pair of Oontz's and be ready for anything. Toss in a small battery pack and it can play all day. Every day I'm amazed at how easy it is to surround yourself with great music all the time. I don't know why people make it so hard on themselves.
    Last edited: Dec 26, 2019
    gorman likes this.
  5. bigshot
    If you use AAC and then stream using AAC, even if it is bumped to PCM and then back to AAC, there won't be any significant loss. The same codec is going to encode it the same the second time. Theoretically, transcoding from one lossy codec to another might shave something off, but in practice, I doubt you would hear it. I don't think it's worth worrying about. Lossy and bluetooth together works great.

    If you're at home, plug in.
    Last edited: Dec 26, 2019
  6. Harold999
    On my Androidphone i can only get aptx and ldac working. In the developers menu i can select aac, but that setting won't stick.
    The bluetooth receiver i got on the external dac/amp should support aac though.
  7. Sgt. Ear Ache
    Totally agree. Bluetooth sounds perfectly fine for pretty much any portable use. I have one of these things...

    it looks ridiculous. But it sounds remarkably good...put it in a corner on a wooden table and it will fill a room commendably. Combined with my little Fiio dap its a great portable sound system.

    I also have a couple of Fiio BTR1 Bluetooth amp/dac receivers. They are super-portable and work like a charm. With a set of cheap (but shockingly good sounding) earbuds it's a great setup that fits in a pocket. Lossless be damned. Nobody but nobody can actually hear a difference between 320kb mp3 and lossless in any real-world on the go listening situation.
    Bytor123 likes this.
  8. Harold999
    Got AAC working now on the smartphone, had to disable a certain setting in the Android developers menu.

    I did my best to try to hear any difference between Tidal high (320kbps AAC) by bluetooth, versus USB cabled Tidal hifi.
    I couldn't. Bluetooth rocks. Well, at least for my ears.
    Last edited: Dec 28, 2019
  9. bigshot
    AAC 256 VBR is transparent for me. AAC rocks to human ears!
    Bytor123 likes this.
  10. gregorio
    TBH, it wasn't a strange choice. AptX was originally designed as an intermediate codec for professional use, initially in the broadcast industry. Lossy codecs such as MP3, AAC, etc., are "perceptually" lossy codecs, meaning they mainly rely on auditory masking to reduce file size, IE.They remove audio frequencies that would not be heard/perceived. Perceptually lossy codecs are therefore particularly good as an end use codec but not so good as an intermediate codec. For example, let's say there is some frequency based processing downstream of the codec, say EQ or multi-band compression or limiting; we potentially have a problem with perceptually lossy codecs because we obviously can't process frequencies that have been removed. In other words, an EQ setting could cause certain masked freqs to no longer fall within the masking threshold (and therefore be audible) but if a perceptual codec has been used those masked freqs have been removed and can't be EQ'ed (and potentially unmasked). Compression/Limiting processes may also be impacted as the energy contained in various freq bands is altered by removing the masked freqs. This potential problem is lessened if we don't use a perceptual codec, if instead we just use a time domain codec which maintains all the freq content (albeit at less resolution). In addition, a time domain codec such as aptX is computationally far quicker to execute than perceptual codecs (which require a transform to the freq domain, the application of a psychoacoustic model and a transform back again) and therefore has lower latency, a consideration for certain applications such as gaming or audio/video over IP for example.

    1. The same codec isn't going to encode the file the same the second time because the second time it has a very different frequency content. Converting the AAC to PCM obviously does not restore the frequencies that have been removed by the initial conversion to AAC. This AAC to PCM data therefore has a significantly different freq content than the original file and will be processed significantly differently by another, subsequent AAC conversion.

    2. The transcoding process obviously doesn't/can't consider the freqs which have already been removed by the first encoding process. So, transcoding from one lossy codec to another WILL ALWAYS "shave something off".
    2a. That all depends on the content of the original file, how well a particular encoder deals with an already lossy encoded files and of course listening skill and environment. I tested transcoding and two pass encoding (transcoding using the same codec): In the case of AAC two pass encoding, I could blind test against the (uncompressed) original but it was extremely difficult and I could only manage it with one of the samples I used (all of which were chosen to highlight potential issues). Two pass MP3 or transcoding between different codecs was generally easier to blind test than two pass AAC, although still difficult. I didn't include aptX in my test but taking into account what I said above (to castleofargh), transcoding between a perceptually lossy codec and aptX would generally be somewhat better (in theory) than between two perceptually lossy codecs. However this is an assumption that could be incorrect. It should also be noted that I was testing at levels higher than normal, under extremely good listening conditions (in a studio) and I did this testing in the early 2000's (and all the encoders have improved since then). So today I agree, in practice very few people (and possibly none at all) would hear a difference at reasonable listening levels.

    In theory "yes, definitely". In practice I think it's very unlikely you will ever encounter a set of circumstances which could result in you being able to hear a difference!

  11. Harold999
    I have just read elsewhere that even if you have an AAC source (Tidal high Q) and a AAC codec in your Bluetooth device, Android and Apple iOS as well will decode the stream to pcm first so they can mix notifications in the music played.
    That's a bummer.
  12. Harold999
    In 'do not disturb' mode Android doesn't mix notifications in the music anymore, but the question is, does it still do the AAC > PCM conversion then?
  13. gregorio
    1. Why is that "a bummer"?

    2. I don't understand your question, there will always have to be a conversion to PCM.

  14. Harold999
    Well, in an ideal scenario that happens only once, in the end dac.
    In case of bluetooth streaming with an Android/Apple phone, it happens twice in case of an AAC source.

    I wonder if it also happens twice in 'do not disturb' mode, because Android doesn't mix the notification sounds in the music stream then. The AAC to PCM conversion in the phone itself isn't necessary then, only in the end dac.
    Last edited: Dec 28, 2019
  15. bigshot
    I've encoded back and forth from AAC 256 to AIFF and back to AAC 256 ten times and there is little or no change in the file. I've been told that transcoding from AAC to MP3 to Ogg Vorbis to whatever else would introduce more signal degradation than just re-encoding AAC over and over, but I haven't tried that myself. It would be an interesting experiment if someone wanted to try it. The way it was explained to me was that once AAC throws out the masked sound, if you bounce to AIFF and encode to AAC again, it's just trying to throw out the same stuff again, and it isn't there to throw out, so there is very little change to the file. If you change codecs, it throws out something different every time and you end up with what's left.

    In practice, I haven't found any audible degradation of the signal at all using bluetooth. I did back when it first started, but the newer ways of handling it are audibly transparent for me and my purposes. No reason to worry about it though because most serious listening is probably done at home wired. I think worrying about this is just another reason for audiophiles to encourage their OCD.
    Last edited: Dec 28, 2019
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