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AAC/M4A File - Elements Distorting over Bluetooth
Old 19th March 2017
  #1
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AAC/M4A File - Elements Distorting over Bluetooth

This issue deals with masters I've created recently. I don't notice the issue over any playback system except as described below. I've been "faux mastering" my own material for some time and never run into this until recently.

I have been noticing tracks that exhibit distortion when playing AAC/M4A versions via Bluetooth in my car (it's usually electric guitars). The same tracks played via USB/flash drive do not contain the distortion nor do commercial tracks of similar AAC quality played via Bluetooth. I have tried lots of other trial and error approaches with none removing the distortion.

I've been very careful to mix (in all of these "tests") to an integrated loudness level of approx -16 LUFS with true peaks of no more than -0.9 dBF. Gain staging was done very carefully end-to-end and I am not aggressively limiting the tracks (using Elephant for limiting chores which is, given the settings I use, very transparent).

Using Reaper DAW on Windows 7 Pro, 8 GB memory, Dell Inspiron 620 (i5 processor). Master buss has TDR Kotelnikov compressor with only slight gain reduction, TDR Slick EQ for Mastering and Elephant as mentioned above.

"Car" versions are m4a/aac files converted from 24-bit, 44.1 kHz files using QAAC (iTunes) to produce 256 kbps VBR files (roughly "iTunes Plus" quality).

Any ideas? I never had this problem before January of this year and I can't think of any changes made to tools or process that would have caused this.

Thanks much for any ideas/suggestions. Let me know if this should be in "Mix Checks."

Last edited by nyaben; 19th March 2017 at 08:42 PM..
Old 20th March 2017
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Old 20th March 2017
  #3
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Thanks, I'd agree that the Bluetooth streaming is compounding data compression (since the files I'm playing are lossy AAC); however, most of the files I have done in the past sound fine this way as do commercial artist recordings. Is it possible to clip a mic preamp, recording it at an appropriate level into a DAW and then only be able to hear it via this bluetooth file? I don't hear the clipping any other way (studio monitors, home stereo).

I need to check streaming of the 24-bit wave file.
Old 4 weeks ago
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I have since tried the same files via streaming Bluetooth from dropbox and they sound fine, distortion free. I can only conclude that Apple Music is somehow causing this as I stream from my digital library. It is not noticeable streaming from their official music library but I do now recall songs in my own library (by professional label artists) occasionally sounding this way in the past.
Old 4 weeks ago
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Okay, I have finally figured this out. For some reason, even though I have an average of -16 LUFS (Apple's target volume level for radio and sound check), Apple Music's sound check is adding gain/volume on playback to the point of making my song distort. I remastered this track to -23 integrated LUFS and it no longer distorts. Any add'l comments or opinions would be welcomed from you professionals from your experiences.
Old 4 weeks ago
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Suffice to say eliminating Sound Check & Sound Enhancer from playback is usually one of the first troubleshooting steps. Glad you got an answer.
Old 4 weeks ago
  #7
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Thanks for the thoughtful replies! Appreciate it.
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