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Discussion Starter #1 (Edited)
Took car in for a service and afterwards the sound was awful. I have the Signature NAV so the Bose speakers should not sound awful. I put 1 + 1 together after dealer did a fantastic job cleaning my dash and car and was very worried they'd vaccumed the tweeters (they look like air vents on the dash).

If I could describe the sound it would be crackly / crackling / crackle / scratching / resonating at higher volumes, particularly with cymbals and higher frequencies coming exclusively out of the front 2 dash tweeters (all the other speakers were fine as they do lower frequencies only) mainly when the volume is louder than a typical conversation in the car (and it's worse the louder you go). If you know what a 64kbit MP3 sounds like imagine that but with crackling.

Dealer wasn't that helpful (although to be fair the issue wasn't as bad during the 5 minutes I had their attention when I was trying to demonstrate it) however I have now worked out what was triggering all this and I doubt I am the first to question the audio quality.

Terrible Spotify Mobile Bluetooth sound coming from my Bose SoundLink Mini speaker with the same phone made me think it might not be Zoe related.. but yet the same song played on Spotify Desktop with the same Bluetooth speaker was fine.

Sometimes the crackling sound was not as noticeable sometimes it was so unbearable I had to turn the volume down.

Test songs that seem to really show it up are:
  • the sweeping sound in "The Nights - Felix Jaehn Remix" (4 to 11 seconds)
  • "Not Alone (From "Final Fantasy IX") - Nobuo Uematsu" (around 1:15 minutes with the violin)
  • "Hey Now - Zero 7 Remix" (3:13 minutes onwards)
  • "The Sound Of Silence - Disturbed"
  • "Stand Up - The Prodigy"
This led me to the first discovery:

1. Recent Spotify updates have turned on "Normalize volume". This means the volume should constantly be the same and you won't have to keep turning up and on the volume but does this by compressing the range, this can when the volume is loud cause a loss of fidelity. It also appears you need to reboot your phone after making this change. This seemed to make the sound a bit better. I'm also assuming you've already changed the default audio quality to "Very High" (formerly known as "Extreme").

2. I then trashed all of my paired devices on my phone and still had no success. Then I remembered reading online somewhere someone asking if a user with similar terrible sound had a fitness tracker. Bluetooth 4 (aka. Bluetooth Low Energy, BLE) does not show up in the paired list. I turned off my Fitbit Versa (Go to Settings, About and select Shutdown on the watch itself) sound instantly improved.

Then comes more Googling:
Why did the sound vary so much and why is this a recent observation? Well, Fitbit syncs longer and is more chatty if you've done more walking/activity etc. or it's been a while since you synced (you've left your phone on charge and wandered off). If you've been relatively sedentary for a while then the sound should be okay.. however faster driving (can throw off the step counter), incoming notifications and it being a while since it last synced can cause the issues to occur for quite a while. Fitbit also just before the service updated their Android app (4th September) and it would appear the app now syncs more aggressively with the device than before. I have turned off "All Day Sync" so it will now only sync when I open the app and it sometimes makes it better but not always.

So I'm not sure whether we can blame Android/Bluetooth chipset Manufacturer for not being able to allow multiple connections to co-exist gracefully or whether this is strictly a limitation in Bluetooth but if you're experiencing poor audio performance (particularly if you have crackling and particularly if you have a smart watch) consider reducing the load on your Bluetooth connection by dropping some devices or the frequency at which they communicate :).

I've seen a lot of information about AptX vs A2DP.. but I suspect this is all down to one codec being more bandwidth heavy than another.. issue seems to be contention / dropped packets. Same with people saying disabling WiFi helps, that is just because WiFi uses the same 2.4Ghz as bluetooth, so knocking that off might free up a tiny tiny amount of bandwidth (Does Bluetooth Really Interfere with Wi-Fi? - 82303 - The Cisco Learning Network).

Also we now know crackle is bluetooth packet loss (/contention) and not strictly a blown speaker.
 
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