||09-25-2018 01:05 PM
Likely because Bluetooth HFP (phone calls) audio quality is transferred via a mono data stream with other modulation and whatnot going on as well (look up SBC codec), plus there's an upstream microphone component. A2DP (music) on the other hand is capable of stereo 320kbps+ audio quality, but it's largely dependent on if the head unit is coded to receive that data/what the Bluetooth receiver is capable of. Since our Mazda HUs have to receive other library data there's also overhead (AVRCP 1.5 probably) which could prevent full 320kbps from happening, but I would absolutely expect at least 256kbps stereo to be possible via Bluetooth. For most people I don't think this is terrible.
Wired CarPlay on the other hand is whole different ballgame. Wired CarPlay is obviously transferred via USB but is limited to USB 2.0 data spec (theoretical sequential data speed of ~480Mbps, but expect about half that and substantially less for simultaneous upstream and downstream transfers). Nonetheless audio will sound better in this case because there's still more bandwidth than Bluetooth that can be used, and the audio is processed entirely differently/better. Now you can take advantage of VOIP quality phone calls, and higher quality music.
Wireless Carplay however can actually provide the best quality. From my experience Bluetooth is only required to pair in this case, but WiFi is what's primarily used (can verify this), making for a substantially better bandwidth allowance than USB. This is why the screen resolution is much higher when using CarPlay via WiFi than USB (no 800x480 res limit). While I can't say for sure audio quality isn't lost because I'm sure on some super minimal degree it is, I haven't noticed any distinguishable audio quality degradation via wireless Carplay. Since CarPlay is basically an AirPlay extension of your iPhone's screen anyways, it could be possible Apple is transferring it using their lossless codec.
So really wireless CarPlay is what we really want, unless the default Mazda system can start supporting lossless audio files. :)