Every Bluetooth headphone and speaker compresses sound into a codec before sending it over the air, then decompresses it again the instant before it plays. The codec is not something you choose. macOS and the device agree on one automatically the moment they connect, and that choice quietly decides two things: how good the audio sounds, and how much delay sits between your Mac playing something and your ears hearing it.
Most people never think about codecs until something feels off, a pair of headphones that lag noticeably behind video, or a call where their voice comes through AirPods sounding thin and distant. Both have the same root cause. Here is what a codec actually does, the three you'll realistically run into on a Mac, and the separate bandwidth problem behind AirPods' microphone dropping quality mid-call.
What a Bluetooth codec actually does
Bluetooth's wireless connection has a limited amount of bandwidth, far less than a wired cable carries. Uncompressed audio, the kind stored on a CD or streamed at full quality, is too large to fit down that connection in real time. A codec solves this by compressing the audio into a smaller stream before it's sent, then decompressing it back into sound the instant it arrives at the headphones or speaker.
That encode-send-decode round trip is not instant. It takes a small but real amount of time, and different codecs are built with different priorities: some aim for the broadest compatibility, some for better sound quality, and some specifically for cutting that round trip as short as possible. Which one your Mac ends up using depends entirely on what the connected device supports.
SBC, AAC and aptX: the three you'll actually meet
SBC is the one codec every Bluetooth audio device is required to support, so it's the fallback that guarantees any two devices can talk to each other. It prioritises compatibility over everything else, which means it's also the lowest quality of the three and carries the most delay.
AAC is the codec Apple uses heavily. AirPods and most Apple-made or Apple-certified accessories negotiate AAC automatically, and on Apple's own hardware it's noticeably better than SBC on both counts: cleaner sound and less delay.
aptX is a codec developed by Qualcomm, common on Android phones and on some third-party headphones built with Android in mind. Apple devices don't use aptX natively, so a Mac generally won't negotiate it even if a pair of headphones lists it as a feature, unless that specific device also falls back to a codec macOS does support.
Why the codec is a genuine, physical reason for lag
Because each codec's encode-send-decode round trip takes a different amount of time, the codec in use is a real, measurable cause of delay, not something imagined. A pair of headphones running SBC has to do more work to compress and decompress the same audio than a pair running AAC, and that extra work shows up as extra milliseconds between the sound leaving your Mac and reaching your ears. Two headphones doing the exact same task, playing the exact same video, can feel meaningfully different simply because they negotiated different codecs.
This is why some Bluetooth headphones feel consistently laggier than others even when nothing else about the setup has changed. It's also why the fix, when there is one, is usually a different device rather than a setting: since macOS chooses the codec automatically, the only lever you actually have is which accessory you're connecting in the first place. For the fuller picture on what causes audio lag on a Mac, including HDMI processing and what happens when a laggy Bluetooth device gets grouped with others, see why your Mac's audio lags behind video.
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Download for MacThe other bandwidth problem: why a call makes AirPods sound worse
Codec choice explains lag, but it doesn't explain why AirPods' microphone sounds noticeably worse than AirPods' speakers the moment a call starts. That's a separate, related limit: Bluetooth's classic audio profiles split the connection's available bandwidth between the outbound stream carrying audio to you and the inbound stream carrying your microphone back.
Playing music only uses one direction, output only, so it runs on A2DP, the stereo profile built for streaming audio, using whichever codec was negotiated, AAC on AirPods. The instant a call starts and the microphone activates, both directions need to share the same connection at once, and Bluetooth's classic profiles were never built to carry two full-quality streams simultaneously. macOS switches to HFP, the Hands-Free Profile built for calls, which typically drops to a narrower, mono codec built for voice rather than music. That's the trade forced by having to fit input and output down one connection at the same time, and it's why voices come through thinner and more compressed the moment a call begins.
This isn't unique to AirPods. It happens on effectively any Bluetooth headset with a built-in microphone, because it comes from how the Bluetooth profiles themselves share bandwidth, not from anything specific to Apple's hardware. For the practical fix, keeping AirPods for listening while a different microphone handles the call, see AirPods microphone sounds bad? Use a different one.
Where Chorus fits, and where it can't reach
Codec negotiation, profile switching, and reconnecting after a drop all happen below the level any app can see or touch. A Bluetooth speaker that disconnects repeatedly usually isn't a codec problem at all, it's radio interference, a corrupted pairing, or a macOS reconnection bug; see why your Bluetooth speaker keeps disconnecting for those causes. macOS and the connected device handle all of this entirely between themselves. Chorus sits above that layer: it gives you one list to group outputs, switch inputs, and send individual apps to different devices, with a keyboard shortcut for each, but it was never going to change a codec or force a different Bluetooth profile, because no app running on macOS has that lever.
For the full pairing and troubleshooting detail, see Apple's own guide to connecting Bluetooth devices.
Frequently asked questions
What is a Bluetooth audio codec?
A codec is the compression format a Bluetooth device and a Mac agree on so audio can fit down a wireless connection with limited bandwidth. It compresses sound before sending it and decompresses it again before it plays, and which codec is in use changes both the sound quality and the delay between the two.
What is the difference between SBC, AAC and aptX?
SBC is the baseline codec every Bluetooth audio device supports, and it gives the lowest quality and the highest delay of the three. AAC is used heavily by Apple, including AirPods and most Apple-made accessories, and gives noticeably better quality and lower delay than SBC. aptX is a Qualcomm codec common on Android phones and some third-party headphones, and Apple devices don't use it natively, so it rarely comes into play on a Mac.
Why does my AirPods microphone quality drop during a call?
Bluetooth's classic audio profiles split the available bandwidth between the outbound audio stream AirPods send to you and the inbound microphone stream you send back. Music only needs one direction, so it runs on a high-quality stereo profile. The moment the microphone activates, both directions have to share the same limited connection, so macOS switches to a lower-quality, often mono profile to fit them both, which is what makes voices sound thin on a call.
Can I choose or change which Bluetooth codec my Mac uses?
No. macOS and the Bluetooth device negotiate the codec automatically the moment they connect, based on what both sides support, and there is no setting in System Settings that lets you force one. The only way to get a different codec is to use a device that supports a better one, such as AAC or aptX.
Does Chorus control which Bluetooth codec my Mac uses?
No. Chorus groups outputs, switches inputs, and routes individual apps to different devices, but it has no influence over Bluetooth codec negotiation, which happens automatically between macOS and the device itself, outside any app's control.
