Bluetooth's soft limits in OS X: Count your devices

A reader finds a common problem: Too many Bluetooth devices connected to one Mac can perform inconsistently.

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Steve Johnson wrote in with a complaint I hear regularly (and experience myself):

I have a Mac mini with a Bluetooth trackpad and keyboard. I recently received a Bose radio and play music from the Mac via Bluetooth. Sometimes the radio signal gets garbled. I can temporarily disable the keyboard and trackpad and get the music working, then re-enable. Is there a way to correct this situation without the workaround?

Bluetooth is a great short-range wireless technology that uses frequency hopping (FH) to rapidly switch narrow bands of frequency used to carry data, avoiding interference and competing uses. This should mean you can have a lot of Bluetooth and other wireless in the same 2.4 gigahertz (GHz) band in the same place. (A version of FH was first developed in the 1940s by actress and genius Hedy Lamarr and avant-garde musician George Antheil.)

In practice, though, a combination of saturation of those frequencies and Bluetooth’s functional limits makes it all work less well than you’d hope. You can try to reduce competing use of the spectrum—if you have a baby monitor, remote-door bell, older cordless phone, or other wireless stuff that uses 2.4 GHz, you might move or replace it. (Check the labels and manuals for frequencies used.)

A Wi-Fi base station too close to your Bluetooth equipment and computer can be a problem, too. Every consumer Wi-Fi access point uses 2.4 GHz, and many also use 5 GHz. They use a swath of 2.4 GHz that can deny about one-quarter to one-third of the band to nearby Bluetooth devices. If you’re in a highly congested area, like an apartment building in an urban area, you may have so many Wi-Fi base stations and devices active, you simply can’t avoid it.

Even then, the Bluetooth controller on a Mac may not be able to keep up. I’ve spoken many times to folks at the Bluetooth SIG, which handles standards and certification for devices using that technology, and despite the ostensible capability of Bluetooth, you may see more limits in practice.

Apple is quite blunt on its page about Bluetooth input devices and Macs in describing constraints:

The official Bluetooth specifications say seven is the maximum number of Bluetooth devices that can be connected to your Mac at once. However, three to four devices is a practical limit, depending on the types of devices used. Some devices require more Bluetooth data, so they’re more demanding than other devices. Data-intensive devices might reduce the total number of devices that can be active at the same time.

If you can’t make the Bluetooth work reliably and there’s an audio input, you might switch to running an audio cable for more reliability.

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