Summary
The SIG's design guide for transmitter makers, covering stream settings, Presentation Delay, airtime, encryption, volume and transmit power.
Key findings
- Public transmitters must send at least one Standard Quality stream (16 or 24 kHz). For live use, where listeners also hear the room, the guide recommends the 24 kHz low-latency setting to limit echo.
- All Auracast receivers must support a Presentation Delay of 40 ms, and receivers built to the Hearing Access or Telephony and Media Audio profiles must support any value from 20 to 40 ms. Transmitters should not set less than 20 ms, but may set a flag asking receivers to play as early as they can; different pairs of receivers may then play at slightly different times.
- Transmitters for professional installation may let the installer adjust Presentation Delay to cope with latency in larger venues.
- One stream uses about 11 to 22% of the airtime at Standard Quality and 30 to 41% at High Quality, so a transmitter can carry only a few streams; the guide suggests defaulting to 24 kHz, or mono, in public locations. Settings with 7.5 ms frames should not be used for public broadcasts.
- Transmitters need no volume control: the specifications assume the level passes through unchanged and each listener sets the volume on their own receiver. The guide warns that turning a transmitter down can lead to painfully loud audio when a listener switches to another stream.
- Transmit power should be the lowest that gives reliable coverage of the intended area. Confidential content should be encrypted because the signal passes through walls.
- With the recommended 30 ms advertising interval, most scanning devices should find a transmitter within 10 seconds. A built-in test sound is suggested to help check an installation.
- Appendix A, Tables 8.1-8.2, calculates 10.6-21.7% airtime per Standard Quality BIS and 29.7-40.7% per High Quality BIS, using 10 ms frames on the 2M PHY and excluding advertising overhead (about 0.6% per BIG). These are per-BIS figures, not a venue-wide channel limit.
- Appendix B says Program_Info and Language metadata describe subgroups, but assistants and receivers may ignore optional metadata. The Assisted Listening Stream marker identifies processing intended to improve intelligibility; the guide cautions that hearing aids already apply personalised processing and may prefer another stream.
- Appendix A, pages 30-32, shows higher airtime with more retransmissions and examples of stereo carried by two BISes and alternative programmes by additional BISes. The guide also warns that all BISes within one BIG receive the same airtime allocation, sized for the largest packet, so combining codec settings can use more airtime than placing them in separate BIGs.
Limits
Design guidance for manufacturers. The airtime figures are the SIG's calculations for one stream on the 2M PHY, not measurements. The cover gives 16 September 2026 and the version history 14 September 2026.
Read the original
www.bluetooth.com/wp-content/uploads/2024/05/2403_How_To_Auracast_Transmitter.pdf
Cited in
- Should the Auracast stream be delayed to line up with the loudspeakers?Open
- How far does one transmitter really reach in a full room?Partly answered
- What audio level should the stream carry: -35 dBFS or louder?Disputed
- Auracast, hearing loop, infrared, RF or Wi-Fi: which suits which room?Disputed
- How do many transmitters share one broadcast across a large venue?Open
- How much do crowds, phones and Wi-Fi reduce reliability?Open
- Open or protected: when should a venue broadcast need a code?Open
- What works in courtrooms, council chambers and other meeting rooms where speech must be heard by all and kept in the room?Partly answered
- What works in hospitals, clinics and pharmacies, where a misheard word can matter most?Partly answered
- Can one venue offer several Auracast streams at once (languages, audio description, a mix for hearing aids), and how do listeners choose?Partly answered
Cite this source
Bluetooth SIG Market Development. How to build an Auracast transmitter: technical overview, version 3. Bluetooth SIG. 16 September 2026. https://www.bluetooth.com/wp-content/uploads/2024/05/2403_How_To_Auracast_Transmitter.pdf (accessed 13 October 2026).
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