Signal Routing, Mixers, Crosspoints, Automixers & Acoustic Echo Cancellation
Learning objectives
- Route audio with crosspoints and mixers in a DSP.
- Configure an automixer for multiple mics.
- Explain and enable acoustic echo cancellation.
- Distinguish AEC from feedback suppression.
Vocabulary
- Crosspoint
- A routing switch in a matrix mixer: input × output.
- Mixer
- A block that sums selected inputs to an output.
- Automixer
- A mixer that auto-adjusts mic levels (gates NOMA) for talkers.
- AEC
- Acoustic echo cancellation — removes far-end audio heard by mics.
- Reference signal
- The signal an AEC uses to identify and remove echo.
Lesson content
A DSP routes audio with matrix mixers: each crosspoint connects an input to an output. For conferencing, you mix mics to the codec, return far-end audio to the room, and use AEC so the far end does not hear themselves back.
Routing and mixers
- A matrix mixer has inputs on one axis and outputs on the other; each crosspoint is a gain.
- Route mics → codec (send to far end); route far-end audio → speakers.
- Send program audio (share content) to the codec so far end hears it.
Automixing
- An automixer opens mics for active talkers and attenuates the rest.
- It keeps the mix at a steady level as mics open/close (NOMA compensation).
- Set thresholds and attack/release so it sounds natural, not choppy.
Acoustic echo cancellation (AEC)
- 1Far-end audio comes out the room speakers.
- 2The room mics pick that audio up and send it back — that is echo to the far end.
- 3The AEC compares the mic signal to a reference (the far-end signal) and subtracts the echo.
- 4Each mic channel needs its own AEC and the correct reference.
AEC removes echo returning to the far end in a call. Feedback suppression tames local howl (room feedback). They are different problems with different tools; both may appear in a conferencing room.
AEC needs the right reference signal and enough headroom to converge. Feed it a clean reference, do not overdrive the room speakers, and allow convergence time before judging the result.
Worked example
Four ceiling mics each through their own AEC (reference = far-end RX), summed by an automixer, sent to the codec transmit. Far-end receive goes to the ceiling speakers. A test call confirms the far end hears no echo and the local talkers are clear.
Implementation checklist
- ✓Matrix crosspoints route mics→codec, far-end→speakers, content→codec.
- ✓Automixer configured with sensible thresholds.
- ✓Each mic has its own AEC with the correct reference.
- ✓Room level set so AEC can converge.
Downloadable study guide
01. Matrix mixer = input×output crosspoints; route mics→codec TX, RX→speakers. 02. Automixer opens active mics, keeps level steady (NOMA). 03. AEC subtracts far-end echo using a reference; one AEC per mic. 04. AEC ≠ feedback suppression; both may be present.