AEC Reference Is the Far-End Signal, Not Another Microphone
Echo cancellation only has a useful reference when the reference represents what the loudspeaker actually played.
TOPIC
Embedded Audio & Voice
Lessons, explainers, experiments, and implementation notes.
Clocked Playout Beats Packet-Driven Audio
Network packets arrive when the network allows; speakers need samples when the audio clock demands them.
Jitter Buffer Depth Is a Latency Budget
Every extra buffered frame trades conversational responsiveness for tolerance to arrival variation.
When Downlink Suppression Makes Speech Worse
A processor that removes noise can also remove speech detail when its assumptions do not match the signal.
Separate Codec Bring-Up from Acoustic Tuning
Prove clocks, framing, routing and gain before judging microphones, speakers or enclosure acoustics.
Downsampling 16 kHz Microphone Audio to 8 kHz PCMA
The physical capture clock and the network codec did not run at the same sample rate.
Why 8 kHz Telephony Can Sound Robotic After 16 kHz Playback Conversion
The downlink could be packet-complete and still sound gritty or artificial after conversion to the physical playback rate.
An AEC-Off Run Is a Control Experiment, Not a Product Setting
AEC was implicated in several symptoms, but disabling it permanently would remove a required speakerphone function.
The PBX Scheduler Bug That Looked Like an ESP32 Audio Bug
The device received media in repeating bursts that looked like a local queue or I2S starvation problem.
timerfd Gave Me 21–26 Ticks When I Needed About 50
Asterisk was configured for 20 ms media timing, yet packet forwarding arrived in scheduler-sized bursts.
Killing sngrep Was Not Enough: Finding the Orphan Process Behind 80 ms Bursts
A long-running packet tool looked suspicious, but removing it did not fully restore 20 ms scheduling.
Reading RTP Sequence, Timestamp and Arrival Gaps Together
Packet loss, reordering and burst arrival can sound similar but require different fixes.
Why a Clean PBX Forwarding Trace Still Did Not Mean Zero Lag
After server cleanup, Asterisk forwarding became fast, but conversation still felt delayed.
The Geography Was Larger Than the Firmware Optimization
Engineering time was being spent chasing tens of milliseconds in firmware while the media route crossed Bangladesh and Ohio twice.
Why V116–V131 Did Not Automatically Replace V115
Later queue, PLC and timer experiments looked more sophisticated on paper but repeatedly introduced crackle, echo or additional delay.
V117 Taught Me to Revert a Bad Audio Architecture Quickly
A rebuffer/AEC/volume experiment produced obvious bad crackle instead of the intended stability improvement.
V132A: Turning Clear Audio into a Preserved Release Asset
A working phone is not a release unless the exact artifact and source context can survive the next experiment.
Long-Call Drift Is Different from Short-Call Sound Quality
A call can sound excellent for thirty seconds while two media clocks slowly walk apart.
A Tinny Speaker Is Not Automatically a Codec Problem
Field feedback described the speaker as very tinny even though the digital call path was working.
From Demo-Ready Audio to Production Acceptance
One strong call can prove a candidate is promising but not that it is ready for manufacturing or field release.
The Audio Bug Was Not One Bug
Robotic speech, cutouts, lag and echo initially collapsed into one vague complaint called bad audio.
Reconstructing V115 as a Known-Good Behavioral Control
Later experiments had changed queues, PLC, timing and logs until nobody could safely say which behavior belonged to the last clear build.
Why PCAP Evidence Cannot Prove Acoustic Latency
It was tempting to use server packet timing as proof of the complete mouth-to-ear delay.
The Measurement Stack That Made Subjective Audio Debuggable
Words like robotic, delayed and crackly were useful user reports but poor root-cause evidence.
When a Warning Is Not the Root Cause: The I2S Mode1 Conflict
An I2S Mode1 conflict warning looked suspicious enough to become a candidate explanation for crackle.
How I Separate Confirmed Facts, Inferences and Failed Experiments
Fast-moving firmware work made it easy for an attractive theory to become remembered as fact.
When UART Logging Became a Real-Time Audio Bug
The firmware sounded worse while producing the very diagnostics intended to explain it.
Speaker Writes Over 20 ms Became My Local Deadline Signal
Crackle and cutouts needed a device-side timing metric that was closer to the DAC than packet arrival.
Why UDP Arrival Should Not Be the Speaker Clock
RTP arrived in bursts even when packet sequence was mostly healthy, so directly pacing I2S from receive callbacks made network jitter audible.
V126 to V127: What 68.201 ms to 0.457 ms Actually Proved
A large first callback-to-speaker delay suggested work was accumulating between RTP reception and physical output.
High-Volume Echo Moved the Bug into the Physical Product
After digital timing became stable, echo increased at high speaker volume and could no longer be treated as only a network or queue problem.
Why More Jitter Buffer Was Not Automatically Better
Increasing jitter tolerance seemed like the obvious response to bursty packet arrival.
AEC Starts with a Truthful Reference Signal
Echo tuning was meaningless if the AEC reference did not represent what the loudspeaker actually played.
ES7210 Channel Mapping Was an Audio-Architecture Problem
Codec detection alone did not prove the capture channels meant what the DSP assumed they meant.
128-Sample AEC Blocks vs 160-Sample G.711 Frames
The AEC library consumed a block size that did not divide evenly into the telephony frame size.