AEC Starts with a Truthful Reference Signal

The useful breakthrough was not another codec setting. It was recognizing that echo tuning was meaningless if the AEC reference did not represent what the loudspeaker actually played.

The test platform was the LOUP ESP32-S3 voice device with ES8311 playback, ES7210 capture, SIP/RTP media and a small speakerphone enclosure. The network codec was G.711 A-law/PCMA at nominal 8 kHz with 20 ms / 160-byte RTP payloads, while the physical audio path ran at 16 kHz in the recovered playback design. That mismatch between network time, device time and acoustic time is exactly why a vague word like “crackle” is not a diagnosis.

The evidence that matters for this article is specific: The trusted V115 path published the exact PCM selected for physical playback into the AEC reference path before the I2S speaker write. I keep those observations tied to the build and test where they were recorded. They are not universal ESP32 performance claims.

The engineering result was also specific: Reference provenance became a non-negotiable invariant of the clear-audio path. This article is about how I got from the symptom to that bounded conclusion, what the data did not prove, and what I would monitor before touching the same path again.

How I framed this case

I treated this case as a boundary-identification problem. The failure was audible at the end of the chain, but the useful question was which boundary first contained evidence of the defect. Starting from Echo tuning was meaningless if the AEC reference did not represent what the loudspeaker actually played., I walked backward until The trusted V115 path published the exact PCM selected for physical playback into the AEC reference path before the I2S speaker write. could either confirm or reject the current hypothesis. That approach kept later layers from being blamed for defects they only reproduced. The mechanism that mattered was AEC estimates the acoustic echo path from a known far-end reference to the microphone; a stale, differently processed or misaligned reference makes adaptation solve the wrong system. The false lead—Treating AEC parameters as the first thing to tune when reference routing was still uncertain.—was attractive precisely because it could explain the symptom without explaining the evidence. The accepted result, Reference provenance became a non-negotiable invariant of the clear-audio path., mattered because it changed the earliest failing boundary rather than merely changing how the failure sounded.

Signal identity before DSP tuning

My first capture/AEC question is now “what exact sample is this?” I want sample rate, channel/slot, bit width, scaling, physical source and timestamp relationship. A DSP block receiving perfectly formatted samples from the wrong microphone is still wrong. An AEC receiving a far-end signal that differs from physical playback is still wrong. A resampler that drops tails can be wrong only at frame boundaries and therefore sound intermittent.

Once those identities are proven, algorithm tuning becomes meaningful. Before that, tuning can hide routing defects and make the next hardware revision harder to reason about.

Case notebook

I use this table as a compact incident contract. If a later retest changes the evidence but not the written conclusion, the conclusion needs review.

A useful follow-up is to ask what would falsify the retained result. For this case, a repeat run on the same controlled topology should reproduce the relevant observation. If The trusted V115 path published the exact PCM selected for physical playback into the AEC reference path before the I2S speaker write. disappears while the symptom remains, then the old explanation no longer covers the new incident. If the observation returns without the symptom, then it may be contextual rather than causal. That is why I keep mechanism-level counters beside the listening test.

Instrumentation sketch

raw slot 0 -> verify physical source before naming it
raw slot 1 -> selected MIC1 in the recorded path
raw slot 2 -> verify / unused in historical mapping
raw slot 3 -> historical MIC2

AEC reference := exact PCM chosen for physical playback
AEC input blocks := 128 samples
network voice frame := 160 samples @ 8 kHz
carry remainder; never silently drop the tail

The snippet is not presented as drop-in production code. It documents the measurement model. I want the instrumentation to be cheaper than the deadline it observes, explicit about units, and easy to disable or summarize after the call. The most dangerous diagnostic is one that silently changes scheduler behavior while appearing to measure it.

For AEC Starts with a Truthful Reference Signal, the next retest would therefore preserve the same topology and change only the variable tied to AEC estimates the acoustic echo path from a known far-end reference to the microphone; a stale, differently processed or misaligned reference makes adaptation solve the wrong system.. I would collect the same observation again, compare it with the known-good control, and only then decide whether a new firmware branch deserves to replace the baseline.

The tempting explanation I did not accept

The attractive wrong turn was: Treating AEC parameters as the first thing to tune when reference routing was still uncertain.

Embedded audio is full of these traps because many failure modes sound alike. Packet bursts, resampler artifacts, AEC suppression, output starvation, clipping and acoustic echo can all be described as “robotic” by a listener. A large queue can hide packet jitter while making conversation sluggish. Turning AEC off can remove one processing cost while making the product unusable as a speakerphone. A warning in the log can look causal simply because it is the only visible abnormality.

The rule I now use is that a theory must predict another observable fact. If I believe packet loss is causing a missing word, I should find the corresponding sequence gap or payload absence before the device. If I believe I2S is stalling, speaker-write timing should show it. If I believe the PBX is batching media, its receive/forward timestamps should expose the batch before the ESP32 sees it. If I believe echo is acoustic, changing volume or geometry should change the failure even when packet timing remains stable.

This is slower than guessing for the first ten minutes and much faster than carrying a wrong theory through ten firmware versions.

How I interpret the numbers

The measurements in this series are deliberately tied to their recorded tests. They describe one board, one firmware revision, one network path and one observation window unless the evidence says otherwise. I do not turn 0.457 ms into a product-wide latency claim, or 175.6 seconds into proof of indefinite stability, or a 20–28 ms network variation into a codec property.

I use distributions and boundaries wherever possible. A maximum speaker write tells me a deadline was missed, while incidence tells me how common the miss was. Packet p50/p95/p99 and maximum gaps reveal whether a path is usually healthy with isolated excursions or continuously unstable. Drift is a slope, not a single latency. AEC-off stability is a control result, not a shipping configuration. A binary hash proves identity, not quality.

For AEC Starts with a Truthful Reference Signal, the important interpretation is: AEC estimates the acoustic echo path from a known far-end reference to the microphone; a stale, differently processed or misaligned reference makes adaptation solve the wrong system. The number is useful only because it narrows the fault domain.

When exact current data is not available, I would rather repeat the test than invent a value. The same applies to acoustic latency: without synchronized physical capture, the correct statement is that the network/device measurements bound parts of the delay, not that they measure mouth-to-ear time.

What this result proves—and what it does not

The result I am willing to claim is narrow: Reference provenance became a non-negotiable invariant of the clear-audio path. It is supported by the recorded observation: The trusted V115 path published the exact PCM selected for physical playback into the AEC reference path before the I2S speaker write.

It does not prove that every LOUP board, every network path or every future firmware build behaves the same way. It does not turn a server-side packet capture into an acoustic measurement. It does not turn a stable AEC-off test into permission to remove AEC from a speakerphone. It does not make a hash a quality metric. Those distinctions sound obvious in hindsight and are easy to lose when a demo deadline rewards a simple story.

The useful causal statement is the one consistent with the mechanism: AEC estimates the acoustic echo path from a known far-end reference to the microphone; a stale, differently processed or misaligned reference makes adaptation solve the wrong system. If another experiment changes that mechanism, I expect the evidence to change too. If the evidence stays the same, I should question the theory before rewriting more code.

I also keep the rejected explanation visible: Treating AEC parameters as the first thing to tune when reference routing was still uncertain. That is part of the result. Knowing which layer did not create the step change prevents future debugging from starting at the same dead end.

Reproducing the experiment without changing the experiment

If I had to hand this case to another engineer, I would ask them to preserve the same evidence boundary before attempting a fix:

• record codec/sample format and physical channel mapping
• verify the exact PCM used as AEC reference
• test capture with AEC both enabled and disabled as controlled experiments
• preserve partial DSP blocks/tails across frame boundaries
• repeat mute and full-duplex tests after any mapping change

The point is not ceremony. Embedded audio is sensitive to hidden changes. A different softphone setting, a different PBX region, a new enclosure revision, a verbose log level or a queue added “for safety” can all change the result while leaving the test name unchanged.

For AEC Starts with a Truthful Reference Signal, the pass condition should be written in terms of the mechanism and evidence, not just “sounds good.” The subjective call still matters—it is the product—but the engineering result has to survive comparison.

The production rule that survived the incident

Before tuning an echo canceller, prove the reference sample stream, timing and channel identity.

I translate that sentence into an operational rule. The metric or artifact that revealed the failure must remain available in future debug builds, but it must not create the same realtime cost. The known-good binary must remain recoverable. A new audio experiment must identify exactly which layer changed. A release candidate must be tested against the same user-visible behaviors that made the earlier baseline valuable.

This keeps debugging cumulative. Instead of starting every audio complaint with “maybe packet loss” or “maybe AEC,” the next investigation begins with a fault tree and a set of already-proven boundaries. The value of the V115/V127/V132A history is not the version numbers themselves; it is the accumulated map of which measurements are trustworthy and which changes have already regressed the product.

What I would do differently on the next product

I would design the measurement points earlier. The RTP callback, playout clock, queue, I2S boundary, exact AEC reference, capture slots and acoustic test points would all have named interfaces and low-cost counters from the beginning. That would reduce the amount of forensic reconstruction needed after subjective complaints arrive.

I would also separate debug verbosity from realtime instrumentation by architecture, not convention. Realtime code would update fixed counters or ring-buffer events; a lower-priority task would export summaries. If a UART or network logger can block the media task, the design has already allowed the observer into the deadline path.

For multi-device validation I would automate the call matrix and preserve a compact evidence bundle per run: firmware identity, hardware revision, test endpoint, RTP summary, device timing summary and subjective/acoustic result. The point is not to collect everything. It is to make two runs comparable.

Most importantly, I would preserve the same failure-model discipline. AEC estimates the acoustic echo path from a known far-end reference to the microphone; a stale, differently processed or misaligned reference makes adaptation solve the wrong system. That principle remains true whether the next product uses ESP32-S3, a Linux SoC, a different codec or a cloud media service.

The result I keep from this incident

Reference provenance became a non-negotiable invariant of the clear-audio path.

The deeper value is the method. I started with a perceptual symptom, located the earliest layer that could create it, chose evidence that could see that layer, changed one variable, and checked the known-good invariants afterward. The process is slower than random tuning for the first build and dramatically faster by the tenth.

The short version of the lesson is: Before tuning an echo canceller, prove the reference sample stream, timing and channel identity.

That is the standard I now use for embedded audio work. A fix is not convincing because the call sounds better once. It is convincing when the mechanism, measurement, artifact identity and regression behavior all agree about why it got better.