AS3460
ALC Tuning Guide
Demo Kit Manual
• PUBLIC
UG000480
• v3-00 • 2021-Nov-08
126
│ 82
Does Not Reach gm_state_left = 0
●
If the algorithm does not reliably reach the gm_state_left = 0 state in all leakage conditions,
either the ANC performance achieved with the current filters is not good enough or a corner
case prevents the state transition. Make sure to turn off all corner cases in the ALC
configuration at this state of the tuning process.
7.7.4
Stability Problems
It can occur that, when going rapidly from highest to lowest leakage, i.e. pushing the earbud quickly
into the ear ("push-in"), an instability occurs, because the high leakage filter gain is high enough to
cause a feedback loop when used at low leakage. A sub-algorithm named "FF stabilizer" is in place to
avoid it. Enable it in the ALC configuration if necessary. If instabilities still occur, you can tune its
sensitivity in the "FF stabilizer" section of the ALC configuration. Use the pre-defined watchers (eye-
symbol at "FF stabilizer", then click on the eye symbol of Flex's top bar to open the watcher window) to
observe the instability metric during a push-in and adjust the thresholds accordingly. If the headphone
is very prone for this kind of instability it might be a hint for assembly problems that cause a large
acoustic bleed from driver to feed forward microphone.
7.8
Feedback Filter
Once the feed forward only adaption tuning is complete and proven to work as desired, the next step
is to add feedback noise cancellation.
7.8.1
Feedback Filter Design
In general, the feedback filter can be designed in the same manner like on a non-adaptive headphone,
but must meet the following requirements:
●
stable above 3 kHz in
all
leakage conditions
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a maximum performance of 10 to 15 dB
●
a simple shape of the performance curve with only one minimum
●
the downsampler settings must be the same as for the feed forward filter
