AS3460
ANC Tuning Guide
Demo Kit Manual
• PUBLIC
UG000480
• v3-00 • 2021-Nov-08
126
│ 62
Equation 4:
𝐻
𝐹
(𝑠) = −
𝐻
𝐻
(𝑠)
𝐻
𝑀
(𝑠) ∙ 𝐻
𝐴
(𝑠) ∙ 𝐻
𝐿
(𝑠)
It is not necessary to measure these transfer functions separately, as they are all included in the
characterization measurement, yielding the equivalent expression:
Equation 5:
𝐻
𝐹
(𝑠) =
𝐻
𝐴2𝐸
(𝑠)
𝐻
𝐴2𝐹𝐹
(𝑠) ∙ 𝐻
𝐷2𝐸
(𝑠)
Where D2E is the “Driver to Ear”, A2E is the “Ambient Noise to Ear” and A2FF is the “Ambient Noise
to Feed Forward Microphone” transfer function. This measurement procedure is described in detail in
the App Note on “Characterization”.
The goals when designing the feed-forward filter are simple:
●
Match the target both in phase and magnitude over a frequency range as large as possible
●
Where a match is not possible, reduce the gain
6.2.2
Tuning Guide
Before starting the filter tuning it is important to evaluate the target curve for several samples and fits,
to know what parts of the transfer function a reproducible and should be taken into account for the
filter design and what parts change for every sample or due to the fit of the headphone. This way
overfitting can be avoided and the filter will work on all samples with a similar performance.
To match the ideal filter or target curve it would usually be necessary to have an extremely high low
frequency gain. The first step when starting a feed-forward filter design is to introduce a low shelf and
sometimes also a wide peak at low frequencies, to identify the frequency at which the phase can be
matched without producing too much low-frequency overshoot. The next step is to use peak and notch
filters to follow the target curves. Finally, a low pass should be used to suppress the higher
frequencies where no phase match is possible.
6.3
Example
The figures below document such a filter design procedure:
