AS3460
ALC Tuning Guide
Demo Kit Manual
• PUBLIC
UG000480
• v3-00 • 2021-Nov-08
126
│ 79
Within this first test, you can already rule out many common mistakes. For example if you do not hear
any cancellation, but instead nothing or even a noise boost. This could be due to either a gain offset or
an inverted signal. Use the Live Mode to play around with the feed forward gains and inverting the
signal. If decreasing the gains improves cancellation, it means that your measurements are not
calibrated correctly. If cancellation occurs after inverting the feed forward signal, it means that your
measurements were measured with an inverted speaker polarity or the measurement calibration did
not account for the microphone polarity. A howling noise could mean that the signal routing is not
correct and the feedback microphone signal is going to the feed forward filter
.
Information
●
Do not invert the signal while the earphone is in your ear.
●
Do not use the "Gains" tab for playing around with gains. The gains in the "Gains" tab are
meant for production calibration only. These values persist on the chip independent of the
configuration. This can lead to mysterious gain offsets and confusion.
At this point, you can also evaluate the quality of the ANC filter. Test if the low frequency and high
frequency overshoot is within acceptable levels and if different people experience the same ANC
performance. With regular listening setups, it can be hard to spot a low frequency overshoot. We
recommend using a very large subwoofer to reproduce low frequency sound pressure levels as they
can occur inside a bus or aircraft. If you do not have a very large subwoofer or an aircraft, you might
need to purchase a bus ticket.
Information
●
If you want to measure the ANC performance, make sure to use a broadband noise test
signal, since special signals, e.g. a sine sweep, would distort the adaption.
●
The commonly available ear simulators (e.g. HATS) which are, as mentioned before, not
suitable for characterizing leaky headphones, are also not suitable for measuring ANC
performance in leaky headphones. Please use the
ams
leakage adapter or measure the
performance in a real ear by probing the feedback microphone.
7.7
Feed Forward Tuning
– High Leakage and Adaption
After proving that the low leakage feed forward filter works, the next step is to design the high leakage
feed forward filter. The filter design can be approached similar to the low leakage filter, but you should
make sure to not use too much bass boosting. When using too much bass boosting, it could happen
that there is not enough signal headroom left to handle situations with excessive low frequency noise,
such as sitting on a bus or in a car. Too much means more than 40dB filter gain for typical microphone
combinations.
