Technical Article. Feed-forward active noise cancellation: practical considerations for wired headset designs. By Joel Gehlin.

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1 Technical Feed-forward active noise cancellation: practical considerations for wired headset designs By Joel Gehlin Page 1 / 9

2 Feed-forward active noise cancellation: practical considerations for wired headset designs By Joel Gehlin Field Applications Engineer (Scandinavia), ams AG Active noise cancellation (ANC) technology is in transition from being an expensive luxury to an affordable, mainstream feature of earphones and headphones. More and more manufacturers of consumer and professional devices are therefore now starting to adopt ANC. Like almost every design project in electronics, bringing a new ANC design to market is easier said than done. Of course, off-the-shelf ANC speaker drivers from suppliers such as ams provide an obvious starting point for a new project. But there is more to the implementation of ANC than simply dropping an ANC IC into an existing earphone or headphone design. This article provides a designer s hands-on guide to two important aspects of ANC system design using the feed-forward topology: optimising the filter network; and providing user controls. Matching the acoustic properties of the phones Table 1 shows the three topologies used today in ANC systems. In a feed-forward system, a microphone exposed to the environment listens to the ambient noise. The system s transfer function simulates and subtracts the noise from the audio signal fed to the speaker inside the unit. A feedback system by contrast uses a microphone beside the speaker inside the ear cup or bud. Rather than sensing the ambient noise around the wearer s head, it listens to the actual noise at the ear. Both the feed-forward and feedback topologies have their advantages and drawbacks: the hybrid design approach aims to get the best of both. Page 2 / 9

3 Table 1: ANC topologies for various headphone types In the feed-forward topology, the noise signal sensed outside the headphone by the exterior microphone is not identical to the noise apparent to the wearer s ear: the noise signal is attenuated to some extent by the bud, pad or cushion that fits in or on the ear. So before the microphone s output can be inverted (so as to cancel it), it has to be filtered. The purpose of the filter network is to mimic as closely as possible the attenuating effect of the chosen headphone design, within the frequency band audible to human hearing. The attenuating effect of the headphones can be effectively represented by phase and gain curves plotted against frequency: these may be provided by any professional acoustic testing laboratory. With these curves in hand, the design team can now set to work on designing the filter circuit. Figure 1 shows a typical filter circuit for use with the AS3410 feed-forward ANC speaker driver IC from ams. In the green circle is the filter schematic for the left audio channel. As can be seen, the filter architecture consists of T-notch filters, band-pass filters, low- and high-pass filters and shelf filters. Page 3 / 9

4 Fig. 1: typical feed-forward filter implementation The filter design should start with the two T-notch filters: their values may be modified until the filter s output roughly matches the headset s phase curve. Starting with the twin T-notch filters on the QMICL OP output, a band-stop filter is created that helps the design to reach the lower db peaks in the headset s gain curve; it also boosts the phase curve at the given notch frequency. The upper part is the low-pass circuit and the lower part is the highpass circuit. Using resistor R7, it is possible to change the gain of the system. For example, if R7 has a small value, the gain of the whole curve is increased, and vice versa. With resistor R8 it is possible to smooth the notch curve a little. After this there is another notch filter, which works at higher frequencies, and a kind of low-shelf filter (C11 and R12). The low-shelf filter works on the low frequencies of the audio signal (20Hz to 1kHz); C11 increases or decreases the gain curve below the shelf frequency. In this case, when C11 has a high value it increases the gain curve, and decreases the curve at lower frequencies when C11 has a low value. ams provides a simulation software tool to users of its ANC ICs: Figure 2 shows the phase and gain simulations of the filter circuit in Figure 1. After a first simulation like this, the filter can be fine-tuned Page 4 / 9

5 to get an even closer match to the attenuation properties of the headset. It is particularly important to optimise the gain and phase match between 100Hz and 1kHz, the range in which the feedforward topology attenuates noise most effectively. As Figure 2 shows, at very high frequencies it becomes impossible to achieve an adequate phase match. Here, it is important that the calculated gain curve is below the measured gain curve, otherwise the headset could suffer from oscillation problems. Fig. 2: simulation results for a typical feed-forward filter circuit Once the filter design has been fine-tuned, it is important to test the system s ability to cancel ambient noise in the real world. To do this, the designer should measure the headset s response with an Audio Analyser. A unit such as the 2700 Series from Audio Precision is suitable for this task. Figure 3 shows a typical example of a feed-forward headset s attenuation performance when using the AS3410 ANC IC. The red line shows the passive attenuation provided by the headset alone, without ANC enabled; the green line shows the noise attenuation achieved with ANC enabled. At its peak, noise is attenuated by a huge 35dB. Page 5 / 9

6 Fig. 3: noise attenuation of a typical feed-forward headset using the AS3410 Implementing user controls It is important for the headset s user to be able to turn the ANC function on and off, so that they can choose to hear external sounds (such as public announcements). Headset designs today typically use a slide switch placed on the ear cup or dongle. But if a Monitor function is required, the hardware design can become tricky. Monitor function is an assisted hearing mode: it can be implemented in a feed-forward system, because it can amplify, as well as cancel, the ambient noise sensed by the microphone. The schematic in figure 4 shows how a slide switch has been combined with a push button for controlling the Monitor mode. Page 6 / 9

7 Fig. 4: schematic showing a typical feed-forward ANC design with slide switch and push button A brief press on the Monitor switch toggles the output state of the NAND gate latch, changing the AS3410 s function to assisted hearing mode. In effect, this enables the R3/R4 resistor divider so that the voltage on the MODE_SCSL pin is half of the battery voltage. The benefit of this circuit is that it is flexible: almost any CMOS NAND gate can be used, including Schmitt trigger gates and/or open collector inverters, NAND or NOR gates, transistors, FETs or MOSFETs. Its function is very simple: when power is first applied, the capacitor at the input of the first gate (C1) guarantees that the latch output (Node 2 on the U2A inverter) is initially latched low. Thereafter, button presses toggle the output high and low and open/close the PMOS switch, which enables/disables the R3/R4 resistor divider. Page 7 / 9

8 Another user control that may be required in ANC headsets is an indication of battery status. An LED indicator can also signal whether the ANC function is on or off. Support for indicator LEDs is integrated into the AS3410: it includes a current sink for an LED that can be controlled in three steps, and be switched off, by setting the PWM to 0%, 25%, 50% or 100% duty cycle of a 50kHz signal. This LED driver function can be combined with battery voltage monitoring to provide an indication to the user of the battery s state of charge. If low battery monitoring is implemented in the AS3410, the ILED switches to blinking at a 1Hz, 50% duty cycle and 50% current setting when the low-voltage threshold is crossed. It is also possible to configure the low battery threshold to be 100mV higher than the actual battery level, to provide advance warning of a low battery. Figure 2 shows a circuit providing a low-battery indicator: it uses a discrete voltage sensor from ams, the AS1926, with two reset signals (one inverted) and manual reset (MRN - not used in this example). Fig. 5: the AS3410 offers an integrated LED driving capability Page 8 / 9

9 When the battery power source is good, ILED blinks the green LED at 1Hz. As soon as the AS1926 detects that the battery voltage has fallen to 1.0V or below, ILED stops blinking the green LED and starts switches to 1Hz blinking of the red LED. It is important to note that VBAT should be kept high enough to support both the forward voltage of the red LED and the voltage drop over PMOS1. To decrease power consumption while the battery is good, the green LED s light output can be reduced by choosing a larger value of the current set resistor RISET. Taking a design into the real world The circuit and filter design approaches described above provide a guide to the process of implementing an ANC headset with a high-quality ANC speaker driver. But every design will have its own characteristics, derived from the mechanical construction of the headphones and the choice of microphones and speakers. This means that, to achieve the optimal acoustic performance, the designer will need to go through multiple iterations of the feed-forward filter design. This makes simulation software an extremely valuable tool. In addition, the use of highquality microphones and speakers reduces the complexity of the filter design. It can happen that a design team starts a project with the intention of using the AS3410 for a feedforward design, but finds that the quality of microphones and speakers specified require an excessively complex filter circuit. In this case, it can replace the AS3410 with an AS3430, which integrates two additional op amps. This extends the capability of the filter design, even though the AS3430 is normally used in feedback ANC designs. When bringing a simulated ANC design into the real world, it is therefore helpful to work with an ANC expert, both to provide technical guidance, but also, as in the case of the ams ANC portfolio, to be able to port a filter design from one ANC speaker driver to another. Page 9 / 9