One of my business colleagues at UBM recently wrote about a pair of noise cancelling headphones (NCH) he owned. He did a teardown of the phones to see what technology was used.
You might think that the best approach to get effective noise cancellation would be based on a powerful DSP. That way, you could tailor the response needed to the ambient noise characteristics and the signal characteristics (likely music but possibly voice). And you could even add some user interface to allow selection of certain noise cancellation characteristics.
If you did all of this, you would likely have a dandy set of cans that exhibited superb noise cancelling ability. Sadly, they would do so for perhaps only a few hours, probably very few. The problem is that the delightfully powerful DSP cited above would be quite the power hog. Typically, these headphones are powered from one AAA alkaline cell. The nominal capacity of such a cell is around 1.0A/hr. A typical NCH will have a published operating time spec of perhaps 30 hours. This implies a current draw of about 33mA.
So, how do they do it? Well, it's mostly analog circuitry that does the audio processing. The heart of the NCH analyzed in this example is the AS3501 from Austria Micro Systems.
This device contains preamps for the noise-sensing microphones (one located in each earphone housing). Following the preamps, there are automatic gain control blocks and an “ANC block” (the automatic noise cancelling block where the IP magic takes place). There are provisions in the ANC block to limit those annoying clicks and pops associated with powering up the device. Also included is clip detection circuitry to keep the output voltage swing within the rails. This is done by reducing the gain of the line in amplifiers through which the program material passes. There is also filter circuitry here to tailor the frequency response and a very low power charge pump to develop additional supply voltages.
There is a small amount of digital circuitry on the AS3501 — for example, a PROM that contains calibration info related to the two microphones and the two earpieces. That device is one-time programmed in the NCH manufacturing process. The PROM uses very little power.
The tiny speakers are driven either as single-ended (grounded load) devices or as bridge-tied load devices. That's the version where they are driven by an H-bridge of output transistors and neither side of the speaker is tied to ground. Configuration here depends on how much power is needed for the speakers and their impedance.
Overall power consumption of the IC is quite low: Current draw varies from around 5mA to several tens of mA depending on program material.
I am not telling you all this because I want to sound like an advert for Austria Micro Systems. Instead, I want to bring to your attention a good example of what we are going to discuss here in the ongoing months and years regarding integrated analog. In another blog, More Thoughts on Integrated Analog, I mentioned analog front ends with specialized bridge circuitry to work with strain gauges. But that may be a bit too esoteric an application. But everyone is familiar with headphones and probably most of you have had a chance to see how they work — generally, pretty well. And the reason is parts like this.