Noise-Reducing Headphones Hide Analog Heart

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.

That's the AS3501 in a 4mm X 4mm package in the middle of the circuit board.

That's the AS3501 in a 4mm X 4mm package in the middle of the circuit board.

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.

OK, so there is a little bit of digital circuitry. But not much.

OK, so there is a little bit of digital circuitry. But not much.

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.

Comments welcome.

22 comments on “Noise-Reducing Headphones Hide Analog Heart

  1. brentbutterworth
    January 24, 2013

    Sony has a new headphone, the MDR-1RNC, that uses DSP for noise cancelling. The results aren't super-impressive, but not bad for a first attempt. Here's a review with isolation measurements (which I did):

    I'm told by a buddy at Analog Devices that new ultra-low-power DSPs will make their use in headphones much more common, and achieve results “far better than the Bose QC15.” That's a tall order; I hope he's not just boasting.

  2. goafrit2
    January 24, 2013

    I think Bose owns the world of great headphones though the sector has come under attack with many cheaper products from Asian firms. Yet, I always think you pay for what you get. If you spend $200 for a  headphone, you should expect better quality than one that is sold for $30.

  3. Michael Dunn
    January 24, 2013

    On my last flight, they were selling what they called  noise-cancelling earphones for $10! Surely they were just passive phones with higher isolation, wouldn't you think?

  4. eafpres
    January 25, 2013

    A little known fact is that, at least in business and first class, and at least before the Airbus, all United head sets are noise reducing.  I have tried it and they do in fact reduce cabin noise.  As I recall there is a way to check, switching to a particular channel or something like that.  Since they presumably have power in the connector, power drain wouldn't be an issue but they have been around so long it seems they must be analog.  Not as good (nearly) as high end ones but somebody was thinking.

  5. Brad Albing
    January 25, 2013

    Altho' the $30 cans do work surprisingly well. Not bad if you just want to knock down the roar of the jet engines just outside your window.

  6. g_money99
    January 25, 2013

    More and more airlines are starting to offer these kind of noise cancelling headphones, especially in first and business class.

    They are able to make them low cost by putting all of the noise cancelling electronics in the actual headphone jack in the armrest. The headphones themselves only contain the microphones.

  7. TheMeasurementBlues
    January 25, 2013

    I once tried the Bose noise-cancelling headphones while in the Bose cafeteria. I was doing interview for a story on their automotive sound systems. The headphone really did cut down on noise from other conversations. Bose wouldn;t let me take them headphones home, but at least I got a Bose music CD (blues, or course).

    The Bose NCH are just too expensive, so I bought a set of NCH from the Sony outlet store for $25. The NC did help a little on planes, but once the battery ran out, I never replaced it. Recently, I bought a set of Bose over-the ear non-NH headphones for $109 at Costco. I'll give them a try on the way to DesignCon next week. The bose work great on a 10-hour train ride, though.


  8. eafpres
    January 25, 2013

    Speaking of noise reduction, I read that Cirrus might have 2 chips in the iPhone 5; the 2nd one possibly adding many enhanced features including the better ambient noise reduction Apple is advertising about.  Cirrus has had the codec chip in the iPhone for a while, but the 2nd one is new.

    Has anyone tried the new noise reduction in iPhone5?  The standard codec seems all digital after ADCs; however I can't find much on the 2nd chip.  Analog or DSP?  Apple is certainly power sensitive.

  9. Brad Albing
    January 25, 2013

    I've seen the ads for the phone. I have to ask around and see if anyone knows specifically how it's being done.

  10. goafrit2
    January 26, 2013

    >> A little known fact is that, at least in business and first class,

    One remarkable things about most of those sets is that they work fairly well in the aircraft but when used outside, they offer limited value. I have tried to use of these systems at home in the old day when they were free and they allowed you to take them and found them not useful

  11. goafrit2
    January 26, 2013

    >> The headphones themselves only contain the microphones.

    @g_money99, why will an airline headphone have a microphone? Largely they are speaker units. You hadly interact through voice. So including a micr is a waste of resource

  12. goafrit2
    January 26, 2013

    >> Has anyone tried the new noise reduction in iPhone5? 

    I think they combined many things in iPhone5. In one of the articles by IDC, they were able to show how they got the noise highly suppressed by using the second micr. Yes, Apple does run ads explaining that in one of their orchestra ads. But looking deep into what they did I have no idea. My understanding is that there is a way you can align those MEMS micrs and get this done.

  13. eafpres
    January 27, 2013

    Well, at low frequencies the wavelength of sound is a lot larger than the phone, so I'm not sure they could do it with spatial positioning and some kind of interference cancellation.  Also, that would only work over limited frequencies.  So it almost has to be an analog circuit as Brad described or a DSP.  Based soley on the mechanical design and the advertised performance, my guess is that they listen with one MEMS microphone, and create cancelling signals they add into the other speaker your ear listens to.

    But there could be even more subtle things going on.  Suppose they excite the whole handset at very low amplitudes but with correct phase to cancel sound incoming from the back side?  That would be susceptible to being disabled by a cover.

    Because they appear to be using a Cirrus chip, if that chip is available to others, then the solution must be “generic” and not too platform specific.  However, I don't know yet if the chip is sole sourced to Apple.

  14. g_money99
    January 28, 2013

    @goafrit, The cheap noise cancelling headphones that airlines are offering these days are different than standard NC headphones. Normaly everything is contained in the headphones, the electronics and microphones as discussed in the blog. To save money airlines are incorporating the 'expensive' electronics part into the headphone jack in the armrest. The headphones now only need the microphone portion of the noise canceling system making them much lower cost.

  15. goafrit2
    January 28, 2013

    >> To save money airlines are incorporating the 'expensive' electronics part into the headphone jack in the armrest.

    This is the first time someone has successfully explained that. Thanks.

    It seems the airline industry is crunched that even $2 makes a lot of difference for them.

  16. RedDerek
    February 22, 2013

    Mic pick-up location is the key, having to be close to the ear. The processing time and the wire length clearly are not an issue to respond at audio frequencies. Analog, once integrated, can be very elegant – as in this case.

  17. aefrgqergqwergqerg
    February 22, 2013

    Active noise cancellation is all high-zoot, but if you want about 30bD of reduction just plug in Etymotic in-ear headphones. Even with no music playing, the crying baby in the seat behind you is un-noticiable, and if you play music at low volume, nothing will disturb you. Shure makes a cheaper version– I have not tried those.

  18. Brad Albing
    March 20, 2013

    Martin – still waiting for your report on those inexpensive Costco cans.

  19. Brad Albing
    March 20, 2013

    An addendum – besides the processing cktry, I'm pretty sure they have multiple mirophones. It's like a Greatful Dead concert.

  20. Brad Albing
    March 20, 2013

    Good info – I'm researching them now.

  21. Brad Albing
    March 28, 2013

    Also the screaming kids behind you. Sadly, these cans won't help suppress the kicking-the-back-of-the-seat phenomena from the screaming kids behind you. And they won't give you any more seat room due to the fat guy sweating next to you.

  22. Brad Albing
    March 28, 2013

    Got to find some that aren't painful to leave stuck in my ear canal for 5 to 8 hours.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.