How do noise cancelling headphones *really* work? - Page 1
Forum section: electronics / projects

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 06:28:39 am
content: Just curious... I have Googled and read a bit, but find the explanations I came across unsatisfying. Sure, in principle you want to measure the incoming environmental noise with a microphone mounted to each ear's headphone; then add an inverted copy of the signal to the signal driving the respective speaker. But how does this actually work in practice?(a) I assume that there is a phase delay in the microphone, and in driving the speaker. So if you simply invert the microphone's signal, your resulting speaker output will probably be "too late" -- right?(b) So, do real-world noise cancelling headphones actually delay the microphone signal further, by a bit less than 180°, so that thepreviouswave cycle received on the microphone drives the speaker, to interfere with thenextincoming wave? The fact that noise cancelling headphones apparently can't handle one-off impulse signals is consistent with that assumption.(c) But, of course, a "bit less than 180° delay" means a different delay time for each frequency component. Does the signal processing actually include an FFT, frequency-dependent time delay, and re-synthesis? Or some clever all-pass filter with the right frequency-dependent delay characteristics?(d) Finally, how do you determine theamplitudeof the cancelling signal you need to feed to the phones? Even without the active cancellation, only a dampened version of the incoming sound will reach the ear, due to the headphone's passive noise dampening. One needs to supply an inverted signal with the same (frequency-dependent) amplitude. Do the manufacturers just characterize the frequency-dependent passive dampening once, in the lab, and assume that it will be the same for each person wearing the headphone? Or is something more clever going on?I'd appreciate any insights you might have on this -- and any references to technical literature. Thanks!EDIT: Fixed mangled sentence in point (a) above...And adding one more question:(e) Several publications and DSP app-notes deal with "active feedback" noise cancellation schemes, which assume the presence of another microphone (behind the speaker), to detect and minimize an error signal. But do commercial ANC headphones have such error detection microphones?

author: Mr. Scram
author_rank: Super Contributor
timestamp: June 17, 2018, 07:55:37 am
content: Interesting question. I won't pretend I have a clue, but I'd love to see the input of others.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: June 17, 2018, 08:22:02 am
content: I don't know, I'm only guessing but... if you sample the inputs and delay the output just a bit (some ms) it seems easy to substract the "ambient noise" sampled signal from the "voice input" sampled signal and deliver that some ms later to the output.

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2018, 08:26:16 am
content: IME you betcha they work, well the good stuff does.In those I have used they were both Bose, a professional Aviation headset and their QuietComfort 35.

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 08:37:32 am
content: Quote from: tautech on June 17, 2018, 08:26:16 amI don't doubtthatthey work -- buthowdo they do it?

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 08:40:48 am
content: Quote from: GeorgeOfTheJungle on June 17, 2018, 08:22:02 amNo, that won't work. Some of the environmental noise reaches your ears directly, via the headphone's less-than-perfect passive dampening. And you have to cancel that out immediately, via a speaker output signal that interferes destructively with the "real" soundwaves reaching your eardrum. No delay allowed!

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: June 17, 2018, 08:59:38 am
content: Ahh, ok, I see. I was not thinking a pilot in a cockpit but the mobile phones' noise cancellation. Perhaps that's a different beast?

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 09:22:04 am
content: Quote from: GeorgeOfTheJungle on June 17, 2018, 08:59:38 amRight -- subtracting background noise (captured via a second microphone) from the speech signal you want to send down the line to a remote party is probably an easier task. The source signals are better defined and "symmetrical", since they are both captured via microphones, and you can allow for a few milliseconds of delay for digital signal processing.But I was thinking of the noise-reducing headphones, as used by pilots. (Or airline passengers, office workers wanting a quiet environment, etc. -- Bose's "QuietComfort" headphones are the best-known example of consumer versions.)

author: wraper
author_rank: Unknown
timestamp: June 17, 2018, 09:28:35 am
content: QuoteWhy do you think there is significant delay between microphones and speakers? On top of that, electrical signal from microphone to speaker travels almost instantly (if we assume there is no digital processing delay) while actual ambient sound still has to travel the distance to your ear.

author: wraper
author_rank: Unknown
timestamp: June 17, 2018, 09:34:08 am
content: Where phase shift will matter the most is high frequencies but, first of all, damping them is much easier and ear is much less sensitive to them as well.

author: wraper
author_rank: Unknown
timestamp: June 17, 2018, 09:39:24 am
content: https://www.infineon.com/dgdl/Infineon-AN538_Infineon_microphone_in_Noise_cancelling_headsets-AN-v01_00-EN.pdf?fileId=5546d462602a9dc801607a0e96951aa6

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 09:42:07 am
content: To answer the last of my own questions (point (e) above), the Bose headphones do indeed have another microphone which captures the speaker output. See this teardown video, from 9:45 onwards:That does open up a lot of possibilities, of course -- e.g. optimizing the amplitude (as a function of frequency) of your correction signal, depending on how well the passive dampening fits the current user's ears etc.I wonder whether even the small in-ear phones (e.g. Bose QC 20) have room for those error detection mics? @Dave, how about a teardown video?

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 09:50:39 am
content: Quote from: wraper on June 17, 2018, 09:28:35 amRight, the microphone probably contributes the smallest delay; but I would assume that driving an inductive speaker does incur non-negligible phase shifts? And even more so the A/D and D/A conversion required for digital signal processing.I agree with your other post that this becomes more relevant at higher frequencies. And indeed at frequencies above a few kHz the typical consumer ANC headphones seem to rely on passive dampening only. But still, even at the frequencies below that limit, they seem to do more than just "invert and add" -- as witnessed by the presence of an error detection microphone.Thanks for the Infineon appnote! Hadn't found that one.

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2018, 09:51:25 am
content: Quote from: ebastler on June 17, 2018, 09:42:07 amWell those aren't exactly the TOL set, are they ?From the QC35 blurb:Quotehttps://www.bose.co.nz/en_nz/products/headphones/over_ear_headphones/quietcomfort-35-wireless-ii.html#v=qc35_ii_black

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 10:11:25 am
content: Quote from: tautech on June 17, 2018, 09:51:25 amThe Bose QC 25 aren't quite the top of their product line; the QC 35 adds wireless operation as an option.Adjusting the "level of noise cancellation", as described for the Q C35, could also be offered in a "naive" analog implementation, I think: Add the inverted noise signal at your best estimate amplitude for "optimum" noise cancelling, add a bit less of it for "medium" noise cancelling. But apparently Bose -- who tend to come out in reviews as having the most effective noise cancelling -- do something smarter than that, based on active feedback of the noise cancelling result.

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2018, 10:26:48 am
content: Quote from: ebastler on June 17, 2018, 10:11:25 amYet when you look at their aviation headsets they don't seem to offer the same flexibility of settings.Maybe aviation noises are 'within' a range they can characterize and apply the right global settings for.Quotehttps://www.bose.co.nz/en_nz/products/headphones/aviation_headsets/a20-aviation-headset.html#ProductTabs_tab0

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 10:46:06 am
content: Quote from: ebastler on June 17, 2018, 09:42:07 amTo answer my own question yet again: Indeed, even the small in-ear ANC phones have two microphones for each ear -- one to detect the incoming environmental noise, and one to monitor the efficiency of the noise cancellation behind the speaker, near the ear. Pretty impressive!There's a nice sequence of tear-down photos here:https://imgur.com/gallery/OGKO0

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 10:56:04 am
content: Quote from: tautech on June 17, 2018, 10:26:48 amTheir latest offering for pilots seems to have very similar functionality to the QC 35 -- including "talk through", selectable levels of noise cancelling, and a Bluetooth option:https://www.bose.com/en_us/products/headphones/aviation_headsets/proflight-aviation-headset.html(Bluethooth?! Are airline pilots allowed to listen to music or podcasts from their iPhones if nothing interesting is going on?)

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2018, 11:00:01 am
content: FMS looks after their duties once they're properly airborne.They just need watch for warning lights and the gauges.(daughter's a Captain)

author: InterestedTom
author_rank: Regular Contributor
timestamp: June 17, 2018, 11:20:17 am
content: IMO First step is a good mechanical design, with some degree of passive Noise Rejection. Any electronic filter is then orders of magnitude less complicated, since less cancellation is demanded of it for passable performance.Having said that, given they can fit Active Noise Cancellation into earphones as well as headphones, it seems the filters work pretty well.You mentioned something about FFT followed by amplitude adjustment followed by IFFT synthesis, this would be much more computationally intense than a simple digital filter implementation. There would be a strong compromise between having enough FFT points to cover low frequencies and having a high enough sample rate to cover high frequencies. It would also take a pretty fast FPGA/ASIC/DSP to do all that in real time, it would also have to be small and low power, to operate within a battery device.Look up Digital Signal Processing, a FIR or IIR filter with 3 sets of coefficients for different environments would be very simple to implement, and much less computationally intense than full FFT followed by multiplication for each point in the FFT followed by IFFT.

author: djnz
author_rank: Regular Contributor
timestamp: June 17, 2018, 11:23:47 am
content: One way to get rid of the delay is to develop a model for environmental noise, created by constantly sampling ambient sounds with microphones. If you are smart and lucky, you can then predict future noise levels and subtract them in realtime.

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 17, 2018, 11:29:26 am
content: This TI app notehttp://www.ti.com/lit/an/spra042/spra042.pdfcontains a lot of good info on active noise cancellation methods and their implementation.

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 11:56:34 am
content: Thank you all for a lot of good input so far.Looking at wraper's comments, and the TI appnote Wolfram linked to, maybe questions (a) to (c) from my original post above are indeed non-issues: Quite possibly, the feed-forward approach is only used to cancel lower frequencies, where slight delays in generating the inverted waveform can be neglected. (The appnote also describes that the microphone can be placed further upstream, so that the sound propagation to the target gives you some extra processing time. But I don't think that is very significant in the small geometry of headphones.)So maybe my question (d) is the tricky one to solve for an ANC designer. And it seems that another microphone downstream from the speaker, as mentioned in (e), provides the typical answer: Tune your frequency response actively, based on the error signal of that microphone, so you can react e.g. to the user changing the placement of the headphone over their ears.

author: Howardlong
author_rank: Super Contributor
timestamp: June 17, 2018, 12:10:44 pm
content: Quote from: ebastler on June 17, 2018, 09:42:07 amThey do, and you have to be incredibly careful when repairing them. I accidentslly rolled my chair’s castor over a QC20 earbud once. The bud was understandably deformed, but still worked when noise cancellation was off. With noise cancellation on, not only did the noise cancellation not work at all well, there was also a continual white noise element.I did manage to repair it up to its original performance, but when trying to tidy up my handiwork, I did a partial disassembly again and was not able to get it working well again. I keep meaning to go back and have another go, but I’m pretty sure the way it’s constructed it’s pretty critical precisely where the component parts sit. There’s no easy way to adjust either, you can only really test them when reassembled.

author: InterestedTom
author_rank: Regular Contributor
timestamp: June 17, 2018, 12:19:39 pm
content: Quote from: djnz on June 17, 2018, 11:23:47 amNah... This would only work for synthesising noise. Noise isn't subtractive, it's additive. It might be possible to make the background noise appear less audible by adding noise with a different frequency content to make the cumulative noise seem more psychoacoustically inaudible to a given subset of animals.I should clarify that in Engineering terms noise usually means a signal which when sampled over a sufficiently long (mathematically speaking infinite) time has a fixed frequency content, it's amplitude at any given time is random but it's amplitude distribution when sampled over a sufficient (again mathematically infinite) length of time is fixed.Environmental/Ambient Sound is time variant in frequency content and/or amplitude content. Not all Sound is Noise, not all Noise is Sound.

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 12:32:03 pm
content: Quote from: InterestedTom on June 17, 2018, 12:19:39 pmI would put it another way: You need to add a noise signal with the correct (complex) amplitude, with exactly the opposite phase of the real-world noise. If you do that, desctructive interference will result and the noise is indeed "subtractive". But adding noise with the rightintensityspectrum only, and random phase, will not do the job.

author: InterestedTom
author_rank: Regular Contributor
timestamp: June 17, 2018, 02:00:01 pm
content: Quote from: ebastler on June 17, 2018, 12:32:03 pmNoise by it's definition is unpredictable, it is not periodic, periodic signals repeat periodically ad infinitum. Noise cannot have phase because it has no period.

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 02:11:31 pm
content: Quote from: InterestedTom on June 17, 2018, 02:00:01 pmYou are applying a narrow definition of noise here. I am not sure whether, even as a technical term used in signal processing, this would be generally accepted as a property for all kinds of "noise".https://en.wikipedia.org/wiki/Noise_(signal_processing)More relevant to the present discussion: I am not sure whether "noise cancelling headphones" can actually deal with "noise" which meets your criteria. That would exactly lead back to the question whether the feed-forward correction works by simply inverting the present signal (and hence only works if the processing and output delay can be neglected), or whether it assumes a mix of frequencies which are repetitive at least over short time scales. For a pure feed-back approach, based on a microphonebehindthe speaker, I can't see how it would deal with random noise at all.I think the manufacturers of "noise cancelling headphones" apply the term "noise" in the colloquial sense of "unwanted sounds" -- most of which will be periodic in nature, at least over short timescales.

author: InterestedTom
author_rank: Regular Contributor
timestamp: June 17, 2018, 02:17:28 pm
content: Quote from: ebastler on June 17, 2018, 02:11:31 pmYes. Sorry, I assumed djnz meant noise in the engineering/physics sense since most sounds are unpredictable (with just a microphone).

author: InterestedTom
author_rank: Regular Contributor
timestamp: June 17, 2018, 02:30:52 pm
content: Quote from: ebastler on June 17, 2018, 02:11:31 pmYou are right, it only works because the processing delay is sufficiently small to allow the interference signals to be sufficiently attenuated, even if they are noise-like.

author: Bassman59
author_rank: Super Contributor
timestamp: June 17, 2018, 03:46:40 pm
content: Quote from: ebastler on June 17, 2018, 06:28:39 amThey use optimized variants of the Least-Mean-Squares algorithm to implement an adaptive filter. Read all about it in Widrow and Stearns’ classic text “Adaptive Signal Processing.” A microphone is used to capture the noise signal outside of the ear cup, another microphone is used to capture the signal+noise (at the ears, essentially). Since you know the noise signal and you know the desired signal (after all, it’s what you’re playing through the headphones), the loop can easily derive the cancellation signal.Such noise cancellers work well eliminating the low-frequency continuous drone of an airline cabin. They can’t follow transients, so the addition of cups that seal the ears from the environment help with that.I did an adaptive noise canceller in a TMS320C30 a long time ago, basically implementing the example shown in the book (a fan masking the sound of a person talking through a microphone). It worked well enough.

author: David Hess
author_rank: Super Contributor
timestamp: June 17, 2018, 03:52:11 pm
content: It really is not that difficult. Propagation through the mechanical structure of the headphone is slow compared to electronics so delay only becomes a problem at higher frequencies where phase becomes very sensitive to the environment and this is where the effectiveness of cancellation falls off but since dampening improves at higher frequencies, this is not as much of a problem.Analog implementations work great and only require a filter with the proper phase and amplitude response which is a textbook exercise. If anything the largest problem in low power designs is excess noise in the active circuitry which results in an objectionable hiss. A very noticeable improvement of 10 to 20dB up to a couple kHz is feasible.Digital designs work the same way with phase and amplitude matching implemented with an FIR filter or equivalent. Just as with an analog filter, delay depends on the lowest frequency of operation. Digital designs have the advantage of being capable of adaptation.It is instructive to think about how echo cancellation in a POTS type of modem works. A large part of the 120 millisecond latency is delay through the FIR filter which has to be long enough to *wait* for the far end echo so that it may be cancelled. For a noise cancelling headphone, the delay through the filter has to match the speed of sound delay through the headphone structure which is not a difficult task.

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 05:12:23 pm
content: Quote from: David Hess on June 17, 2018, 03:52:11 pmAs I said earlier, that's one possibility. But I am not sure whether I quite believe it:Let's say the distance between the outside microphone and the speaker is 30 mm in a headphone -- that only gives you 0.1ms of acoustical propagation delay to work with. The Infineon microphone alone which wraper linked to (reply #10 above) has 30° phase delay at 50 Hz; that's your electronic path laggingbehindby 1.5ms. The speaker might add more lag; let alone digitization, if you apply digital filtering.How do you make up for that without some predictive approach for your cancellation signal? Or what am I overlooking?EDIT: Probably the group delay (along the electronic path for the cancellation signal) is what one should be looking at, rather than the phase delay. But that has a similar order of magnitude for the Infineon microohone: Extrapolating from the range shown in Fig. 10 of the application note, I would estimate 1.5ms group delay at 50 Hz.

author: IanB
author_rank: Super Contributor
timestamp: June 17, 2018, 05:22:52 pm
content: Quote from: ebastler on June 17, 2018, 06:28:39 amI think there are only two options for this technology. Either it is a trade secret held by Bose, Sony and others, or there are patents filed for IP protection. In the first case you won't easily find out, but in the second case you should be able to look up the relevant patents and find a description.There are at least some patents filed in this area.

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 05:51:40 pm
content: Quote from: IanB on June 17, 2018, 05:22:52 pmI agree, and admit that -- although I do often enjoy studying patents -- I have been too lazy to do so in this case, so far. There aremanypatents in this field... Bose claims that its most recent pilot headset alone is covered by 30 patents (or applications, I assume).And one never quite knows which of the patented ideas are actually used in real-world products. E.g. while the second, downstream microphone (used to control the adaptive filtering) is described in DSP application notes and probably patents, I had not expected to see them used in consumer headphones. The teardown photos on the web, and Howardlong's report on having taken one apart himself, were most helpful -- and surprising to me!

author: coppice
author_rank: Super Contributor
timestamp: June 17, 2018, 06:12:42 pm
content: Quote from: Bassman59 on June 17, 2018, 03:46:40 pmWhat you describe is the basis of classic noise cancelling with speakers. However, headphones can generally much simpler, because the setup has a lot less variability. I know that most noise cancelling headphones are simple analogue designs, and its not clear to me that even the high ones are any different.

author: IanB
author_rank: Super Contributor
timestamp: June 17, 2018, 06:31:36 pm
content: Quote from: coppice on June 17, 2018, 06:12:42 pmWhat is clear, is that some headphones work better than others.I think part of this is down to engineering. You can innovate and come up with all sorts of clever ideas, but if you don't engineer the product well the clever ideas will come to nothing. I suspect the difference between products that perform more or less well is often down to how good a job the engineers did in implementing the solution.

author: David Hess
author_rank: Super Contributor
timestamp: June 17, 2018, 07:06:53 pm
content: I have used a couple of analog implementations over the years and they all worked about the same and had the same design flaw with excess noise. Micropower CMOS operational amplifiers just suck for audio but that is what they use and I have no faith that the digital ones are any better in this respect and they are probably even worse.

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 07:34:45 pm
content: Quote from: coppice on June 17, 2018, 06:12:42 pmWhy would you say that? As mentioned above several times, and shown in the teardown video and pictures, the approach with two microphones, upstream and downstream, seems common in consumer ANC headphones. (At least the ones from Bose.)QuoteThis looks pretty digital to me. (Bose QC35)

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2018, 08:39:12 pm
content: This technology is in wider common use than you imagine, shooting for example:https://www.3m.com/3M/en_US/company-us/all-3m-products/~/Peltor-Sport-RangeGuard-Electronic-Hearing-Protector/?N=5002385+8740615+3293173751&rt=rud

author: coppice
author_rank: Super Contributor
timestamp: June 17, 2018, 08:46:19 pm
content: Quote from: ebastler on June 17, 2018, 07:34:45 pmWhat relevance does that have to whether a complex adaptive scheme is needed or not?Quote from: ebastler on June 17, 2018, 07:34:45 pmThe QC35 is a bluetooth headset, so it clearly has considerable digital and RF hardware. That doesn't mean the cancellation is digital. What looks digital about the board, anyway?

author: ebastler
author_rank: Super Contributor
timestamp: June 17, 2018, 09:02:19 pm
content: Quote from: coppice on June 17, 2018, 08:46:19 pmI assume the second microphone (error detection, behind the speaker) is there to provide the input for the adaptive scheme. Otherwise, why would it be needed in addition to the first microphone (outside, to detect the environmental noise)?QuoteLargish chips and a quartz clock?Bose does state that they have moved to digital signal processing, apparently starting with the QC25 (which is not wireless).https://media.americanmusical.com/ItemFiles/Manual/Bose_QC25_FAQ.pdf

author: tautech
author_rank: Super Contributor
timestamp: June 17, 2018, 09:08:44 pm
content: The QC35 has corded input too, BT is only there if you need to use it and selectable. Battery life is of course less when you do.

author: coppice
author_rank: Super Contributor
timestamp: June 17, 2018, 09:46:42 pm
content: Quote from: ebastler on June 17, 2018, 09:02:19 pmThat's what most bluetooth solutions look like.Quote from: ebastler on June 17, 2018, 09:02:19 pmThe description in that document is odd. It seems to imply they changed to digitising the signal only to get the size of the hardware down, but still using the techniques from the older headphones. They also say their approach to digitising is low latency, to avoid problems other digital solutions cause. That doesn't sound like an adaptive solution.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: June 18, 2018, 03:40:52 am
content: One thing that helps in wrapping your head around how these things work is to remember the wavelength of the sounds involved. As mentioned previously the cancellation works best at low frequencies, typically a couple of thousand Hertz and lower. With sound velocity about 340 meters/second, a 2000 Hz "noise" has a wavelength of about 17 cm, which is huge compared to the headphone volumes.The second thing that helps is recognizing that they don't help nearly as much for white noise. They are really good for engine rotation frequencies and their overtones, resonant frequencies of aircraft structures and the like. My Bose headphones are magic for reducing cabin noise in an airliner. Not nearly so magical for office noise, unless that noise is dominated by things like the hum from fluorescent lights, fan noise and the like.

author: ebastler
author_rank: Super Contributor
timestamp: June 18, 2018, 05:37:49 am
content: Quote from: coppice on June 17, 2018, 09:46:42 pmYes, but they wouldn't use two identical chips on the same PCB, would they? (I'm not sure what these are in the context of digital signal processing either, of course.)There are a few more disassembly photos in this thread:https://community.bose.com/t5/Wireless-Headphones/QC35-Bluetooth-Stuttering-Drop-outs/td-p/23495/page/23. Having seen a few PCBs of analog ANC implementations, I would say there is simply not enough analog stuff present in the QC35 to do the filtering etc. in analog hardware.

author: InterestedTom
author_rank: Regular Contributor
timestamp: June 18, 2018, 04:27:58 pm
content: Quote from: ebastler on June 18, 2018, 05:37:49 amErm could either be digital or analog.Digital: two DSPs running independent algorithms.Analog: Switched capacitor filters.But I doubt a company would get away with saying they are using one technology when actually they are using another.

author: mark03
author_rank: Frequent Contributor
timestamp: June 18, 2018, 05:36:51 pm
content: As far as I am aware, no one is using digital, LMS adaptive-filter-based noise cancelation in a headphone form factor. If anyone has definitive evidence to the contrary I would be *very* interested to see it. The reason is fairly obvious, as already pointed out in this thread: the acoustic delay from a microphone mounted *on* the headphones, to the inside of your ear, is nowhere near long enough to do DSP, unless you gin up something really crazy with analog-based discrete-time processing. The A/D and D/A delays alone would kill you. Now, of course this doesn't matter if the noise is periodic, but in practice those systems don't perform very well, in fact not as well as the analog-based feedback/feedforward combos. (Austria Microsystems, or whatever they are called now, have a whole range of chips intended for this application, and some nice app notes to learn more about it.)I believe the better analog ANC systems, like the ones from Bose, have a certain amount of digital tunability to optimize performance across different users and varying fit. That's more than enough for a marketing department to claim "digital ANC!!!!"

author: m98
author_rank: Frequent Contributor
timestamp: June 18, 2018, 07:36:28 pm
content: In terms of aviation headsets, Lightspeed Zulu 3 has, at least subjectively to me a little better noise canceling performance than the Bose A20. And they seem to use a lot of discrete analog signal processing for their ANR technology.https://fccid.io/2AFOMLSA01/Internal-Photos/Internal-Photos-3759648

author: bson
author_rank: Unknown
timestamp: June 19, 2018, 03:36:18 am
content: Quote from: ebastler on June 17, 2018, 06:28:39 amReactive phase, i.e. an angle between voltage and current is a delay (on either voltage or current). But this is not the only use of "phase"; if you go back to the basics:S = M*sin(wt+p) where p is the phase, you can see there is no "delay" here. For any t that produces S=M, making p=pi makes S=-M for exactly the same t. Hence, signal inversion is a 180 degree phase change, just not areactiveone. Of course, inverting a signal without delaying a full cycle is trivial. (Other non-reactive phase changes are much harder in a circuit, but trivial using DSP.)

author: David Hess
author_rank: Super Contributor
timestamp: June 19, 2018, 03:44:17 am
content: Quote from: mark03 on June 18, 2018, 05:36:51 pmIf it can be done in the analog domain then it can be done in the digital domain. The right converters can have a maximum latency of one sample and nothing prevents oversampling and decimation to reduce converter latency below that but diminishing returns are quickly reached because the latency of a single cycle converter starts out low. Then it becomes a matter of clocking data through a finite impulse response filter which has the same group delay as the analog filter it replaces.Unfortunately the only thing this improves is the ability to tune the filter digitally and it requires more power and is noisier when these are already problems in analog designs.

author: InterestedTom
author_rank: Regular Contributor
timestamp: June 20, 2018, 09:13:49 am
content: Quote from: ebastler on June 17, 2018, 05:12:23 pmThe infineon microphone is a silicon microphone, I think you will find more conventional construction microphones will have a better phase response.