On Thu, 6 Aug 2009 06:08:50 -0400 Mark Neidorff <m...@neidorff.com> wrote:
> On Wednesday 05 August 2009 04:42 pm, John Hasler wrote: > > M writes: > > > i was considering to buy headphones with Active Noise Cancelling / > > > Reduction. > > > > > > But before spend money, i'd like to know if there's a software that > > > could do the same job (for free). > > > > No. Not feasible. > > -- > > John Hasler > > Is it technically not feasable, meaning that a room is too large to do noise > cancelling in, or not feasable from the linux software prespective? > Pretty much technically not feasable and from a scientific point of view, not even mathematical Think of it this way, if you want to cancel a sound wave you and another sound wave traveling in the same direction but inverted. Think of sea waves, you need to invert the wave. If they travel at different directions they will cancel at some points and add at others and may not even exist together at some places. You would also get different effects at different wave lengths due to the differing relative error. This mean that you need to cancel the wave exactly at the source or on a complete sphere around the source (with an accurate rendition on that sphere which would mean nano speakers). The second problem is that you also need exact measurements to create the cancellation wave, also on the entire sphere, and to take account the delay between measuring and reacting (even assuming zero time computation). This is only partially feasible at the headphone level where the listener and speaker are close together with a know orientation relative to the microphone, minimizing the relative error, this also explains why you get noise reduction and not noise cancellation and different effective with different noises (depending on the uniformity, pitch and direction). A good algorithm also needs to take into account where the speaker, microphone and listener are all relatively located and take an assumption on the direction the noise is coming from. It may be feasible to improvise noise cancellation headphones though, by sticking a microphone on the headphones and feed the input back inverted with the correct delay and volume. At this level it would only take some electronics, no processor at all. I don't have the time though to create a simulation at the moment to see if you need some processor based optimization of not. It does sound like a fun test though. > Mark > > -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
