Develop Reference

Speaker output to mic input without cable on Android?

I want to connect speaker output to mic input on Android phone without using any physical cable. Such routing should be transparent to all apps. With this function I can design a bunch of interesting apps, e.g., measuring round-trip delay of a voip call, automatically replying to an incoming call etc.
Some similar questions can be found here:
Background music for call
Outputting audio stream into microphone
It seems that there is no solution on Android right now, but the tool JACK running on Linux is the closet one. Can someone provide me some scripts or codes to create a JACK client that realizes this (speaker -> mic) on Linux? And how hard would it be to port JACK server and client to Android?
Thanks!

Getting Jack on Linux to connect the mic to the speakers:
Start Jack with QJackCtl. This is not as easy as it sounds:
Start QJackCtl.
Click the "Setup..." button.
Choose "alsa" from the "Driver:" dropdown.
Choose your soundcard/soundcards that you want to work with from the "Input Device" and "Output Device" dropdowns. Note that if you have a relatively capable soundcard, there may be a number of possible devices for it (my Audigy 2 ZS has four). Choosing the correct one is beyond the scope of this tutorial.
Check the latency in the bottom-right corner. Realize that Jack itself will take some time to process the sound. This gives you information about that (and I'm sorry, but I don't know precisely what it means). Running Jack is often a tradeoff between getting a super low latency and avoiding "xruns", which are where Jack has to give up on processing a section of audio. Xruns often result in a "click" sound.
Click "OK."
Click the "Connect" button.
This opens the "Connections" panel. This is basically a place that you can plug different Jack applications into one another so they process audio from each other.
Choose the input ("system" on the left) and the output ("system" on the right) and click "Connect". You may need to look through the various things listed under "system" on one or both sides to find the thing that's actually capturing from your mic/outputting to your speakers, in which case . Again, this is beyond the scope of this tutorial; Google is your friend, as is trial and error.
Enjoy the noise of feedback when you bring the microphone near the speakers.
As far as porting Jack to Android goes, you might take a look at this, which basically seems to indicate that Android isn't able to handle the low-latency requirements of Jack (which might also cause you problems in trying to do latency measurements). You could also look at this and this, which might offer an alternative way to do what you're trying to do (I'm not sure where it stands regarding the latency issues, also, I've never used either PD or libpd myself).

How do I simultaneously record audio from every mic on a microphone array into a separate buffer in Android?

Many tablets and some smart phones use an array of microphone for things like noise cancellation. For example Motorola Droid X uses three microphone arrays and even allows you to set "audio scenes". An example is discussed here.
I want to be able to record from all the microphones that are available on the tablet/phone at the same time. I found that using AudioSource we can choose the mic (I do not know which mic this is specifically but it might be the one facing the user) or the mic that is in same orientation as the video camera, but could not find anyway of accessing all the other mic in the mic array. Any help that points me in the right direction to investigate this will be great. Thanks in advance for your time.

It's seems like you've verified that there isn't a standard Android API for accessing specific mics in an array. I couldn't find anything either.
As is the case with custom additions to the Android system, it's up to the manufacturer to release developer APIs. Motorola has done this before. I took a look at all of the ones they have listed and it seems they simply don't expose it. Obviously, they have code somewhere which can do it (the "audio scenes" uses it).
So the quick answer: you're out of luck.
The more involved answer: you can go spelunking around the source code for the Droid X because it's released as open source. If you can actually find it, understand that you're using an undocumented API which could be changed at any time. Plus, you'll have to do this for every device you want to support.

Android and Sound processing?

I am taking this crazy class on Moble Programming. We have to do a final project and I would like to do some sort simple guitar processor app.
I wanted to do this in IOS, but it seems like the learning curve for IOS is to impractical for a short class.
No offense to anyone but Droid is easier to program, at least to me, but I am confused if you can even get guitar input from a jack (not mic) and then do some processing on the input and feed it to the output.
I'm aware of latency, which may or may not be a big deal for a class.
Does anyone know if Droid can do anything like this? If so any articles or somewhere to start? I know with IOS you can at least buy a jack and it seems to have tons of open source processing code, but I can't seem to find anything for Droid. All I have seen is "Ghetto Amp" for guitar stuff.
Any ideas?
Thanks

You may want to look at this project:
http://code.google.com/p/moonblink/wiki/Audalyzer
should be pretty useful :)
However the core class you will be using to pick up and look at audio streams is: http://developer.android.com/reference/android/net/rtp/AudioStream.html

I wrote a MIDI guitar for a college project a long time ago, in assembly for a Texas Instruments DSP. As long as you just played exactly one note, and were really careful about it, it could tell what you'd played.
Not much amplification was needed. In fact, I could get some notes even on an unamplified signal. I had oscilloscopes and a pretty generalized ADC to work with, you might have to amplify the signal...but if you do, be careful not to fry your input. Start low...and really, the more you can read up on the tolerances the better.
Looks like they never made any hi-fi micro-USB 24-bit ADCs or wrote drivers for them. I guess there's no market. :) But if you're doing a school project and not producing the latest Muse album, get a path from your guitar to the headset line in:
http://androidforums.com/android-media/194740-questions-about-audio-recording-droid.html
I'd probably just sacrifice a cheap or broken headset to get the headset plug. ( Maybe they sell appropriate tips at Radio Shack but I've learned not to assume such things anymore :-/ ) After building a cable I'd I'd feed it an amplified signal from the guitar so I could control the gain level to whatever I wanted.
Depending on latency requirements you can use Java or NDK. Note this answer:
Need help about sound processing
(I have one of the original Droids sitting around in a drawer, I'm sure I could use it for something but I just haven't figured out what!)

How does noise cancellation work in android?

I came across this relatively old post which describes how impressively Nexus One's noise cancellation works and I was wondering where can I find more information about its implementation in the OS software.
In particular:
How much of it is done using software and how much of it is done in
hardware?
Which modules in the Android source code are responsible for noise
cancellation?
Can I control its behavior via Android's API? (if so, which ones)
Does it also work with the microphone in the headset that comes with
Nexus One (4-pin 3.5mm jack) or does it work with the built-in
microphone only?

I only know the answer for the Nexus One, but:
It's done in hardware.
Not sure.
Nope.
Maybe?
For the N1, it works using a second microphone in the back, and comparing the two signals. I don't know exactly how this process is done (hardware or software), but I know there isn't an API for it. Also, it probably doesn't work for the external headset, since there's no second sound source to compare the first one to (unless the headset has two mics too, but I don't think it does).

About the Nexus One:
All hardware only configuration in software.
Sound drivers and sound system but only configuration.
No API possibly some prop configuration but I haven't been able to get that to work.
No, longer reply following.
I haven't found any indication that it uses the other microphone to do noise reduction for the headset. It wouldn’t make much sense either as it would most likely just try to cancel out with the noise from your pocket.
For most other android phones and for headset on the Nexus One I'm pretty sure that there is only some sort of filter to reduce input of sound that is not speech.
I have done some research on this that I tried to get some help with on the android porting and dev lists. There is a little further info:
http://groups.google.com/group/android-porting/browse_thread/thread/fe1b92065b75c6da?pli=1
With the reservation that I haven't looked at the latest and greatest versions of android.

Microphone input

I'm trying to build a gadget that detects pistol shots using Android. It's a part of a training aid for pistol shooters that tells how the shots are distributed in time and I use a HTC Tattoo for testing.
I use the MediaRecorder and its getMaxAmplitude method to get the highest amplitude during the last 1/100 s but it does not work as expected; speech gives me values from getMaxAmplitude in the range from 0 to about 25000 while the pistol shots (or shouting!) only reaches about 15000. With a sampling frequency of 8kHz there should be some samples with considerably high level.
Anyone who knows how these things work? Are there filters that are applied before registering the max amplitude. If so, is it hardware or software?
Thanks,
/George

It seems there's an AGC (Automatic Gain Control) filter in place. You should also be able to identify the shot by its frequency characteristics. I would expect it to show up across most of the audible spectrum, but get a spectrum analyzer (there are a few on the app market, like SpectralView) and try identifying the event by its frequency "signature" and amplitude. If you clap your hands what do you get for max amplitude? You could also try covering the phone with something to muffle the sound like a few layers of cloth

It seems like AGC is in the media recorder. When I use AudioRecord I can detect shots using the amplitude even though it sometimes reacts on sounds other than shots. This is not a problem since the shooter usually doesn't make any other noise while shooting.
But I will do some FFT too to get it perfect :-)

Sounds like you figured out your agc problem. One further suggestion: I'm not sure the FFT is the right tool for the job. You might have better detection and lower CPU use with a sliding power estimator.
e.g.
signal => square => moving average => peak detection
All of the above can be implemented very efficiently using fixed point math, which fits well with mobile android platforms.
You can find more info by searching for "Parseval's Theorem" and "CIC filter" (cascaded integrator comb)

Sorry for the late response; I didn't see this question until I started searching for a different problem...
I have started an application to do what I think you're attempting. It's an audio-based lap timer (button to start/stop recording, and loud audio noises for lap setting). It' not finished, but might provide you with a decent base to get started.
Right now, it allows you to monitor the signal volume coming from the mic, and set the ambient noise amount. It's also using the new BSD license, so feel free to check out the code here: http://code.google.com/p/audio-timer/. It's set up to use the 1.5 API to include as many devices as possible.
It's not finished, in that it has two main issues:
The audio capture doesn't currently work for emulated devices because of the unsupported frequency requested
The timer functionality doesn't work yet - was focusing on getting the audio capture first.
I'm looking into the frequency support, but Android doesn't seem to have a way to find out which frequencies are supported without trial and error per-device.
I also have on my local dev machine some extra code to create a layout for the listview items to display "lap" information. Got sidetracked by the frequency problem though. But since the display and audio capture are pretty much done, using the system time to fill in the display values for timing information should be relatively straightforward, and then it shouldn't be too difficult to add the ability to export the data table to a CSV on the SD card.
Let me know if you want to join this project, or if you have any questions.