How to transmit modulated voice over a call in Android?

Question

I want to know in Android it is possible to transmit modulated voice like Ultra voice changer app does during a call. I have searched a lot but I only got results for how to change voice after recording. So, please reply me that is it possible to transmit changed voice at a time of call in Android.

Answer 1

It seems not possible. According to this XDA-Post "The calling screen is built within the phone". You can replace a dialer, but are not able to intercept the voice spoken during a call. I cannot find any official API from android, which would make it possible to write your own "calling" App (which means record voice and send it).

Answer 2

The GSM full rate speech codec operates at 13 kbits/s and uses a Regular Pulse Excited (RPE) codec - This means that the microphone and speech detection in GSM is optimised for transmission across a Time Division Multiplexed 'digital' channel that is then modulated across the air interface using GMSK, a continuous-phase frequency-shift keying modulation scheme.

Noises other than the 'average' speech pattern are heavily distorted (or suppressed) - For instance DTMF (tones) are not well received on a device, and must be transmitted by the network core, but tones designed for the hearing impaired work well. Voice is shaped (filtered) on entry to the codec (microphone design) for best codec detection and reproduction at the other end.

In summary - It is not possible to 're-modulate' across the GSM system, because the entry point is not a radio (air interface), or even access the GSM digital frame. Your only access for a voice call is the GSM codec which is expecting a voice in a confined audio spectrum.

Answer 3

I know there are apps that act like voice changer where they change your voice and transmit that thru gsm voice. Maybe you can make an app that takes the voice then modulate it to something like phase shift keying or digital radio monodiale like what hams used for digital vhf, hf, dpmr, mototrbo radio communication and transmit the audio thru gsm voice channel then demodulate it back to normal voice but instead of straight forward modulation demodulation you can add pgp, aes, pre shared keys, blowfish or whatever encryption you like. I'm also intrested to see a project like this.

I think it would also be great if we could use this to transmit data thru voice gsm channel like the 56k dial up modems in the past instead of the gprs data channel allowing you to make data connection to other cellphone to transfer files without incurring additional data charges which is really good for subscribers with unlimited call plans.

See reference:

http://freedv.org/tiki-index.php

http://www.aprs.org