Subject: Re: About importance of "phase" in sound recognition From: Bob Masta <audio@xxxxxxxx> Date: Wed, 6 Oct 2010 09:08:23 -0400 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>On 5 Oct 2010 at 19:03, emad burke wrote: > I'm exactly talking about what is called > "in-sensitivity to phase". I'm talking about the phase information that is > discarded in the process of MFCC feature extraction and it has been proven > to be succesfull feature set for speech recognition. The "insensitivity to > phase" that implicitly implies that if you change the order (precedence) of > travelling waves in each cochlear channel among each other, it will not > affect the perception and you can add random phases to different channels > without affecting the perception(?). One classical way to demonstrate this insensitivity is to build up a wave from several component frequencies, and listen to the sum. Then change only the relative phases and see if you can detect a difference. It turns out that you can't, most of the time. (This assumes that you turn the sound off while you are making the changes... it is easy to hear dynamic changes.) You do need to use a bit of caution: Different phase relations can cause large differences in waveform peak heights, and the larger peaks can produce distortion due to nonlinearities in the speaker, the ear, or even the air itself. So you might hear a difference that isn't really due to phase as such, just added components due to distortion. But this is not a problem for "reasonable" listening levels. You can use my Daqarta software to demonstrate the insensitivity for yourself with any Windows system. Click the Generator button to get a default 440 Hz sine, and adjust the controls for a comfortable level. In the Generator dialog, click on the left Waveform Controls button (midway down the dialog) and you will get the control dialog for the left Stream 0. (There are four streams per channel, labeled 0-3, which are summed together by default.) Set the Level for Stream 0 to (say) 50%, since the total for all streams must be no more than 100%. (If you want to use four equal-amplitude components, then set each to 25%. Here I assume you will set up the first four components of a square wave.) Now at the top of this dialog click on '1' to change to Stream 1, and set its Tone Freq to 3 * 440 = 1320. Set its Level to 1/3 * 50 = 16.67%. Now toggle Stream On near the top of the dialog to add it to the output. Repeat as needed for Streams 2 and 3. At this point all components are in phase. To set any component to an arbitrary phase, click on its Tone Freq button to bring up the control dialog, and adjust Main Phase as desired. Please let me know if there are any questions of problems. Best regards, Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!