Subject:Time Stretch/Pitch
Posted by: blackrm
Date:11/30/2004 10:07:02 PM
Hello. Could someone please describe to me what Sound Forge does when you stretch an audio file without changing the pitch? If you were to take a file that was about 43 min. long and stretched it to 46 min. long would it add to the spaces in the audio file to accomplish it? (I assume it's more complicated than that.) Thank you for your time. |
Subject:RE: Time Stretch/Pitch
Reply by: mpd
Date:12/1/2004 7:16:39 AM
I'm not positive how SF accomplishes this, but in general there are two steps involved. 1. Resample the data with an interpolation filter to get more data points. There are a bunch of good ways to do this. Two ways are with a fractional delay filter, or with a high quality polynomial filter. The net result of this is more data points, but the spectrum has been shifted down. 2. Shift the spectrum back to the proper place. This is non-trivial with real data (a real mix produces both the sum and difference products, and the small shift involved with time stretching means the two overlap), hence the different operating modes. One way to do this is to take the real data, convert it to an analytical signal (ie, a complex or quadrature signal), perform a complex spectral shift, and then perform a quadrature to real conversion. That is a high-level look at what gets done, but step two is difficult to implement to perform well in the general case. |
Subject:RE: Time Stretch/Pitch
Reply by: blackrm
Date:12/7/2004 7:18:10 AM
I apologize for not getting back to you sooner. I think I understand what you're telling me concerning time stretch. The process used grabs more data points at a different frequency (probably changing, if you could hear it, the pitch/tone of the wave file - maybe ;-)) and then puts it back out at the rate that was first recorded (in the longer time frame) with the correct pitch. Do you know at what point time stretching a wave file would take it beyond the correct pitch? Or is that even possible? Thank you, Bob |
Subject:RE: Time Stretch/Pitch
Reply by: mpd
Date:12/7/2004 7:36:30 AM
Any time stretching will result in a frequency shift. Whether it is audible or not is another story. Also keep in mind that time stretching results in a linear frequency shift, while musical scales are logarithmic. This means that to the ear, the change isn't the same in all octaves. |