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Spectral Frequency Question

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Matthew Chung
Spectral Frequency Question
on May 3, 2013 at 8:08:49 pm

I have several voice recordings that I am encoding to mp3 to stream at 16kbps. I was given a sample wav file which encodes to 16kbps mp3 fine and which encodes with a lot of sound artifacts. (wav files below)

(wav file that encodes fine)
https://dl.dropboxusercontent.com/u/45836281/speech_orig.wav

(wav file that does not encode well)
https://dl.dropboxusercontent.com/u/45836281/secondsample.wav

When looking at the spectral frequency display of the two files, the one that encodes fine has a frequency under 4k and the one that does not has a frequency over 10k (images below)

(encodes fine frequency)
https://docs.google.com/file/d/0B8U02wD3-W-OWURkdWhKOXFmNnc/edit?usp=sharin...
(encodes not so good frequency)
https://docs.google.com/file/d/0B8U02wD3-W-OeExYR1BoWDNRdHc/edit?usp=sharin...

I believe I understand why the second file encodes to mp3 with artifacts, because of the high frequency, but what I do not understand is why the second file has such a high frequency. I believe the human voice has a frequency of 300hz to 3400hz (http://en.wikipedia.org/wiki/Voice_frequency) so how is it possible that it has a frequency over 10k?

Also, if the frequency is why the encoding is introducing artifacts, is there a way to record audio such that it is within a certain frequency?

Thanks in advance


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Angelo Lorenzo
Re: Spectral Frequency Question
on May 4, 2013 at 7:56:00 pm

The human voice is rich in harmonics and overtones. Anything past 5-6K is considered "air" or "lightness" in terms of adding to the human voice.

To remove it before compression use a low pass filter effect. MP3 encoders usually incorporate a low pass filter, but they're rarely as good as doing it yourself before compression.

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