Audacity adds high freq signature to the waveforms [SOLVED]

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Audacity adds high freq signature to the waveforms [SOLVED]

Permanent link to this post Posted by mfarooq7897 » Mon Apr 18, 2016 11:20 pm

Hello all

I am an avid user of audacity and have been using it at work for a long time. As I work more and more with the tool, I have discovered its very useful features.

I would like to bring your attention to an issue that I am seeing using Audacity.

Every time i export an audio file from audacity, I notice that some high freq component is added to the waveform. Please see the jpeg attached. I took a waveform, named s.wav. exported it and named it s1.wav.
then i exported s1.wav as s2.wav. Then (after importing) exported s2 as s3. Continuing this process until i reached to s6.wav. I see that the RMS of silent sections has a roughly 2.5dB difference.
This means that at every export, audacity does funny things to the waveform.

Has anyone else seen the problem before?

Sorry if I am being dim here but as much of a power user of audacity I am, i am not so familiar with audacity forum rules. Forgive any transgressions I may have made in this process.
Thanks
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Re: Audacity adds small high freq signature to the waveforms

Permanent link to this post Posted by steve » Mon Apr 18, 2016 11:46 pm

mfarooq7897 wrote:Has anyone else seen the problem before?

Yes, it's because you are exporting as 16-bit.
Audacity works internally in 32-bit float format, which is an incredibly high quality audio format. 16-bit format on the other hand is "only" CD quality (which is still very high quality). In order to "reduce the sound quality" from 32-bit to 16-bit, the sample values need to be rounded to 16-bit values. There are several ways that this can be done. The simplest way would be to round down, round up, or round to nearest, but this is not the best way because it causes harmonic distortion (albeit at a very low level). In order to maintain as much audio information as possible during the conversion, Audacity rounds some values up and some values down. This is done in a carefully calculated way that pushes the rounding errors into the very high frequency range where they are least audible. The process is called "dither".
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Re: Audacity adds small high freq signature to the waveforms

Permanent link to this post Posted by mfarooq7897 » Tue Apr 19, 2016 12:34 am

Ok. That makes sense. In fact, i remember reading in manual that Audacity never degrades the quality of signal when reading (as you said, it works internally in 32 bit to preserve fidelity) but its only when exporting at lower (and funny) formats that it dithers to preserve information.

So, if i do so much waveform processing (exporting and importing etc.) using audacity, and i must use 16bit files for my (DSP) processing purposes, then i think the solution to my problem is as follows:
1. set Default Sample Format as 32 bit. Do all exporting and importing as many times in 32bit.
2. After I know that I now have the wave-forms as I want, convert them to 16bit. this will only add dithering effects the last time!


Gracias gentlemen!
Thanks for your support.

p.s. How do i mark this 'SOLVED'?
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Audacity adds small high freq signature... [SOLVED]

Permanent link to this post Posted by steve » Tue Apr 19, 2016 11:31 am

mfarooq7897 wrote:After I know that I now have the wave-forms as I want, convert them to 16bit. this will only add dithering effects the last time!

Yes, that is precisely the way to ensure absolute maximum "quality".
Similarly, if you require an MP3 version, converting to MP3 should be the final step in the production process (from the 32-bit original).

mfarooq7897 wrote:p.s. How do i mark this 'SOLVED'?

Just say that you are happy and a moderator will do that for you (the forum software does not have a "solved" button, so we do it manually :))
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