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MP3 Sample Rate

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brad hebert
MP3 Sample Rate
on Sep 17, 2008 at 6:45:01 pm

Today is the first time I've been asked to deliver a specific sample rate for an mp3. I usually use LAME or Switch to convert audio files to mp3, but I don't think they even have an option for the sample rate.

There is another app, AudialHub, that I had never used, but it does have the option to select different sample rates.

This is just hitting me strange, because I've made hundreds, if not thousands, of mp3s of all different bitrates. I guess I thought that sample rate was irrelevant - that it fluctuated with the compression algorithm or something... but I admit I have noticed sample rates in the details/properties/info about mp3s. I'm guessing that they are almost always 44.1kHz and that most encoders always use 44.1kHz... but what about selecting very low bitrates on LAME or Switch - would they still use such a high sample rate? or do they automatically adjust the sample rate down as is appropriate for the bitrate?

Just curious if anyone has some knowledge about this.




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Daniel Low
Re: MP3 Sample Rate
on Sep 17, 2008 at 10:24:09 pm

There is a relationship between bitrate and sample rate, eg. below a certain bitrate it's not possible to use the higher sample rates. Nothing is set in stone as each encoder offers different levels.

For example you'd switch, or be forced to 32kHz or 22.050kHz below say 64Kb/s.

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brad hebert
Re: MP3 Sample Rate
on Sep 18, 2008 at 12:21:55 am

I'm starting to understand this more now, thanks.

I realized earlier that my version of Switch was not working correctly. If I tried to convert a .aif file from 44.1kHz to 48kHz (and keep it .aif), it looked like it was processing correctly. But when I checked the new file, it was exactly the same as the original - and it would do this for any sample rate, never actually changing it.

That was keeping me from realizing the following:

When converting to mp3, the sample rate of the original file seems to remain the same when Switch converts it (.aif at 44.1k yields .mp3 at 44.1k - .aif at 48k yields .mp3 at 48k). This might be the same in other encoders as well. I haven't tried the lower bitrates - I'm guessing you're correct that past a point, the sample rate is automatically reduced.



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