How do I correctly slow down the track to compensate for wrong sampling rate?

Question

I have some ripped tracks from an audio CD which originally had the music in 44100Hz bitrate, but it's encoded as 48000Hz. The pitch is definitely higher and so is the speed. How do I properly reduce the bitrate without losing audio quality?

I have Sound Forge 10 application I could use to do that if it's possible.

Answer 1

You simply re-sample them from 48 kHz to 44.1 kHz.

I believe in Sound Forge you go to the Process menu and select re-sample.

Or you can do it by using one of the many free audio editing software such as these:
http://www.hongkiat.com/blog/25-free-digital-audio-editors/

Don't apply pitch-shifting or time warp, only plain and simple re-sampling.

You won't normally loose any data re-sampling as it is merely about setting playback speed in the file header. You are only telling the player to play the exact same data, but at a lower rate. The opposite is what you're experiencing, the data played at a higher frequency resulting in higher pitch and faster playback.

However, be aware of that some program look at the system's playback rate and can actually interpolate the data and so forth. Therefor I would advice you to check other programs as well if you run into lowered quality.

If none should allow you there is a more low-level approach you can use by altering the audio file header directly (and header only) to set new playback rate. For this you can use for example sox:
http://sox.sourceforge.net/sox.html

(Audacity apparently has a bug related to re-sampling - dunno if that is fixed by now.)

Answer 2

So, if you have ripped a CD (originally 44.1kHz) but now have 48kHz WAV files where the pitch and speed are higher, you do not need to resample the audio.

You only need to rewrite the sampling rate information in the file header back to 44.1kHz. This will ensure that the files are played back at 44.1kHz with the correct pitch and speed.

If you resample these files back to 44.1kHz (as per the accepted answer) the incorrect high pitch and speed will remain with the file.

By rewriting the file header you fix the pitch and speed problem.

A tool such as "Sound Devices Wave Agent" will assist with rewriting the file header.