What is the minimum MP3 encoding rate?

andrewstr · 01-28-2004, 02:12 PM

I need to maintain a a backlog of a radio station's audio for a period of 90 days. Previously, I have been using RealProducer on a Windows machine. I got tired of the Windows machine crashing so I have upgraded the system to Red Hat 9 and am using SOX and LAME to encoder audio in real time to MP3. The encoding is working but I want to be able to use the highest compression rate available since quality is not that important.

I have tried encoding at a bit rate of 8k but when I play back the file, it plays very quickly. I have tried using several MP3 players but they all play the files too quickly. Here is the command I am using to do the encoding:

sox -r 8000 -t ossdsp -w -s /dev/dsp -t raw -c 2 - | lame -s 44.1 -x -b 8 -m m - sound.mp3

What do I need to do to record the smallest possible MP3 file?

Thank you!

delta9 · 01-28-2004, 03:10 PM

I'm not sure here but could it be that you are setting the audio samplerate to 8000 (instead of eg 44100 for cdquality) instead of the lame compression mp3 bitrate?

If you play a file with a samplerate of 8000Hz at the speed of a 44100Hz one it will play (more than 5 times) too fast.

Looking at the commandline you're using you are converting the input to a 8000Hz stream and recording it to a 44100Hz 8000bit/s mp3.

andrewstr · 01-28-2004, 03:30 PM

I switched the command line to this:

sox -r 8000 -t ossdsp -w -s /dev/dsp -t raw -c 1 - | lame -s 8 -x -b 8 -m m - music.mp3&

Now the file plays at the correct rate but is completely unintelligible. I also get this error from mpg123:

mpg123: Can't rewind stream by 35 bits!

Any other ideas?

delta9 · 01-28-2004, 05:11 PM

Try a higher samplerate (11025, 22050) and/or bitrate (16kbit/s, 32kbit/s), and experiment with them, I think you can't go infinitely low with them and still have have something that sounds good.

andrewstr · 01-28-2004, 05:22 PM

I did a lot of experimenting and this seems to work OK:

sox -r 44100 -t ossdsp -w -s /dev/dsp -t raw -c 1 - | lame -s 44.1 -x -b 8 -m m --lowpass 5 -q 0 - music.mp3&

I really can't encode at a bit rate greater than 8 k because I would run out of space. With the settings above, the size of file is comparable to a RealAudio file at 8k but the RealAudio files sounds better IMHO. Do you have any experience with OGG files? I wonder if they would be better for audio archival purposes.