Audacity Forum

Just testing a just-downloaded Audacity 1.3.11 on a new Windows 7 machine. On exporting a .aup file, recorded at a sample rate of 128k Hz and a bit rate of 128kbps to MP3 format, I get a 'invalid sample' panel which suggests this combination is not supported by the MP3 format and suggests I resample at any of nine rates up to 48000. Any ideas? (It was fine on the old XP with 1.2.6!)

<<<On exporting a .aup file, recorded at a sample rate of 128k Hz>>>

Audacity will not Export an AUP file, you have to Save one of those. It will Export a WAV file if you want and that's what's indicated here.

48000 is the sample rate of television sound and 44100 is the sample rate of Music CDs. 128000 is the sample rate of NASA scientific experiments. If you have Audacity set to default to 128000 you should probably change that to a more earthly number in Audacity Preferences.

Exporting MP3 at a bitrate of 128k is perfectly valid and you can do that at the MP3 Export step.

Koz

48000Hz is the maximum sample rate supported by MP3.
The old 1.2.6 version just downsampled automatically when necessary without offering any options.
For maximum compatibility select 44100Hz as the destination sample rate.

Working at 128kHz is rather excessive if your final format is MP3. The increased frequency response of very high sample rates is totally filtered out when it is encoded.
128kbps is not a high quality MP3 format - it is generally considered to be the minimum for reasonable quality stereo music recordings. The "Standard" preset in Audacity 1.3 is quite a lot better for a relatively small increase in file size.

Thanks for the quick response. I've readjusted my project rate to 41000 and found the .aup to MP3 export went through without a murmur. Being new to this, I thought the higher the sampling rate, the better the MP3 reproduction although, of course, the larger the file. Is that not the case? (Some vinyl/MP3 exports on the XP machine, sampled on 1.2.6 at 128000, uncovered previously 'hidden' sounds. Did that .aup/MP3 export actually ignore the 128000 rate subverting it down to the 41000 region?) Thanks.

Floreat wrote:Did that .aup/MP3 export actually ignore the 128000 rate subverting it down to the 41000 region?

Audacity 1.2.6 with the current version of Lame will shove it down to 48kHz.
There is probably no harm with using 48kHz but there is a small chance that some applications will not play it back at the right speed. If you do have a program that does not like 48kHz MP3s then there is a good chance that it will not like VBR MP3s either. Personally if I'm planning to output only in MP3 format then I work at 44100Hz throughout the project. There may be a marginal improvement in very high frequencies using 48kHz but at any particular kbps you have the same number of bits per second regardless of the sample rate, so a marginal improvement in one aspect is likely to be a marginal degrading of another. The only way to really improve the quality is to use more bits per second. VBR encoding is clever in that it will increase the number of bits per second when it is most needed, and will reduce the number of bits when it can get away with it - subjectively improving the overall sound quality.

I think I'll stick to the 40000 band for simplicity's sake. I must say that, on tests, my ears detected little difference betweeen 128K and 44K. I am still knocked out by the fact, that as a novice, I can record off the internet radio using Audacity to save a .aup file and then switch that into MP3 format. Add in the ability to take all the crackle, hiss and pop out of a Frankie Laine 78rpm and transfer that to MP3 and I'm still in awe! Thanks.