Convert Audio into MP3

Question

I've got two 45min audio files that I'd like to compress/convert to MP3 files. In compressor I chose AAC Audio High Bit Rate High Quality DL. What I got was a 60 mb MP4 QT file which I took into iTunes to convert into an MP3, bringing my file down to 52 mb (down from 500+ mb)
A couple of questions: Is there an easier way to do this and is 52 mb a good size for a 45 min stretch of VO? I've got solid quality at this point. I'm sure I could shave a few MBs. How do they get a Harry Potter audio book into a reasonable download???
Thanks - Gene

Answer 1

Stereo or mono? AAC will give you very good quality at 64Kbps mono, and good quality at 32Kbps or even slightly lower. The latter would give you about 4KB/s which means 45min x 60sec/min x 4KB/s = 10800KB = 10.8MB for mono with a 45 minute recording. In any case, AAC should give you better quality per byte than does MP3. However, MP3 is a more widely used and a more PC-compatible format. Of course, you need to double those data rates if you want to use stereo and maintain equivalent quality.
You can also gain some control over the quality by adjusting the audio's sample frequency (or sample rate -- the approximate values 8KHz, 11KHz, 22KHz, 32KHz, 44KHz, and 48KHz being common). If you want the best quality make certain that the sample rate you choose for the encoding matches that of the original recording or that it is a simple integer ratio below the original (thus "good" ratios from 44KHz would be 22KHz and 11KHz, or 1/2 and 1/4 the original).
Note, however, that if you use lower bit rates (e.g. 32Kbps) for the encoding you may be limited to lower sample frequencies (e.g. 22KHz), which is okay as long as the resultant quality is good enough. The encoder may also force fixed limits on the bit rate depending upon the requested sample rate (and vice versa).
It also depends upon what the source material is -- spoken word, music, etc. -- since you can probably use a higher compression rate (smaller output file) for plain spoken word.<small><hr width="75%"><small>If this suggestion helps in any way, a confirmation or acknowledgment would be appreciated, since that would also help others who may be having the same difficulty. Do for others as you would have them help you.<center>Thanks for sharing, Waymen.</center></small>