Q: Will batch converting using the above bit-rates result in:
A - Smaller AAC files with the same quality as the MP3s
YES, if it's ripped from either CD or uncompressed AIFF/WAV/SDII format
NO if it's converting an MP3 to AAC, it's already compressed, you'd lose a lot of fidelity.
B - Smaller AAC files but worse quality
NO, an AAC file at 128Kbps is reported to be the equivelent to a 256Kbps mp3, it's certainly better than a 192Kbps mp3 to my ears.
YES if it's MP3 -> AAC
YES if you're encoding at anything less than 128Kbps (or 64Kbps in mono). AAC lowers the samplerate at lower bitrates, that means you're losing far more quality than Mp3 would because you can encode a mono mp3 at 48Kbps and it will still be 44.1Khz but AAC would drop that down to 32Khz, defeating the purpose of the higher quality codec.
Generally speaking, with any audio codec, encoding mono audio at exactly half the bitrate of stereo audio offers equal quality.
It's claimed that AAC is twice the quality of MP3 at equal bitrates, here's some examples for comparison :
I ripped the audio directly from CD using an external CD writer and a sample editor, I then converted the file from stereo to mono and cropped it to a few bars, saving it as a 44.1Khz mono AIFF to encode from. AAC files encoded with Quicktime 6, MP3 encoded with iTunes.
Uncompressed 44,1Khz mono AIFF file : The file all other files were encoded from (1.2Mb)
64Kbps Mono AAC : Equivelent to 128Kbps stereo AAC (120K)
64Kbps Mono MP3 : Equivelent to 128Kbps stereo MP3 (120K)
48Kbps Mono AAC : Equivelent to 96Kbps stereo AAC (90K)
The 48Kbps AAC file, even though it's been encoded from the 44.1Khz AIFF file, is only 32Khz. Here is an
Uncompressed 32Khz mono AIFF (920K) version for comparison.
The difference between 44.1Khz and 32Khz is VERY apparent whether it's AAC or Uncompressed AIFF.
The difference between the 64Kbps MP3 and AAC files is also quite obvious.
The AAC file is very close to the quality of the original 44.1Khz AIFF file.
C - Similar size files, with the same quality
A 256Kbps AAC file sounds identical to the original CD track. A 256Kbps Mp3 sounds close to an original CD track.
D - similar size files but with better quality
A 128Kbps AAC is far higher quality compared with a 128Kbps MP3.
The music in the example files is a few bars from "finished symphony" by
Hybrid. It features a lot of very distinctive, high octave synth sounds along with a 90 piece Orchestra and breakbeats. The tune has huge frequency range and taxes any codec.