i've been pondering on this subject for a while... but let's start at the beginning... the AAC (Advanced Audio Codec) codec was invented as a substitute for the MP3 codec. the AAC+ (HE AAC - High Efficiency Advanced Audio Codec) codec on the other hand wasn't envented to be used commercially. it's purpose was to be used in the new standard DAB (Digital Audio Broadcasting) radio stations, i.e. DAB+ radio stations. the new DAB+ receivers are embedded with the AAC+ codec and can decode HE AAC, as well as the regular AAC coded stream. the AAC codec has a very similar coding scheme as the MP3 codec, except the AAC codec differs from the MP3 standard in some coding algorithm: - higher frequency resolution: the bandwidth of the audio file is devided in 1024 subbands, compared to MP3 which has only 576. - prediction: some test have shown that better coding efficiency can be achieved by predicting the amplitude of the next sample in the audio stream. this can be done by knowing the amplitude of the previous sample of the audio stream. - improved joint stereo coding: compared to MP3, both mid/side and intensity stereo coding is more flexible, allowing lower bit-rates to be applied more frequently without any change to audio quality, which in the end is represented as a smaller (in size) audio file. - improved Huffman coding: in AAC coding, the Huffman code is applied to quadruples of frequency subbands, which in MP3 coding was done to a single frequency subband. this allows larger flexibility of the values given in the Huffman coding tables, which results in higher audio quality of the encoded stream. on the other hand, the AAC+ codec has a whole new algorithmic block added in the coding scheme, so is in no way similar to the AAC codec. that's why most players that had no problem with AAC streams couldn't decode AAC+ streams when the codec was first standardized. the AAC+ codec uses a coding scheme which is know as SBR (Spectral Band Replication). SBR doesn't replace the core codec, but rather operates in conjunction with it, to create a more efficient coding engine that can cut the required bit-rate in half to achieve the same audio quality. actually SBR interlaces the corelation between the low and the high frequency spectrum so that the high frequencies of the audio signal are described using a verry small amount of data. the SBR data describing the high frequencies is combined (coupled) with the low frequency data from the AAC stream. a simple block diagram of a AAC+ coder/decoder (codec) is show below. mp3PRO encoding is done in very much the same way, except the high frequency SBR data is coupled with the low frequency MP3 data. so, is AAC+ better than mp3PRO, or is it the other way around? the most commonly used two methods to determine this are: 1) blind listening tests: i think this method is more reliable, although there is not much science involved in it. it simply shows a statistical value if one codec is better than the other at same bit-rates. 2) signal analysis: this is mostly done by FFT (Fast Fourie Transform) spectral analysis of the coded signal of both codecs. since most of the audio is lost in the high frequency range during encoding, a spectral analysis will reveal the pros and cons of each codec. i haven't done a blind listening test of both codecs, but mp3PRO does seem to sound better than AAC+ at the same bit-rate. but since most of my files are mp3PRO encoded, this result is bias, so it may as well be disregarded. the signal analysis was done using the mp3PRO Fraunhoffer IIS encoder and the Nero MP4 HE AAC encoder using Cool Edit Pro 2.0. the audio file was encoded from wav to mp3Pro and AAC+ at 64kbps, 44.1KHz. the spectral analysis is done using a Blackmann-Harris FFT algorithm with 1024 sample frequency resolution. both left and right channel are scanned. here are the results of the spectral analysis of the same audio track (Pigs In Space - Solar). original wav file (logarithmic view) original wav file (linear view) AAC+ encoded audio file (logarithmic view) AAC+ encoded audio file (linear view) mp3PRO encoded audio file (logarithmic view) mp3PRO encoded audio file (linear view) the results are obvious, but let's go through the pros and cons of AAC+ and mp3PRO encoding. AAC+ pros: 1) larger bandwidth: the graph clearly shows that the cutoff frequency of the AAC+ encoded file is around 20KHz, whereas in the mp3PRO encoded file is around 16KHz. so, AAC+ has a 4KHz plus bandwidth advantage over mp3PRO. unfortunately, most of us have lost their sense of hearing above 16KHz, but nevertheless, the bandwidth gain is not neglectible. AAC+ cons: 1) bandwidth stepping effect: this is common with most codecs that use frequency subband division as a technique to generate the Huffman coding tables. MP3 uses the same technique, but unlike AAC, the subbands are not attenuated as the frequency of the signal is increased (rises). i guess that is why we can see that "climbing stairs" effect in the linear view of the AAC+ encoded file. mp3PRO pros: 1) no bandwidth stepping effect: unlike AAC+, there are no steps in the frequency analysis. the sound hasn't lost his sharpness (in the higher frequencies) like AAC+. mp3PRO cons: 1) loss of bandwidth: AAC+ has a 4KHz plus bandwidth advantage over mp3PRO, which makes mp3PRO the inferior codec in bandwidth handling. i guess that covers about everything i could think of on the subject. i'll see if i could make a blind listening test of the two codecs and compare the results.