MP3 Reviews

MP3 limitations

There are several limitations inherent to the MP3 format that cannot be overcome by any MP3 encoder. Newer audio compression formats such as Vorbis, WMA Pro and AAC no longer have these limitations. In technical terms, MP3 is limited in the following ways:

1. Joint stereo limitations:
Mp3 can not switch joint stereo mode for specifics scalefactor bands. If joint stereo is used, it has to be used for all the bands. This is rather inoptimal, and is limiting the use of joint stereo. As an example, imagine the following situation:
The lower frequencies are featuring an instrument playing on the far left, and frequencies around 1500Hz are featuring a singer in the middle of the stage.
In such a situation, it is not possible to use joint stereo with Mp3 because of the lower frequencies part which is too different between both channels. A further bitrate reduction could have been achieved if it was possible to toggle joint stereo mode on a scalefactor band basis. (in this case regular stereo would have been used for the lower frequencies, and Middle/Side stereo for the remaining part of the frequency spectrum)

2. Maximum frame size
Even if a buffer is available (the bit reservoir), the total size of information belonging to a frame (data inside the frame + data from the bit reservoir) is limited. The ISO standard defines the maximum size to be the size of the buffer for 320kbps frame. Unfortunately, in some (limited) cases this limit seems to be too low, leading to unavoidable degradations of the sound quality.

3. Time resolution can be too low for highly transient signals, may cause some smearing of percussive sounds.

4. Due to the tree structure of the filter bank, pre-echo issues are made worse, as the combined impulse response of the two filter banks does not, and can not, provide an optimum solution in time/frequency resolution.

5. The combination of the two filter banks creates aliasing issues that must be handled partially by the “aliasing compensation” stage, but that create excess energy to be coded in the frequency domain, thereby decreasing coding efficiency

6. Frequency resolution is limited by the small long block window size, decreasing coding efficiency

7. No scale factor band for frequencies above 15.5/15.8 kHz