MP3 - Design Limitations

Design Limitations

There are several limitations inherent to the MP3 format that cannot be overcome by any MP3 encoder. Newer audio compression formats such as AAC, WMA Pro and Vorbis are generally free of a number of these limitations. In technical terms, some limitations include:

• Time resolution can be too low for highly transient signals and may cause smearing of percussive sounds.
• Due to the tree structure of the filter bank, pre-echo problems are made worse, as the combined impulse response of the two filter banks does not, and cannot, provide an optimum solution in time/frequency resolution.
• The combining of the two filter banks' outputs creates aliasing problems that must be handled partially by the "aliasing compensation" stage; however, that creates excess energy to be coded in the frequency domain, thereby decreasing coding efficiency.
• Frequency resolution is limited by the small long block window size, which decreases coding efficiency.
• There is no scale factor band for frequencies above 15.5/15.8 kHz.
• Joint stereo is done only on a frame-to-frame basis.
• Internal handling of the bit reservoir increases encoding delay.
• Encoder/decoder overall delay is not defined, which means there is no official provision for gapless playback. However, some encoders such as LAME can attach additional metadata that will allow players that can handle it to deliver seamless playback.
• The data stream can contain an optional checksum, but the checksum only protects the header data, not the audio data.