Block switching

The Psychoacoustic model decides when to switch to short blocks and back to long blocks (block switching).

In MP3, switching to short blocks will isolate the attack to 192 sample window, so the oscillations are only spread over 192 samples instead of 576. Block switching is also used by Ogg Vorbis and AAC, (except on windows that can be a power of two only). MPC, being a subband coder doesn't make use of block switching, (although it is implemented and can be used)

Switching to short blocks and increasing of the bitrate to encode a series of MDCT coefficients of a particular transient is one way to combat Pre echo.

Additional reading:

Some Java applets to examine fourier series behaviour (note that MDCT behaviour is not totally identical):

• http://www.jhu.edu/~signals/fourier2/

  You can draw a sharp attack here, and use calculate with a large number of coefficients... if you start to reduce the number of coefficients you will see the pre echo appear.

• http://www.gac.edu/~huber/fourier

  Doesnt seem to work in netscape. iirc this one lets you listen to the sound.

• http://www.univie.ac.at/future.media/moe/galerie/fourier/fourier.html

  Maths online fourier series applet.