Re: MDCT


Any reasons on why MDCT is preferred over FFT in audio codecs(eg. Ogg
Vorbis).

Couple reasons, first question is why is the DCT used instead of the
DFT (fast version of both transforms exist, image compression
literature is a good resource for DCT vs. DFT). This is mainly due to
the property of energy compaction. On average the DCT represents the
energy of a signal in less bins than the DFT, this leads to easier/
better compression.

Also as mentioned, TDAC (time domain aliasing cancellation) is where
the MDCT comes into play. Also, version of the "TDAC" for the DFT,
usually use the general overlap and add algorithms (similar to the
MDCT algorithm).

Later compression standards (AAC) doesn't use the poly-phase filter
bank that MP3 does, I am pretty sure Ogg Vorbis is the same. The
audio signal is sent directly to the MDCT and the outputs of the MDCT
is the data that is compressed. The perceptual coding (decision what
to keep) usually uses a standard FFT.


.

Relevant Pages

  • Re: Query DCT and DFT
    ... The DCT is very commonly used in Image Processing for compression. ... I want to know where else we can use DCT predominantly what is the diff betweeb DCT AND DFT. ... DCT output is just a suitable linear combination of the complex DFT output such that one sees real numbers again. ...
    (sci.image.processing)
  • Query DCT and DFT
    ... I have some very basic doubt about dct and dft. ... The DCT is very commonly used in Image Processing for compression. ...
    (sci.image.processing)
  • Re: dct implementation with blocks
    ... > one of the main step of dct based compression of a data sequence X, ... Then DCT computation for each block is applied. ... > DCT directly the whole data sequence X? ... There is more correlation between close-by pixels than between a pixel ...
    (comp.soft-sys.matlab)
  • Re: dft : property of symmetry for real & even seq
    ... you should use a discrete cosine transform (DCT). ... is why people normally have a complex-output DFT (even if they have ... positive frequencies (as I understand it when the time domain samples ... The relationship between the real DFT polar form and the complex DFT ...
    (comp.dsp)
  • Re: Query DCT and DFT
    ... So to compute the fourier series, ... For DFT, ... For DCT, the sequence is mirrored and then ... applications) or of type-I corresponds to even boundary conditions at ...
    (sci.image.processing)