Products and Demos

The coders available today are:

The LD_CELP at 16 kb/s (ITU-T G.728)
(Low-Delay Code Excited Linear Prediction).

  • Characteristics:

  • CELP: Analysis-by-synthesis, excitation of the synthesis filter by the best excitation vector chosen in a codebook.

    The encoder.

    The input signal is partitioned into blocks of 5 consecutive input signal samples. For each input block, the encoder passes each of 1024 candidate codebook vectors (stored in an excitation codebook) through a gain scaling unit and a synthesis filter. From the resulting 1024 candidate quantized signal vectors, the encoder identifies the one that minimizes a frequency-weighted mean-squared error measure with the respect to the input signal vector. The 10-bit codebook index of the corresponding best codebook vector (or codevector) which gives rise to that best candidate quantized signal vector is transmitted to the decoder. The best codevector is then passed through the gain scaling unit and the synthesis filter to establish the correct filter memory in preparation for the encoding of the next signal vector. The synthesis filter coefficients and the gain are updated periodically in a backward adaptive manner based on the previously quantized signal and gain-scaled excitation.

    The decoder.

    The decoding operation is also performed on a block-by-block basis. Upon receiving each 10-bit index, the decoder performs a table look-up to extract the corresponding codevector from the excitation codebook. The extracted codevector is then passed through a gain scaling unit and a synthesis filter to produce the current decoded signal vector. The synthesis filter coefficients and the gain are then updated in the same way as in the encoder. The decoded signal vector is then passed through an adaptive postfilter to enhance the perceptual quality. The postfilter coefficients are updated periodically using the information available at the decoder.

  • Advantages:

    • Very good quality of the coded-decoded speech.
      This is the consequence of the combination of the efficiency of the vocoder to suppress the redundancy and the quality of the waveform coder.

      CELP
      =
      vocoder (Analysis of the input signal)
      +
      waveform coder
      (Criteria = minimization of the difference between the input signal and the quantized signal).

    • Little delay (less than 2 ms).


  • Demonstrations:

  • Several test-files are given. The first file is the original file, the second is the coded-decoded file. This last file is decoded with the postfilter described in the norm ITU G.728. A third file is given without this filter.(16 bits/ 8kHz)

    Original file.
    Coded-decoded file.
    Coded-decoded file without postfiltering.

    The SNR of the coded-decoded file with postfiltering = 18 dB.
    The SNR of the coded-decoded file without postfiltering = 24 dB.

    All these files are coded and decoded on PC.The LD_CELP at 16 kb/s is in real time on Pentium 90 MHz. See Ongoing research projects).
    The demonstrations are also available on SUN.

The LD_CELP at 8 kb/s.

This coder essentially follows the same architecture of the LD_CELP at 16 kb/s. The main difference comes from the selection of the codebooks.Three excitation codebooks are used: two stochastic codebooks and one adaptative codebook.

Several test-files are given. The first file is the original file, the second is the coded-decoded file. This last file is decoded with a postfilter. A third file is given without this filter.(16 bits/8 kHz)

Original file.
Coded-decoded file.
Coded-decoded file without postfiltering.

The SNR of the coded-decoded file = 16 dB.
All these files are coded and decoded on PC.The LD_CELP at 8 kb/s is not yet in real time on PC. We are working on the porting of this coder on TMS320C32 and Pentium. (See Ongoing research projects).

The CELP at 8 kb/s

This coder, constructed from our new Coding ToolKit (CTK), is very efficient. Is is a low computation application permitting real-time applications. News technologies, developed by the Coding Group, has been included in this coder. The very good quality of the coded-decoded files given by this coder is attractive for all the multimedia applications that ask compression on hard disk and very fast decompression. This coder is employed in the new Voice Mailing implemented by the recognize group of this lab.

The CELP at 4.3 kb/s

This new coder uses also the CTK. Its structure is the same that the CELP 8 kb/s but several simplifications has been realised. The quality of the CELP 4.3 kb/s is impressive.

Original file.
Coded-decoded file.

A coder at 2.4 kb/s.

Last updated December 17, 1999, send comments to dutoit@tcts.fpms.ac.be