05-01-2013, 02:55 PM
Sub-band Coding of Speech Signals using
Multirate Signal Processing
Sub-band Coding of Speech Signals.pdf (Size: 131.78 KB / Downloads: 230)
THEORY
A variety of techniques have been developed to efficiently represent speech
signals in digital form for either transmission or storage. Since most of the speech energy
is contained in the lower frequencies, we would like to encode the lower-frequency band
in more bits than the high-frequency band. Sub-band coding is a method where the
speech signal is subdivided into several frequency bands and each band is digitally
encoded separately.
An example of a frequency subdivision is shown in the Figure 1. Let us assume
that the speech signal is sampled at a rate Fs samples per second. The first frequency
subdivision splits the signal spectrum into two equal width segments, a lowpass signal (0
< F < Fs/4) and a highpass signal (Fs/4 < F < Fs/2). The second frequency subdivision
splits the lowpass signal from the first stage into two equal bands, a lowpass signal (0 < F
< Fs/8) and a highpass signal (Fs/8 < F < Fs/4). Finally, the third frequency subdivision
splits the lowpass signal from the second stage into two equal bandwidth signals. Thus,
the signal is subdivided into 4 frequency bands, covering 3 octaves, as shown in the
SIMULATION and RESULTS
Then a third order sub-band coder is used to get three outputs. The lowest octave
(containing the information for the frequency ranges 0 to 1200 Hz) is coded using 16
bits, the second octave (containing the information for the frequency ranges 1200 to 2400
Hz) is encoded by using 8 bits and the final octave containig the least amount of data is
encoded using 4 bits if data. The subband spech encoder thus made is as follows in Fig.
5a. The corresponding decoder is made and the speech signal is recovered.