5.1 Pre‑processing

26.1903GPPAdaptive Multi-Rate - Wideband (AMR-WB) speech codecSpeech codec speech processing functionsTranscoding functionsTS

The encoder performs the analysis of the LPC, LTP and fixed codebook parameters at 12.8 kHz sampling rate. Therefore, the input signal has to be decimated from 16 kHz to 12.8 kHz. The decimation is performed by first upsampling by 4, then filtering the output through lowpass FIR filter H_decim(z) that has the cut off frequency at 6.4 kHz. Then, the signal is downsampled by 5. The filtering delay is compensated by adding zeroes into the end of the input vector.

After the decimation, two pre-processing functions are applied to the signal prior to the encoding process: high-pass filtering and pre-emphasizing (and signal down-scaling).

(Down-scaling consists of dividing the input by a factor of 2 to reduce the possibility of overflows in the fixed-point implementation.)

The high-pass filter serves as a precaution against undesired low frequency components. A filter at a cut off frequency of 50 Hz is used, and it is given by

( 4 )

(Both down-scaling and high-pass filtering are combined by dividing the coefficients at the numerator of H_h1(z) by 2.)

In the pre-emphasis, a first order high-pass filter is used to emphasize higher frequencies, and it is given by

( 5 )