5.1 Input signal pre-processing

26.2903GPPAudio codec processing functionsExtended Adaptive Multi-Rate - Wideband (AMR-WB+) codecRelease 17Transcoding functionsTS

Input signals are pre-processed in order to bring them to the internal sampling frequency of the encoder Fs kHz. The signal is upsampled by a factor K (related to the desired internal sampling frequency), filtered by a a low pass filter and then downsampled by a factor 180. This operation is efficiently implemented by a polyphase filter implementation.

The resulting signals are further decomposed into two equal critically sampled bands as shown in the following figure:

At an internal sampling rate of Fs kHz, the lower band signals are obtained by first low-pass filtering to Fs/4 kHz critically downsampling the low-pass filtered signal to Fs/2 kHz. The higher band signals are obtained by band-pass filtering the input signals to frequencies above Fs/4 kHz, and critically downsampling the high-pass filtered signal to Fs/2kHz sampling frequency.

5.1.1 High Pass Filtering

The lower band signals are high pass filtered. The high-pass filter serves as a precaution against undesired low frequency components. A high pass filter is used, and it is given by

where the filter parameters are dependent on the internal sampling rate.

5.1.2 Stereo Signal Downmixing/Bandsplitting

When the input audio signal is stereo, the lower band mono signal is obtained by downmixing the left and right channels according to the following

where , resp. , is the lower band signal from the left, resp. right, channels. The lower band mono signal is supplied to the core low band encoder for TCX/ACELP encoding.

For stereo encoding, the obtained downmixed mono signal and the right channel signal are further split into two bands: a critically sampled low frequency band and a residual high frequency band according to the following diagram

The critically sampled low band output signals, and are fed to the stereo low band encoder, while the signals and to the stereo mid band encoder.