Before introducing the signal processing flow, the user must understand the following terminologies:
Downlink/Downstream/Far-end/Speaker path – Bluetooth® device receives bitstream and pass to DSP for decoding process
Uplink/Upstream/Near-end/MIC path – Bluetooth device transmits the bitstream, encoded by the DSP processor
The following figure illustrates the block diagram of the DSP processing flow for speakerphone and headset applications for speech and audio signal processing. The DSP part mainly focuses on speech encoders/decoders and audio decoders along with their corresponding signal processing functions. The embedded Analog-to-Digital Codec (ADC) provides high Signal-to-Noise Ratio (SNR) data conversion with 88 dB, and the Digital-to-Analog Codec (DAC) provides high SNR data conversion with 98 dB. The Bluetooth and RF/modem processors deal with the Medium Access Control (MAC) and the wireless data transmission. For the speaker and speakerphone applications, use the external audio amplifiers to amplify the audio signal.
The speech codecs, Continuous Variable Slope Delta (CVSD) with supported bandwidth of 8 kHz and Modified Sub-Band Coding (mSBC) with supported bandwidth of 16 kHz are available for Bluetooth speech applications (see the following figure). mSBC is the mandatory speech codec defined in Bluetooth Hands-Free Profile (HFP) 1.6 to provide High Definition (HD) voice quality. As long as the host device establishes the Bluetooth link through the HFP 1.6 profile, mSBC is the speech codec regardless of the cellular network conditions (3G network or VoLTE).Figure 2-1. Speech Signal Processing
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