1 Introduction

The TAC5x1x and TAC5x1x-Q1 family of audio codecs have separate signal chains for recording (though analog and or digital microphones) and for playback (through DAC) of audio signals.

Each channel of the recording path follows the signal chain shown in Figure 1-1. This signal chain supports an analog differential or single-ended signal or a digital pulse density modulation (PDM) digital microphone. In TAC5x1x and TAC5x1x-Q1 device families, the analog input signal is converted by a high-performance ADC into a digital signal. The digital signal has a programmable phase calibration to adjust the phase delay of each channel in steps of one modulator clock cycle. This allows the system to match the phase across different channels. The phase-calibrated digital signal is then decimated through a set of linear phase filters or low-latency decimation filters. DC offset is removed from the decimated signal through a Digital High-Pass Filter (HPF) with three pre-set cutoff frequencies or a fully programmable cutoff frequency. The output of the HPF is gain calibrated with 0.1dB steps and summed with other channels. The gain calibration matches the gain across different channels, particularly if the channels have microphones with varying gain values. The output is then filtered by the Digital Biquad Filters and, in the case of secondary ASI, is parsed through a sample rate converter, before being gained by the volume control.

Similarly each channel of the playback path follows the signal chain shown in Figure 1-2. the signal chain contains a programmable 8x4 Mixer for the main ASI inputs, and a 2x4 programmable mixer for the axillary ASI inputs. The mixed signal is then gained up through digital volume control, and a high-pass filter similar to that on the recording channel removes any DC component present in the signal. this signal is processed through a series of biquad filters, followed by additional signal processing blocks. The processed signal is then interpolated through interpolation filters, and is converted to analog output through a digital-to-analog converter (DAC).

This application note concentrates on how to configure the Digital Biquad Filters. The Digital Biquad Filters are digitally implemented as a set of IIR filters.