Read audio sample from MEMS microphone available on the STMicroelectronics STM32F4–Discovery board
Embedded Coder Support Package for STMicroelectronics Discovery Boards/STM32F4-Discovery
This block reads the audio samples collected from the MEMS microphone (MP45DT02, ST MEMS audio sensor, omnidirectional digital microphone), available on STM32F4–Discovery™ board. The MEMS microphone produces a Pulse Density Modulated (PDM) signal.
The block output emits int16 Pulse Code Modulated audio frame with the size specified in Frame size (samples) field from the block mask.
The block sample rate is calculated as: Sample frequency/ Frame size.
For example, the Mic in block sample time with 48 kHz sampling frequency and 96 samples frame size is equal to 2 ms (96/48000). The minimum allowed sampling rate for the Mic in block is 1ms.
In the Configuration Parameters dialog, under Coder Target,
you can select
“Mic In” block DMA interrupt as
the Scheduler Interrupt Source. This ensures
that the processing of your algorithm is synchronized with the Mic
In block interrupt.
When you select this option, make sure your model base rate is the same as the Mic In block sample time.
As the Mic In block always outputs 1ms of int16 PCM audio frame data, the values in Frame Size and Sample Time are calculated based on the Sample Rate that you select. The values in these parameters are for display purposes only. You cannot edit these values.
Pulse Density Modulation (PDM) uses the density of a pulse to represent a signal. The relative density of the pulses in PDM corresponds to the analog signal amplitude. In PDM bit stream, ‘1’ corresponds to a pulse of positive polarity and ‘0’ corresponds to a pulse of negative polarity. PDM uses a single bit to convey the audio.
Some of the advantages of PDM processing are:
Simple in concept and execution
Low cost audio transmission in digital form
Efficient systems with good design
Low noise and less signal interference
The Mic In block uses a double buffering to read the audio data from the MEMS microphone.
With double buffering, one buffer is used by the DMA to read audio PDM signal from the MEMS microphone while the other buffer is used to send audio frames to the audio processing algorithm. This helps to prevent overwriting of the buffer while being used by the audio algorithm.
The following describes the double buffering mechanism in the Mic In block.
In the first iteration, when the ‘Mic in buffer1’ sends the data for audio processing, the DMA fills ‘Mic in buffer0’.
In the first iteration, the ‘Mic in buffer1’ does not contain the audio samples. Therefore, the audio samples from the Mic In block are delayed by one audio frame.
After the DMA completes filling Mic in buffer0, an interrupt is triggered.
In the second iteration, when the ‘Mic in buffer0’ sends the data for audio processing and the DMA fills ‘Mic in buffer1’.
Port_1— Output port containing audio samples from MEMS microphone
The output port containing audio samples from MEMS microphone on the STMicroelectronics® STM32F4–Discovery board.
Sample rate (Hz)— Sample frequency to read audio signal
Select a sample rate. It is the sampling frequency to read audio signal from the MEMS microphone.
Frame size (samples)— Audio signal frame size based on sample rate
The audio signal frame size is based on the sample rate that you select.
Sample time— Sample time based on sample rate
The block sample time is calculated based on the sample rate that you select.