PDM Microphone Real-time Loopback Example Guide

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Introduction

This example demonstrates how to use the RA8P1 PDM (Pulse Density Modulation) Interface on Titan Board Mini to read audio from a PDM Microphone and play it back in real-time through the ES8156 Audio Codec, implementing a Microphone → Speaker audio loopback function.

Main features include:

PDM microphone data acquisition using RA8P1 built-in PDM interface
PDM to PCM format conversion (supports 16-bit and 20-bit modes)
Mono to stereo audio processing
Double-buffer mechanism for continuous audio streaming
Audio output via SSI interface and ES8156 codec

Hardware Introduction

1. PDM Microphone

The Titan Board Mini features an onboard PDM Microphone:

Parameter	Description
Type	MEMS PDM Microphone
Interface	PDM (Pulse Density Modulation)
Output Format	1-bit density stream
Sample Rate	Up to 16kHz (this example)
Channels	Mono

2. RA8P1 PDM Interface

RA8P1 PDM interface features:

Built-in PDM demodulator: Hardware PDM → PCM conversion
Configurable filters: SINC filter, HPF, LPF
Multi-channel support: Supports multiple microphone inputs
Configurable output: Supports 16-bit or 20-bit PCM output
DMA support: DMA transfer support, reducing CPU usage

3. Audio Output

Codec: ES8156 (24-bit DAC, supports 8kHz - 192kHz)
Interface: SSI (I2S protocol)
Output: 3.5mm headphone jack / onboard speaker

Software Architecture

1. Data Flow

PDM Microphone
    ↓ (PDM signal)
RA8P1 PDM Interface
    ↓ (Hardware filtering + PDM→PCM)
PDM Buffer (int32_t[])
    ↓ (Software conversion)
DAC Buffer (int16_t[], stereo)
    ↓ (I2S protocol)
ES8156 Codec
    ↓ (Analog audio)
Speaker/Headphone

2. Core Components

PDM Driver

Using Renesas FSP PDM driver:

/* Open PDM module */
R_PDM_Open(&g_pdm0_ctrl, &g_pdm0_cfg);

/* Start data acquisition */
R_PDM_Start(&g_pdm0_ctrl, buffer, buf_size.bytes, samples);

/* Stop acquisition */
R_PDM_Stop(&g_pdm0_ctrl);

/* Close PDM module */
R_PDM_Close(&g_pdm0_ctrl);

Data Conversion Function

/* PDM data to DAC data conversion */
static void convert_pdm_to_dac(int32_t *pdm_data, uint32_t pdm_samples, int16_t *dac_data)
{
    for (uint32_t i = 0; i < pdm_samples; i++)
    {
        int16_t sample_16;
        if (g_pcm_bits == PCM_20BITS)
        {
            /* 20-bit -> 16-bit: shift right 4 bits */
            sample_16 = (int16_t)(pdm_data[i] >> 4);
        }
        else
        {
            /* 16-bit mode: shift left 1 bit */
            sample_16 = (int16_t)(pdm_data[i] << 1);
        }

        /* Mono to stereo */
        dac_data[i * 2] = sample_16;      /* Left channel */
        dac_data[i * 2 + 1] = sample_16;  /* Right channel */
    }
}

3. Double Buffer Mechanism

To achieve continuous audio streaming, a double-buffer mode is used:

Timeline:
    t0      t1      t2      t3      t4
    |-------|-------|-------|-------|
Buffer 0: [Record ] [Play         ]
Buffer 1:                 [Record ] [Play         ]

Buffer 0: Record → Play
Buffer 1: Record → Play
Two buffers alternate for seamless continuous playback

4. Project Structure

Titan_Mini_pdm/
├── src/
│   └── hal_entry.c          # Main program entry
├── libraries/
│   └── HAL_Drivers/
│       ├── drv_i2s.c        # SSI/I2S driver
│       └── ports/
│           └── es8156/      # ES8156 codec driver
└── ra_gen/
    └── hal_data.c           # FSP configuration (PDM, SSI, etc.)

Configuration

1. PDM Configuration

Configure PDM parameters in ra_gen/hal_data.c:

const pdm_cfg_t g_pdm0_cfg =
{
    .unit                              = 0,
    .channel                           = 2,        /* PDM channel */
    .pcm_width                         = PDM_PCM_WIDTH_16_BITS_0_14,  /* 16-bit output */
    .pcm_edge                          = PDM_INPUT_DATA_EDGE_RISE,
    .p_extend                          = &g_pdm0_cfg_extend,
    // ...
};

2. Key Parameters

Parameter	Value	Description
`PDM_FS_HZ`	16000	Output sample rate 16kHz
`PDM_CHANNELS`	1	Mono input
`PDM_PCM_WIDTH`	16_BITS_0_14	16-bit PCM, valid bits [14:0]
`NUM_BUFFERS`	2	Double buffer mode
`PDM_RECORD_DURATION_SEC`	1	1 second audio per buffer

Run Effect

Run Effect

After power-on, green LED lights up indicating system ready
Blue LED lights up indicating audio loopback is running
Speak into the microphone and hear your voice in real-time through headphones/speaker
Blue LED blinks every 50 cycles (heartbeat indicator)

Troubleshooting

1. No Audio Output

Check if headphones/speaker are properly connected
Check volume settings (default 60%)
Verify ES8156 initialization was successful

2. Audio Stuttering

Current uses 1-second buffer, try adjusting PDM_RECORD_DURATION_SEC
Check system load, reduce other tasks

3. Audio Distortion

Verify pcm_width configuration matches hardware
Adjust shift parameters in data conversion function