How to use all 10 audio inputs on Bela GEM Multi?

YHZ

Title: Bela GEM Multi: Channels 2-9 silent despite 10-channel configuration

Text:
I am working with a Bela GEM Multi board and I need to record from all 10 audio input channels simultaneously. I am having trouble getting signals from any channel beyond the first two.

Software Configuration: I have enabled 10 channels using the command line argument -i 10 -o 10. Inside setup(), rt_printf confirms that context->audioInChannels is indeed 10.

Channels 0 & 1 are working: When I connect my analog microphones to the pins for Channel 0 and 1 (IN0/IN1), audioRead(context, n, 0) and audioRead(context, n, 1) work perfectly. The recorded waveform is correct.

No signal on Channel 2+: When I use the exact same wiring logic and microphone on the physical pins for Channel 2 (IN2), audioRead(context, n, 2) returns a constant 0.0 (flatline).

Troubleshooting done: I have increased the input gain to 30dB+ for all channels in the IDE and verified the physical connections.

My Questions:
Does the Bela GEM Multi require additional initialization or specific flags to enable the second codec (Channels 2-9)? Are these pins configured as PDM (digital) by default, and if so, how do I switch them to Analog mode? What is the correct way to get all 10 analog inputs working?

giuliomoro

YHZ I have enabled 10 channels using the command line argument -i 10 -o 10

how so? Those are not supported command line options.

What is the revision number on your Gem Multi cape? You'll find it on the back of the board, underneath the audio input header.

How are you connecting your microphones to the cape?

YHZ

#include <Bela.h>
#include <libraries/Scope/Scope.h>
#include <vector>
#include <fstream>
#include <string>

/**
 * 实验：同步播放二进制信号并录音
 * 采样率要求：96kHz (请在 IDE Settings 中设置)
 */

// --- 参数设置 ---
std::string gInputFile = "zc_signal_525_30.bin";    // MATLAB 生成的二进制文件
std::string gOutputFile = "9.wav"; // 录音保存的文件名
float gScopeGain = 0.1f;                  // 示波器显示增益

// --- 全局变量 ---
Scope scope;
std::vector<float> gPlayBuffer;           // 播放缓冲区
std::vector<float> gRecordBuffer;         // 录制缓冲区
unsigned int gReadPtr = 0;                // 播放指针
unsigned int gWritePtr = 0;               // 录制指针
unsigned int gTotalFrames = 0;            // 总帧数
bool gIsRunning = false;

// WAV 头写入函数
void writeWavHeader(std::ofstream& out, int sr, int ch, int samples) {
    int ds = samples * ch * 2; int fs = 36 + ds;
    out.write("RIFF", 4); out.write((char*)&fs, 4); out.write("WAVEfmt ", 8);
    int s1 = 16; out.write((char*)&s1, 4); short af = 1; out.write((char*)&af, 2);
    short c = ch; out.write((char*)&c, 2); out.write((char*)&sr, 4);
    int br = sr * ch * 2; out.write((char*)&br, 4);
    short ba = ch * 2; out.write((char*)&ba, 2); short bps = 16; out.write((char*)&bps, 2);
    out.write("data", 4); out.write((char*)&ds, 4);
}

bool setup(BelaContext *context, void *userData) {
	    // 打印当前系统识别到的音频输入通道总数
rt_printf("audioInChannels=%d\n",
          context->audioInChannels);

rt_printf("audioOutChannels=%d\n",
          context->audioOutChannels);

rt_printf("analogInChannels=%d\n",
          context->analogInChannels);
    
    // 1. 检查硬件采样率是否为 96kHz
    if(context->audioSampleRate != 96000) {
        rt_printf("❌ 错误：当前硬件采样率为 %.0f，请在 Settings 中改为 96000!\n", context->audioSampleRate);
        return false;
    }

    // 2. 读取二进制文件到内存
    std::ifstream infile(gInputFile, std::ios::binary | std::ios::ate);
    if (!infile) {
        rt_printf("❌ 无法打开文件 %s。请确认已上传到 IDE。\n", gInputFile.c_str());
        return false;
    }
    std::streamsize size = infile.tellg();
    infile.seekg(0, std::ios::beg);
    
    gTotalFrames = size / sizeof(float);
    gPlayBuffer.resize(gTotalFrames);
    if (!infile.read((char*)gPlayBuffer.data(), size)) {
        rt_printf("❌ 文件读取失败。\n");
        return false;
    }
    rt_printf("✅ 信号加载成功！长度: %d 采样点 (%.2f 秒)\n", gTotalFrames, (float)gTotalFrames/96000.0);

    // 3. 初始化录制缓冲区和示波器
    gRecordBuffer.resize(gTotalFrames);
    scope.setup(1, context->audioSampleRate);
    
    gReadPtr = 0; gWritePtr = 0; gIsRunning = true;
    return true;
}

void render(BelaContext *context, void *userData) {
    for(unsigned int n = 0; n < context->audioFrames; n++) {
        float out = 0;

        // --- 播放部分 ---
        if(gReadPtr < gTotalFrames) {
            out = gPlayBuffer[gReadPtr];
            gReadPtr++;
        }

        // 发送到所有通道，确保音箱发声
        for(unsigned int ch = 0; ch < context->audioOutChannels; ch++) {
            audioWrite(context, n, ch, out);
        }

        // --- 录制部分 



		
        float in = audioRead(context, n, 0); // 读取 Audio In 0





		

        if(gWritePtr < gRecordBuffer.size()) {
            gRecordBuffer[gWritePtr] = in;
            gWritePtr++;
            
            // 实时示波器查看
            scope.log(in * gScopeGain);
        } else if(gIsRunning) {
            gIsRunning = false;
            rt_printf("✅ 实验完成，正在保存文件...\n");
            Bela_requestStop();
        }
    }
}

void cleanup(BelaContext *context, void *userData) {
    if(gWritePtr > 0) {
        std::ofstream out(gOutputFile, std::ios::binary);
        if(out) {
            writeWavHeader(out, (int)context->audioSampleRate, 1, gWritePtr);
            for(unsigned int i = 0; i < gWritePtr; i++) {
                float s = gRecordBuffer[i];
                if(s > 1.0f) s = 1.0f; if(s < -1.0f) s = -1.0f;
                short pcm = (short)(s * 32767.0f);
                out.write((char*)&pcm, 2);
            }
            out.close();
            rt_printf("🎉 录音已保存: %s。请下载后用 MATLAB 分析。\n", gOutputFile.c_str());
        }
    }
}

YHZ

The source code, wiring diagram, and version information are provided as attachments

giuliomoro

What's the datasheet of the microphones in use? Do they have differential outputs? Are they AC-coupled or DC-coupled?

YHZ

We use an SPH8878LR5H-1 analog MEMS microphone. The breakout board provides three connections: VCC, GND, and AUD. These are connected to the Bela board’s 3.3 V supply, ground, and audio input channel 0, respectively. During our testing, the microphone operated correctly when connected to audio input channels 0 or 1. However, it did not function properly when connected to channels 2–9.

giuliomoro

Put back the input wing ( the PCB with the input jacks on it) and use the pin sockets on that: do the channels work then?

YHZ

Do you mean connecting the microphone’s AUD pin directly to Bela’s Channel 2+ input, without going through the breadboard?

YHZ

Sorry, it’s still not working.

YHZ

Here are the signals from Channel 0 and Channel 2. As you can see, Channel 2 isn’t picking up any signal at all. I changed the code from “audioRead(context, n, 0)” to “audioRead(context, n, 2)” and also tried swapping the positive and negative wires, but it’s still not working.

YHZ

I just tried using the header pins connected to the 3.5 mm audio jack, and it works. Why doesn’t it work when I use the audio input pins labeled 0+, 0−, 1+, 1−, …, 9+, 9−?

I also noticed that the header pins behind the 3.5 mm jack are only labeled from channels 2 to 9. Where are channels 0 and 1?

giuliomoro

YHZ I just tried using the header pins connected to the 3.5 mm audio jack, and it works.

That's what I was asking you to do.

YHZ hy doesn’t it work when I use the audio input pins labeled 0+, 0−, 1+, 1−, …, 9+, 9−?

Because there's some AC-coupling circuitry on the wing that makes it suitable for input signals that have a DC component referenced to ground. If you want to wire the signals without the wing, you should provide components to emulate the behaviour of the wing, that is, you should add:

a 1µF series capacitor with your signal going into the + input
a 1µF capacitor to ground from the - input

YHZ I also noticed that the header pins behind the 3.5 mm jack are only labeled from channels 2 to 9. Where are channels 0 and 1?

Channels 0 and 1 are not broken out as pins on the input or output wings.

YHZ

Thanks for your reply.

I’m a bit confused about why the microphone works when connected to 0+ / 0- or 1+ / 1-, even though I wasn’t using any coupling capacitor.

I also have the capacitor shown in the attached picture. Would this type of capacitor be suitable?

If yes, could you explain how the capacitor should be wired between the microphone and the Bela input?

Thanks again for your help.

giuliomoro

YHZ I’m a bit confused about why the microphone works when connected to 0+ / 0- or 1+ / 1-, even though I wasn’t using any coupling capacitor.

Those are on a different chip (TLV320AIC3106) and its inputs behave differently.

YHZ I also have the capacitor shown in the attached picture. Would this type of capacitor be suitable?

What value are they ? We recommend 1uF (1 microfarad), but anything above 100nF will work somehow, but it may cut a bit of the low frequencies: this is forming a low-pass filter with the input impedance of the ADC, which is configurable between 2.5k and 10k. You can compute the cutoff fequency in Hz as (1/(2piCAPACITANCE*INPUT_IMPEDANCE)).

YHZ If yes, could you explain how the capacitor should be wired between the microphone and the Bela input?

Here's the diagram from the TLV320ADC3140's datasheet, used on channels 2 through 9:

INxP is labelled + on the Gem cape and INxM is labelled -. Your signal is "line or microphone single-ended input".