FFmpeg Error: Cannot find a matching stream for unlabeled input pad 0 on filter Parsed_amix_54

Question

I'm working on a project to generate a click track using React.js and FFmpeg. The project includes an Express.js backend where I construct an FFmpeg command to combine multiple audio files (count-ins, vocal cues, and click sounds) into a single track. The final output should be an MP3 file.

However, I'm encountering an error when running the FFmpeg command:

Cannot find a matching stream for unlabeled input pad 0 on filter Parsed_amix_54

How can I correctly construct the FFmpeg command to avoid the "Cannot find a matching stream for unlabeled input pad" error and ensure all inputs are properly processed and mixed?

Here is the relevant part of my server.js code:

const express = require('express');
const bodyParser = require('body-parser');
const { exec } = require('child_process');
const path = require('path');
const fs = require('fs');
const ffmpegPath = require('ffmpeg-static');
const cors = require('cors');

const app = express();
app.use(cors());
app.use(bodyParser.json());

app.post('/generate-click-track', (req, res) => {
    const { tempo, timeSignature, clickSound, subdivisions, sections } = req.body;
    const clickSoundPath = path.join(__dirname, 'public', 'click-sounds');
    const vocalCuesPath = path.join(__dirname, 'public', 'vocal-cues');

    const beatsPerBar = parseInt(timeSignature.split('/')[0]);
    const beatsPerMinute = tempo;
    let totalBeats = 0;
    const sectionStarts = [];

    sections.forEach((section, index) => {
        if (index > 0) {
            sectionStarts.push(totalBeats);
        }
        totalBeats += section.length * beatsPerBar;
    });

    const outputFilePath = path.join(__dirname, 'output', 'click-track.mp3');
    const tempDir = path.join(__dirname, 'temp');

    if (!fs.existsSync(tempDir)) {
        fs.mkdirSync(tempDir);
    }

    const countInFiles = Array.from({ length: beatsPerBar }, (_, i) => path.join(vocalCuesPath, 'count-ins', `${i + 1}.wav`));

    let ffmpegCommand = '';

    // Add count-in files
    countInFiles.forEach(file => {
        ffmpegCommand += `-i "${file}" `;
    });

    // Add section vocal cues and click sounds
    sections.forEach((section, index) => {
        const vocalCueFile = path.join(vocalCuesPath, `${section.name}.wav`);
        ffmpegCommand += `-i "${vocalCueFile}" `;

        for (let i = 0; i < section.length * beatsPerBar; i++) {
            const clickFile = i % beatsPerBar === 0 ? `${clickSound}-accents.mp3` : `${clickSound}.mp3`;
            ffmpegCommand += `-i "${path.join(clickSoundPath, clickFile)}" `;
        }
    });

    ffmpegCommand += `-filter_complex "`;

    let inputCount = 0;
    countInFiles.forEach((_, index) => {
        ffmpegCommand += `[${inputCount}:0]adelay=${index * (60 / beatsPerMinute) * 1000}|${index * (60 / beatsPerMinute) * 1000}[a${inputCount}]; `;
        inputCount++;
    });

    sections.forEach((section, index) => {
        const delay = (sectionStarts[index] ? sectionStarts[index] : 0) * (60 / beatsPerMinute) * 1000;
        ffmpegCommand += `[${inputCount}:0]adelay=${delay}|${delay}[a${inputCount}]; `;
        inputCount++;

        for (let i = 0; i < section.length * beatsPerBar; i++) {
            const delay = (sectionStarts[index] ? sectionStarts[index] : 0) * (60 / beatsPerMinute) * 1000 + (i * 60 / beatsPerMinute) * 1000;
            ffmpegCommand += `[${inputCount}:0]adelay=${delay}|${delay}[a${inputCount}]; `;
            inputCount++;
        }
    });

    ffmpegCommand += `amix=inputs=${inputCount}:duration=longest" -codec:a libmp3lame -b:a 192k -y "${outputFilePath}"`;

    console.log(`Executing ffmpeg command: ${ffmpegCommand}`);

    exec(`${ffmpegPath} ${ffmpegCommand}`, (error, stdout, stderr) => {
        if (error) {
            console.error(`Error generating click track: ${error.message}`);
            res.status(500).send('Error generating click track');
            return;
        }

        res.download(outputFilePath, 'click-track.mp3', (err) => {
            if (err) {
                console.error(`Error sending file: ${err.message}`);
            }

            // Clean up temp directory
            fs.readdir(tempDir, (err, files) => {
                if (err) throw err;
                for (const file of files) {
                    fs.unlink(path.join(tempDir, file), err => {
                        if (err) throw err;
                    });
                }
            });
        });
    });
});

app.listen(3001, () => {
    console.log('Server running on http://localhost:3001');
});

I can't include the full error messge as it is too long, but if anyone needs it, I'm happy to link to a text file that includes it.

What I have tried:

1. Verified all file paths and ensured they exist.
2. Escaped file paths to handle spaces in filenames.
3. Logged the constructed FFmpeg command and ran it manually, which still produced the same error.

Environment

• React.js: v18.3.1
• Node.js: v22.2.0
• Express: v4.19.2
• FFmpeg: static build via ffmpeg-static (v5.2.0)

Answer 1

The inputs to the amix filter should be specified e.g. for 4 inputs,

[a0][a1][a2][a3]amix=inputs=4:duration=longest