Merge audio files into one

Usage example

{
  "steps": {
    "merged": {
      "robot": "/audio/merge",
      "preset": "mp3",
      "use": {
        "steps": [
          {
            "name": ":original",
            "fields": "first_audio_file",
            "as": "audio"
          },
          {
            "name": ":original",
            "fields": "second_audio_file",
            "as": "audio"
          },
          {
            "name": ":original",
            "fields": "third_audio_file",
            "as": "audio"
          }
        ]
      },
      "ffmpeg_stack": "v6"
    }
  }
}

Parameters

• interpolate

  boolean | Record<string, boolean>

  Controls whether Assembly Variables are interpolated for individual instruction fields.

  By default, most Robot instruction fields interpolate Assembly Variables. Set this to false to treat every instruction field as literal text, or set an individual field path to false to treat only that field as literal text. For Robot-specific fields that are literal by default, set this to true or set that field path to true to opt back into interpolation.

  Use field names such as path, or dotted paths such as ffmpeg.vf for nested objects.

• output_meta

  Record<string, boolean> | boolean | Array<string>

  Allows you to specify a set of metadata that is more expensive on CPU power to calculate, and thus is disabled by default to keep your Assemblies processing fast.

  For images, you can add "has_transparency": true in this object to extract if the image contains transparent parts and "dominant_colors": true to extract an array of hexadecimal color codes from the image.

  For images, you can also add "blurhash": true to extract a BlurHash string — a compact representation of a placeholder for the image, useful for showing a blurred preview while the full image loads.

  For videos, you can add the "colorspace: true" parameter to extract the colorspace of the output video.

  For videos, you can also add "interlaced": true to detect whether the video is interlaced. This combines the cheap ffprobe field_order flag with a bounded idet sampling pass over the first frames of the source, exposing interlaced, field_order, and a diagnostic interlace_detection object under file.meta. This is computationally expensive and billed accordingly.

  For audio, you can add "mean_volume": true to get a single value representing the mean average volume of the audio file.

  You can also set this to false to skip metadata extraction and speed up transcoding.

• result

  boolean (default: false)

  Whether the results of this Step should be present in the Assembly Status JSON

• queue

  batch

  Setting the queue to 'batch', manually downgrades the priority of jobs for this step to avoid consuming Priority job slots for jobs that don't need zero queue waiting times

• force_accept

  boolean (default: false)

  Force a Robot to accept a file type it would have ignored.

  By default, Robots ignore files they are not familiar with. 🤖/video/encode, for example, will happily ignore input images.

  With the force_accept parameter set to true, you can force Robots to accept all files thrown at them. This will typically lead to errors and should only be used for debugging or combatting edge cases.

• ignore_errors

  boolean | Array<meta | execute> (default: [])

  Ignore errors during specific phases of processing.

  Setting this to ["meta"] will cause the Robot to ignore errors during metadata extraction.

  Setting this to ["execute"] will cause the Robot to ignore errors during the main execution phase.

  Setting this to true is equivalent to ["meta", "execute"] and will ignore errors in both phases.

• use

  string | Array<string> | Array<object> | object

  Specifies which Step(s) to use as input.

  • You can pick any names for Steps except ":original" (reserved for user uploads handled by Transloadit)
  • You can provide several Steps as input with arrays:
    

    {
      "use": [
        ":original",
        "encoded",
        "resized"
      ]
    }
    

  • You can also tag an input Step with as to pass semantic intent to robots:
  • You can also tag input Steps with as to pass semantic intent to robots:
    

    {
      "use": [
        {
          "name": ":original",
          "as": "image"
        },
        {
          "name": ":original",
          "as": "mask"
        }
      ]
    }
    

  Tip

  That's likely all you need to know about use, but you can view Advanced use cases.

• ffmpeg

  object

  A parameter object to be passed to FFmpeg. If a preset is used, the options specified are merged on top of the ones from the preset. For available options, see the FFmpeg documentation. Options specified here take precedence over the preset options.

• ffmpeg_stack

  v6 | v7 | v8 | string (default: "v6.0.0")

  Selects the FFmpeg stack version to use for encoding. These versions reflect real FFmpeg versions. We currently recommend to use "v6.0.0". Deprecated "v5.x" values are accepted for backward compatibility.

• preset

  aac | alac | audio/aac | audio/alac | audio/flac | audio/mp3 | audio/ogg | 35 more ↓

  Performs conversion using pre-configured settings.

  If you specify your own FFmpeg parameters using the Robot's ffmpeg parameter and you have not specified a preset, then the default mp3 preset is not applied. This is to prevent you from having to override each of the MP3 preset's values manually.

  For a list of audio presets, see audio presets.

• bitrate

  string | number

  Bit rate of the resulting audio file, in bits per second. If not specified will default to the bit rate of the input audio file.

• sample_rate

  string | number

  Sample rate of the resulting audio file, in Hertz. If not specified will default to the sample rate of the input audio file.

• duration

  first | longest | shortest (default: "longest")

  Duration of the output file compared to the duration of all merged audio files. Can be "first" (duration of the first input file), "shortest" (duration of the shortest audio file) or "longest" for the duration of the longest input file.

• loop

  boolean (default: false)

  Specifies if any input files that do not match the target duration should be looped to match it. Useful for audio merging where your overlay file is typically much shorter than the main audio file.

• volume

  average | sum (default: "average")

  Valid values are "average" and "sum" here. "average" means each input is scaled 1/n (n is the number of inputs) or "sum" which means each individual audio stays on the same volume, but since we merge tracks 'on top' of each other, this could result in very loud output.

Demos

Related blog posts