Concatenate audio

🤖/audio/concat concatenates several audio files together.

It is possible to concatenate a virtually infinite number of audio files using 🤖/audio/concat.

Imagine you have three file input fields in the same upload form for audio files that you want to concatenate. You can tell Transloadit in which order it should concatenate them using the file input field's name attribute. Just use the value for the name attribute as the value for the fields key in the JSON. Then set the proper order in the as parameter. Valid values here are audio_[[index]]. Transloadit will then concatenate them in the order of the index ascending:

"concatenated": {
  "robot": "/audio/concat",
  "preset": "mp3",
  "use": {
    "steps": [
      { "name": ":original", "fields": "first_audio_file", "as": "audio_1" },
      { "name": ":original", "fields": "second_audio_file", "as": "audio_2" },
      { "name": ":original", "fields": "third_audio_file", "as": "audio_3" }
    ]
  }
}

Parameters

use

String / Array of Strings / Objectrequired
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"
]
```
💡 That’s likely all you need to know about use, but you can view advanced use cases:
› Advanced use cases
- Step bundling. Some Robots can gather several Step results for a single invocation. For example, 🤖/file/compress would normally create one archive for each file passed to it. If you'd set bundle_steps to true, however, it will create one archive containing all the result files from all Steps you give it. To enable bundling, provide an object like the one below to the use parameter:
```
"use": {
  "steps": [
    ":original",
    "encoded",
    "resized"
  ],
  "bundle_steps": true
}
```
  This is also a crucial parameter for 🤖/video/adaptive, otherwise you'll generate 1 playlist for each viewing quality.
  Keep in mind that all input Steps must be present in your Template. If one of them is missing (for instance it is rejected by a filter), no result is generated because the Robot waits indefinitely for all input Steps to be finished.
  
  Here’s a demo that showcases Step bundling.
- Group by original. Sticking with 🤖/file/compress example, you can set group_by_original to true, in order to create a separate archive for each of your uploaded or imported files, instead of creating one archive containing all originals (or one per resulting file). This is important for for 🤖/media/playlist where you'd typically set:
```
"use": {
  "steps": [
    "segmented"
  ],
  "bundle_steps": true,
  "group_by_original": true
}
```
- Fields. You can be more discriminatory by only using files that match a field name by setting the fields property. When this array is specified, the corresponding Step will only be executed for files submitted through one of the given field names, which correspond with the strings in the name attribute of the HTML file input field tag for instance. When using a back-end SDK, it corresponds with myFieldName1 in e.g.: $transloadit->addFile('myFieldName1', './chameleon.jpg').
  
  This parameter is set to true by default, meaning all fields are accepted.
  
  Example:
```
"use": {
  "steps": [ ":original" ],
  "fields": [ "myFieldName1" ]
}
```
- Use as. Sometimes Robots take several inputs. For instance, 🤖/video/merge can create a slideshow from audio and images. You can map different Steps to the appropriate inputs.
  
  Example:
```
"use": {
  "steps": [
    { "name": "audio_encoded", "as": "audio" },
    { "name": "images_resized", "as": "image" }
  ]
}
```
  Sometimes the ordering is important, for instance, with our concat Robots. In these cases, you can add an index that starts at 1. You can also optionally filter by the multipart field name. Like in this example, where all files are coming from the same source (end-user uploads), but with different <input> names:
  
  Example:
```
"use": {
  "steps": [
    { "name": ":original", "fields": "myFirstVideo", "as": "video_1" },
    { "name": ":original", "fields": "mySecondVideo", "as": "video_2" },
    { "name": ":original", "fields": "myThirdVideo", "as": "video_3" }
  ]
}
```
  For times when it is not apparent where we should put the file, you can use Assembly Variables to be specific. For instance, you may want to pass a text file to 🤖/image/resize to burn the text in an image, but you are burning multiple texts, so where do we put the text file? We specify it via ${use.text_1}, to indicate the first text file that was passed.
  
  Example:
```
"watermarked": {
  "robot": "/image/resize",
  "use"  : {
    "steps": [
      { "name": "resized", "as": "base" },
      { "name": "transcribed", "as": "text" },
    ],
  },
  "text": [
    {
      "text"  : "Hi there",
      "valign": "top",
      "align" : "left",
    },
    {
      "text"    : "From the 'transcribed' Step: ${use.text_1}",
      "valign"  : "bottom",
      "align"   : "right",
      "x_offset": 16,
      "y_offset": -10,
    }
  ]
}
```
output_meta

Object / Boolean ⋅ default: {}

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

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.
preset

String ⋅ default: "mp3"

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

Integer ⋅ default: auto

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

Integer ⋅ default: auto

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

Float ⋅ default: 1.0

When used this adds an audio fade in and out effect between each section of your concatenated audio file. The float value is used, so if you want an audio delay effect of 500 milliseconds between each video section, you would select 0.5. Integer values can also be represented.

This parameter does not add an audio fade effect at the beginning or end of your result audio file. If you want to do so, create an additional 🤖/audio/encode Step and use our ffmpeg parameter as shown in this demo.

FFmpeg parameters

ffmpeg_stack

String ⋅ default: "v3.3.3"

Selects the FFmpeg stack version to use for encoding. These versions reflect real FFmpeg versions.

The current recommendation is to use "v4.3.1". Other valid values can be found here.
ffmpeg

Object ⋅ default: {}

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.

Concatenate audio

Parameters

`use`

`output_meta`

`preset`

`bitrate`

`sample_rate`

`audio_fade_seconds`

FFmpeg parameters

`ffmpeg_stack`

`ffmpeg`

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