# Merge audio files into one
Robot: `/audio/merge`
🤖/audio/merge overlays several audio files on top of each other.
Stage: ga
## Usage example
If you have a form with 3 file input fields and wish to overlay the uploaded audios, instruct Transloadit using the `name` attribute of each input field. Use this attribute as the value for the `fields` key in the JSON, and set `as` to `audio`:
```json
{
"steps": {
"merged": {
"ffmpeg_stack": "v7",
"preset": "mp3",
"robot": "/audio/merge",
"use": {
"steps": [
{
"as": "audio",
"fields": "first_audio_file",
"name": ":original"
},
{
"as": "audio",
"fields": "second_audio_file",
"name": ":original"
},
{
"as": "audio",
"fields": "third_audio_file",
"name": ":original"
}
]
}
}
}
}
```
## Parameters
### `interpolate`
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.
Type: `boolean | Record`
### `output_meta`
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](https://blurha.sh) string — a compact representation of a placeholder for the image, useful for showing a blurred preview while the full image loads.
For images, `"thumbhash": true` instead extracts a base64-encoded [ThumbHash](https://evanw.github.io/thumbhash/) into `meta.thumbhash`, together with `meta.has_alpha` (whether an alpha channel exists, even when fully opaque). It describes EXIF-oriented pixels and uses the first frame of animated images. Extraction is best-effort: images above 40 megapixels, unsupported formats, or a failed/bounded decode produce no placeholder. Successful extraction adds a metadata charge equivalent to 20% of that file's bytes. No ThumbHash surcharge applies when disabled or when no hash is produced.
Set this option on the Step producing the image, such as `/upload/handle` for uploaded originals or `/image/resize` for processed outputs. Setting it only on `/transloadit/store` does not request extraction: storage preserves the producer's metadata. Transloadit Storage persists a generated hash with its immutable version and returns it in stored results and native asset reads. Direct S3 uploads do not generate placeholders. A placeholder contains image information, so protect it with the same access controls as the full image.
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.
Type: `Record | boolean | Array`
### `user_meta`
Adds custom JSON metadata to each emitted file without changing its contents. Nested objects and arrays are supported.
Inheritance depends on the Robot. Values merge with existing `user_meta` on the output file; the current Step replaces matching top-level keys. Assign required keys explicitly when a Robot creates fresh outputs.
In processing Steps, `${file.*}` refers to the first input and `${result.*}` to the emitted file. Values are evaluated per output after the Robot runs, before subsequent metadata extraction and temporary storage. On `:original`, values are evaluated per upload before metadata extraction.
Downstream Steps read `${file.user_meta.key}`. See [Custom metadata](https://transloadit.com/docs/topics/assembly-variables.md#user-meta) for a complete example and inheritance rules.
Type: `Record`
Default: `{}`
### `result`
Whether the results of this Step should be present in the Assembly Status JSON
Type: `boolean`
Default: `false`
### `queue`
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
Type: `batch`
### `force_accept`
Force a Robot to accept a file type it would have ignored.
By default, Robots ignore files they are not familiar with.
[🤖/video/encode](/docs/robots/video-encode.md), 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.
Type: `boolean`
Default: `false`
### `ignore_errors`
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.
Type: `boolean | Array`
Default: `[]`
### `use`
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:
```json
{
"use": [
":original",
"encoded",
"resized"
]
}
```
* You can also tag input Steps with `as` to pass semantic intent to robots:
```json
{
"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](/docs/topics/use-parameter.md).
Type: `string | Array | Array