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https://github.com/jsantell/pcm-transform

Transformation stream for PCM data
https://github.com/jsantell/pcm-transform

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Transformation stream for PCM data

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# pcm-transform



[![Build Status](http://img.shields.io/travis/jsantell/pcm-transform.svg?style=flat-square)](https://travis-ci.org/jsantell/pcm-transform)

[![Build Status](http://img.shields.io/npm/v/pcm-transform.svg?style=flat-square)](https://www.npmjs.org/package/pcm-transform)



A transform stream that reads PCM data and transforms with built-in or custom functions.



Batches up samples from a PCM stream to a configurable amount (`batchSize`) and passes them into a configurable transform function so that they can be processed by proximity, like for making waveform data for visualizations.



## Install



```

npm install pcm-transform

```



## Usage



```javascript

var fs = require("fs");

var lame = require("lame");

var PCMTransform = require("pcm-transform");



fs.createReadStream("TesseracT - Concealing Fate, Part One: Acceptance.mp3")

  .pipe(new lame.Decoder)

  .pipe(PCMTransform({ batchSize: 20000 }))

  .pipe(fs.createWriteStream("waveform.data"));



```



## API



### `PCMTransform()`



Constructor for a [transform stream](https://nodejs.org/api/stream.html#stream_class_stream_transform). Takes the following config options:



* `batchSize` - Number of samples to batch together to be transformed. If using 16 bit samples in stereo (2 channels), each sample would be 4 bytes. (required)

* `json` - Whether or not to write downstream a buffer than is JSON.parse'able. (default: `false`). See Rendering JSON section later.

* `inputChannels` - Number of channels on the input PCM stream. (default: `2`)

* `inputBitDepth` - Number of bits per sample (per channel). (default: `16`)

* `outputBitDepth` - Number of bits per value for output. Used to convert buffers from transform functions into JSON. (default: `inputBitDepth`)

* `transform` - Either a string name of a default transform (`min-max`), or a supplied transform function. The function takes as arguments `buffer`, `stream`, and `callback`. Any error or data passed into the callback gets emitted, or written downstream, accordingly. See [Custom Transforms](https://github.com/jsantell/pcm-transform#custom-transforms) for more info.

* `head` - A buffer or string that gets written downstream when the streaming starts.

* `tail` - A buffer or string that gets written downstream when the streaming completes.



## Transforms



### min-max



Takes the batch of samples and returns the min and max value amongst all samples (regardless of channel) per batch. Results in a buffer of data with sequences of min/max pairs in the form of:



MIN_1 MAX_1 MIN_2 MAX_2 .... MIN_N MAX_N



This is similar to [audiowaveform](https://github.com/bbcrd/audiowaveform/blob/master/doc/DataFormat.md) format.



## Custom Transforms



To use a custom transform instead of a built-in one, pass a function instead of the string name to the `transform` option. This function takes a buffer of samples, the stream instance, and a callback that accepts `err, buffer` signature.



Example of a transform that reduces the stream into max value of each batch of samples. Also look at the built-in transforms like [min-max](https://github.com/jsantell/pcm-transform/blob/master/lib/transforms/min-max.js) to see how they work.



```js



function reduceMax (buffer, stream, callback) {

  var max = Number.MIN_VALUE;

  var value;



  for (var i = 0; i < buffer.length; i++) {

    value = buffer.readIntLE(i, stream.inputBitDepth / 8);



    if (value > max) {

      max = value;

    }

    i += (stream.inputBitDepth / 8);

  }



  // Bitshift the final value from input bit depth to output bit depth

  val = val >> (stream.inputBitDepth - stream.outputBitDepth);



  var output = new Buffer(stream.outputBitDepth / 8);

  output.writeIntLE(val, 0, stream.outputBitDepth);

  

  callback(null, output);

}



```



## JSON Rendering



Specifying `json: true` in the constructor will result in a buffer stream that can be `JSON.parse`d. The array of values is stored in the `data` property. Example of default JSON buffer:



```

{

  "data": [0, 100, 200, ..., 400]

}

```



The property name and other fields can be customized by providing a `head` or `tail` configuration to overwrite the defaults for JSON.



## Testing



```

npm test

```



## License



MIT License, Copyright (c) 2015 Jordan Santell