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

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. 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 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 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 to see how they work.

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.parsed. 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