# racket-audio

Integration of common audio libraries in Racket.

The package contains decoder, player and encoder bindings.  Playback uses the
existing audio player modules.  Encoding is provided by `audio-encoder.rkt` with
Opus and FLAC backends.

## Native dependencies

For playback and decoding, install the native libraries used by the selected
backends:

- libao
- libFLAC
- mpg123
- FFmpeg libraries, including libavutil, libavcodec, libavformat and
  libswresample

For encoding, also install:

- libopusenc
- libopus
- libogg
- TagLib with the C binding, usually provided as `taglib` / `taglib_c`

The Opus encoder backend uses libopusenc directly.  The FLAC encoder backend
uses libFLAC directly.  FLAC sample-rate conversion uses the existing FFmpeg
swresample layer.

## macOS

Using Homebrew, install the native libraries before using the package:

```sh
brew install libao
brew install flac
brew install mpg123
brew install ffmpeg
brew install opus
brew install libopusenc
brew install taglib
```

Some Homebrew installations provide FFmpeg as `ffmpeg`; older local setups may
use `ffmpeg-full`.

## Encoder examples

Encode to Opus:

```racket
(require "audio-encoder.rkt")

(audio-encode "input.flac"
              "output.opus"
              (hash 'bitrate 224000
                    'vbr? #t
                    'complexity 10)
              #:encoder 'opus)
```

Encode 96 kHz FLAC to 48 kHz FLAC:

```racket
(audio-encode "input-96k.flac"
              "output-48k.flac"
              (hash 'sample-rate 48000
                    'bits-per-sample 24
                    'compression-level 8)
              #:encoder 'flac)
```

A small test wrapper is available in `encoder-test.rkt`:

```sh
racket encoder-test.rkt --encoder opus --input input.flac --output output.opus --bitrate-kbps 224
racket encoder-test.rkt --encoder flac --input input-96k.flac --output output-48k.flac --sample-rate 48000
```