node-alsa-capture

Node module to record PCM audio data from ALSA capture devices (e.g., microphones).

Emits events about overruns, short reads, deviating sample rates or period sizes.

Dependencies

This module requires libasound2-dev for and libasound2 to link against libasound.so.2.

On debian derivatives system you can install all dependencies with apt:

``bash
sudo apt install libasound2 libasound2-dev
`

Usage


`javascript
const AlsaCapture = require("alsa-capture");

// default values for the options object
const captureInstance = new AlsaCapture({
channels: 2,
debug: false,
device: "default",
format: "S16_LE",
periodSize: 32,
periodTime: undefined,
rate: 44100,
});

// data is an ArrayBuffer
// Buffer size = numChannels formatByteSize periodSize
// Example: 2 Bytes (AlsaFormat.S16_LE) 2 (numChannels) 32 (periodSize) = 128 Bytes
captureInstance.on("audio", (data) => {
console.log(data);
});


// if the requested rate is not available for the capture device
// ALSA will select the nearest available
captureInstance.on("rateDeviating", (actualRate) => {
console.warn(Sound card rate deviates from requested rate; Actual rate: ${actualRate});
});

// if the requested period size is not available for the capture device
// ALSA will select the nearest available
captureInstance.on("periodSizeDeviating", (actualPeriodSize) => {
console.warn(Sound card period size deviates from requested period size; Actual period size: ${actualPeriodSize});
});

captureInstance.on("periodTime", (periodTime) => {
console.log(Actual period time ${periodTime});
});

// if an error occurs, error will contain the message
captureInstance.on("error", (error) => {
console.error(error);
});

// if overruns occur frequently try increasing the period_size
captureInstance.on("overrun", () => {
console.warn("NodeAlsaCapture overrun");
});

captureInstance.on("shortRead", (framesRead) => {
console.warn(NodeAlsaCapture shortRead: Only ${framesRead} read);
});

captureInstance.on("readError", (error) => {
console.warn(NodeAlsaCapture readError: ${error});
});

setTimeout(() => {
// close capture device
captureInstance.close();
}, 10000);

// event is sent after capture devices is closed completely
captureInstance.on("close", () => {
console.log("capture closed")
});

`

API

$3

Creates a new ALSA capture instance which extends eventemitter3 and emits several kinds of events including the PCM audio data. The optional opts object sets the ALSA hardware parameters.

| option | type | description | default |
|------------|---------|---------------------------------------------------------------------|--------------|
| channels | number | select number of channels to capture | 2 |
| debug | boolean | prints debug data to stderr | false |
| device | string | ALSA device ID | default |
| format | string | Sample format (see Supported Sample formats) | S16_LE |
| periodSize | number | A period is the number of frames in between each hardware interrupt | 32 |
| periodTime | number | Set period time near n us. | (no default) |
| rate | number | Sample rate (400 <= rate <= 196000) | 44100 |

Note: snd_pcm_hw_params_set_period_time_near will only be called if the opts object has the periodTime property.

Note: snd_pcm_hw_params_set_access is set to SND_PCM_ACCESS_RW_INTERLEAVED.

$3


Stops the ALSA capture thread. Afterwards the close event will be emitted.

$3

#### .on("audio", (data: ArrayBuffer) => {})
Returns the PCM data in an ArrayBuffer.

The buffer size is derived from the number of channels, the sample format and the period size:

bufferSize = numChannels formatByteSize periodSize

#### .on("close", () => {})
Capture instance closed.

#### .on("rateDeviating", (actualRate: Number) => {})
If the requested sample rate is not available for the capture device ALSA will select the nearest available

#### .on("periodSizeDeviating", (actualPeriodSize: Number) => {})
If the requested period size is not available for the capture device ALSA will select the nearest available

#### .on("periodTime", (periodTime: Number) => {})
The actual period time

#### .on("error", (error: String) => {})
Exceptions in Instance creation. See error messages.

#### .on("overrun", () => {})
An ALSA Buffer overrun occurred.

#### .on("shortRead", (framesRead: Number) => {})
ALSA could not capture enough frames, only framesRead frames.

#### .on("readError", (readError: String) => {})
Read error message. See snd_strerror.

Selecting devices


arecord -l lists all soundcards and digital audio devices:

`
card 0: PCH [HDA Intel PCH], device 0: CX20590 Analog [CX20590 Analog]
Subdevices: 1/1
Subdevice #0: subdevice #0
card 1: CODEC [USB Audio CODEC], device 0: USB Audio [USB Audio]
Subdevices: 1/1
Subdevice #0: subdevice #0
`

To select card 1 use hw:1,0 or if you want to enable the ALSA plugin layer (ALSA will automatically convert the data to a format supported by your sound card 1) use plughw:1,0.

arecord -L lists all list device names

`
default
Playback/recording through the PulseAudio sound server
null
Discard all samples (playback) or generate zero samples (capture)
pulse
PulseAudio Sound Server
sysdefault:CARD=PCH
HDA Intel PCH, CX20590 Analog
Default Audio Device
front:CARD=PCH,DEV=0
HDA Intel PCH, CX20590 Analog
Front speakers
hw:CARD=PCH,DEV=0
HDA Intel PCH, CX20590 Analog
Direct hardware device without any conversions
plughw:CARD=PCH,DEV=0
HDA Intel PCH, CX20590 Analog
hw:CARD=CODEC,DEV=0
USB Audio CODEC, USB Audio
Direct hardware device without any conversions
plughw:CARD=CODEC,DEV=0
USB Audio CODEC, USB Audio
Hardware device with all software conversions
`
So you could also use hw:CARD=CODEC,DEV=0 to write to this external USB audio card or use pulse and use pulse audio settings.

Supported Sample formats

* S8
* U8
* S16_LE
* S16_BE
* U16_LE
* U16_BE
* S24_LE
* S24_BE
* U24_LE
* U24_BE
* S32_LE
* S32_BE
* U32_LE
* U32_BE
* FLOAT_LE
* FLOAT_BE
* FLOAT64_LE
* FLOAT64_BE
* IEC958_SUBFRAME_LE
* IEC958_SUBFRAME_BE
* MU_LAW
* A_LAW
* IMA_ADPCM
* MPEG
* GSM
* SPECIAL
* S24_3LE
* S24_3BE
* U24_3LE
* U24_3BE
* S20_3LE
* S20_3BE
* U20_3LE
* U20_3BE
* S18_3LE
* S18_3BE
* U18_3LE
* U18_3BE
* G723_24
* G723_24_1B
* G723_40
* G723_40_1B
* DSD_U8
* DSD_U16_LE
* DSD_U32_LE
* DSD_U16_BE

License

MIT

Copyright (c) 2018 Bernd Kaiser, feinarbyte GmbH

Acknowledgements

* The ALSA c code examples
* The streaming worker was originally written by Scott Frees.

Compatibility

Tested with Node v8.3.0, v8.9.4`