website/content/blog/recordoutputaudio.md

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2020-03-07 19:54:17 +00:00
---
title: "Record Output Audio via Terminal"
date: 2020-03-01T10:38:06-05:00
draft: false
tags: [ "linux", "audio" ]
---
This post is specific to `PulseAudio` on Linux.
I know of GUI based solutions like [PulseAudio Volume Control](https://freedesktop.org/software/pulseaudio/pavucontrol/) that lets you set up monitor devices. But, what if you want to do this through the terminal?
Luckily, [b-ak](https://askubuntu.com/a/850174) on AskUbuntu gave an elegant answer to this question!
First make sure you have `pulseaudio-utils` installed,
```bash
sudo apt install pulseaudio-utils
```
Next we need to search for the speaker we wish to monitor
```bash
pacmd list-sinks | grep -e 'name:' \
-e 'index' \
-e 'Speakers'
```
It will output something similar to this:
```bash
* index: 0
name: <alsa_output.pci-0000_00_1b.0.analog-stereo>
analog-output-speaker: Speakers (priority 10000, latency offset 0 usec, available: unknown)
```
From here note the name in `<>` of the speaker you wish to monitor. For example for my output above, I wish to monitor `alsa_output.pci-0000_00_1b.0.analog-stereo`.
Next we will use the `parec` command to record the raw audio stream from the PulseAudio server.
```bash
parec --device alsa_output.pci-0000_00_1b.0.analog-stereo.monitor | encoder_command
```
Notice the addition of `.monitor` at the end of the device.
## `lame`
For the `encoder_command`, b-ak used `lame`.
```bash
lame -r -V0 - out.mp3
```
This command takes in a raw pcm `-r` and enables variable bit rates for the highest quality `-V0`. From there it encodes it and puts it in `out.mp3`.
Now `lame` actually makes a couple assumptions about your raw pcm if you didn't specify additional arguments:
- The Raw PCM is formatted in signed 16-bit little endian samples
- The Raw PCM has 2 channels
If you're assumptions don't meet the above, then you will need to add additional arguments.
## `ffmpeg`
We can replace `lame` with the more featureful `ffmpeg` if we take note of the same assumptions above.
```bash
ffmpeg -f s16le \
-ac 2 \
-i pipe:0 \
-b:a 0 \
out.mp3
```
Where we can replace the `.mp3` with whatever file extension `ffmpeg` supports.
Now to show the entire command
```bash
parec --device alsa_output.pci-0000_00_1b.0.analog-stereo.monitor | \
ffmpeg -f s16le \
-ac 2 \
-i pipe:0 \
-b:a 0 \
out.mp3
```