ilugc-meet-14082021-talk3.chatlog

2021-08-14 16:04:55	mohan43u	the third talk is about ALSA
2021-08-14 16:05:07	mohan43u	im the speaker of this tal
2021-08-14 16:05:14	mohan43u	^s/tal/talk/g
2021-08-14 16:05:31	mohan43u	ALSA - Advanced Linux Sound Architecture
2021-08-14 16:06:14	mohan43u	in this talk, Im going to explain about few commands
2021-08-14 16:06:35	mohan43u	you can follow what I execute through https://meet.ilugc.in
2021-08-14 16:06:52	mohan43u	this website is also relayed through jitsi
2021-08-14 16:07:20	mohan43u	I hope everyone joined through either meet.ilugc.in or jitsi will not have problem following my commands
2021-08-14 16:07:39	mohan43u	so lets start talk about alsa
2021-08-14 16:08:02	mohan43u	ALSA is the subsystem in linux kernel responsible for audio
2021-08-14 16:09:00	mohan43u	the device driver people basically need to write audio driver which uses ALSA in order to make their audio device work in linux
2021-08-14 16:09:38	mohan43u	ALSA have all the infra to write audio drivers easily, nowadays, most of the audio devices are USB based
2021-08-14 16:10:22	mohan43u	so kernel have simple standard audio to utilize most of the audio devices in the market
2021-08-14 16:10:39	mohan43u	wen you plug a audio usb device, it is first detected by udev
2021-08-14 16:11:15	mohan43u	then that device will be classified as audio device and respoective drivers will be loaded
2021-08-14 16:12:12	mohan43u-we	once the correct drivers got loaded, you will see the audio device like /dev/snd/pcmC0D0p
2021-08-14 16:13:04	mohan43u-we	you can see the commands which Im executin in the presenter terminal
2021-08-14 16:13:22	mohan43u-we	I just connected to my laptop and listed all the audio devices
2021-08-14 16:13:51	mohan43u-we	in my laptop, there are 5 audio devices
2021-08-14 16:14:16	<--	test68 ([email protected]) has quit (Quit: Connection closed)
2021-08-14 16:14:30	mohan43u-we	the command which I executed shows all the playback devices
2021-08-14 16:14:53	mohan43u-we	if you look carefully, you can see that all the playback devices ends with 'p'
2021-08-14 16:15:15	mohan43u-we	p means playback
2021-08-14 16:15:36	mohan43u-we	as you see, I just listed the capture device
2021-08-14 16:15:43	mohan43u-we	c means capture
2021-08-14 16:15:52	-->	Guest ([email protected]) has joined #ilugc
2021-08-14 16:16:48	mohan43u-we	now I listed another device called control device
2021-08-14 16:17:19	mohan43u-we	so when you plug your usb headset, that usb headset will appear as a audio sound card
2021-08-14 16:17:45	mohan43u-we	you can see all the cards in your linux using /proc/asound/cards
2021-08-14 16:18:07	mohan43u-we	in my current laptop, I only have the built in sound card
2021-08-14 16:18:14	mohan43u-we	so it appears a single ards
2021-08-14 16:18:23	mohan43u-we	^s/ards/card/g
2021-08-14 16:18:50	mohan43u-we	this single cards will create those three type of virtual devices in linux
2021-08-14 16:19:03	mohan43u-we	namely playback, capture and control
2021-08-14 16:19:48	mohan43u-we	see, these are the virtual devices my builtin sound card created
2021-08-14 16:20:24	mohan43u-we	any audio device plugged in into linux will atlease have one playback and one control
2021-08-14 16:20:41	mohan43u-we	capture depends on the type of audio device you plug
2021-08-14 16:20:52	mohan43u-we	any questions till now?
2021-08-14 16:21:21	mohan43u-we	ok, im continuing
2021-08-14 16:21:52	mohan43u-we	this is the raw devices, in order to use this devices, you need to open this devices and use ioctls to talk to them
2021-08-14 16:22:12	mohan43u-we	this is where the libasound comes in
2021-08-14 16:22:46	mohan43u-we	libasound is a library which talk to these devices properly
2021-08-14 16:23:05	mohan43u-we	if you want to use these devices, libasound is the easiest way to talk to them
2021-08-14 16:23:25	mohan43u-we	based on libasound, there are alsa utilities which help us to configure these devices
2021-08-14 16:24:01	mohan43u-we	some of the utilities are aplay, amixer
2021-08-14 16:24:25	mohan43u-we	as you see in the presenter terminal, aplay can also list the audio devices
2021-08-14 16:24:45	mohan43u-we	if you look at the output, you can see that there is only one card (card0)
2021-08-14 16:25:00	mohan43u-we	but there are four sub devices
2021-08-14 16:25:19	mohan43u-we	each subdevices send the output through different ways
2021-08-14 16:25:31	mohan43u-we	the first sub device (device 0) is for the headset
2021-08-14 16:25:36	mohan43u-we	second is for hdmi 0
2021-08-14 16:25:47	mohan43u-we	third is for hdmi 1 and fourth is for hdmi 2
2021-08-14 16:26:22	mohan43u-we	so, if I want to play an audio through 3.5mm audio jack, them I have to send the audio data to card 0, device 0
2021-08-14 16:26:43	mohan43u-we	because that is where the 3.5mm jack is available
2021-08-14 16:27:00	mohan43u-we	in other words, I have to open /dev/snd/pcmC0D0p
2021-08-14 16:27:19	mohan43u-we	card0, device0 in the aplay -l output corresponds to pcmC0D0p
2021-08-14 16:27:42	mohan43u-we	you can say which card and which device from the pcm name itself
2021-08-14 16:27:52	mohan43u-we	pcm<C0><D0>p
2021-08-14 16:28:00	mohan43u-we	the first pcm is literal pcm
2021-08-14 16:28:06	mohan43u-we	the C0 means card 0
2021-08-14 16:28:14	mohan43u-we	and D0 means device 0
2021-08-14 16:28:21	mohan43u-we	and p means its a playback device
2021-08-14 16:28:51	mohan43u-we	any questions? are you guys able to understand how ALSA naming works?
2021-08-14 16:29:11	nihaal	what is pcm?
2021-08-14 16:29:34	mohan43u-we	pcm is the format of the data which device is able to understand
2021-08-14 16:30:02	mohan43u-we	Im not sure, but it expands to something like pulse code modulation or something. i may be wrong
2021-08-14 16:30:18	nihaal	ok, got it
2021-08-14 16:30:48	mohan43u-we	I hope everyone understand how alsa names its devices
2021-08-14 16:31:08	<--	sandeepk ([email protected]) has quit (Ping timeout: 256 seconds)
2021-08-14 16:31:23	mohan43u-we	now, the another command called arecord
2021-08-14 16:31:35	mohan43u-we	arecord -l will show the capture devices available
2021-08-14 16:31:59	mohan43u-we	in my machine, I have 3.5mm jack as capture
2021-08-14 16:32:09	mohan43u-we	that is what is is showing now in the presenter terminal
2021-08-14 16:32:26	mohan43u-we	card 0, device 0, but instead of playback this is capture
2021-08-14 16:32:38	mohan43u-we	so this device corresponds to /dev/snd/pcmC0Dpc
2021-08-14 16:33:15	mohan43u-we	so, the 3.5mm analog audio jack works as playback device, as well as works as capture device
2021-08-14 16:33:58	mohan43u-we	but alsa seperates its functionality into two different virtual devices and expose it to us in /dev/snd
2021-08-14 16:34:21	mohan43u-we	going next
2021-08-14 16:34:37	mohan43u-we	to now more details about these pcm devices, we need to use the proc filesystem
2021-08-14 16:35:38	mohan43u-we	see, I just go to /proc/asound, go to card0, then go to pcm0p and printed its property
2021-08-14 16:35:52	mohan43u-we	as you can see, this is CX20590 Analog
2021-08-14 16:36:47	mohan43u-we	in order to talk to this device, we have to follow the hardware parameters provided in this hw_params file
2021-08-14 16:37:03	mohan43u-we	this fill provides what kind of data this device accepts
2021-08-14 16:37:24	mohan43u-we	as you can see we have to give data to this device through mmap (MMAP_INTERLEAVED)
2021-08-14 16:37:47	mohan43u-we	and the exact digital audio format it uses is called S32_LE
2021-08-14 16:37:59	mohan43u-we	this is the very important parameter
2021-08-14 16:38:28	mohan43u-we	this parameter says, it uses 32 bit for audio sample
2021-08-14 16:39:26	mohan43u-we	so that means, there are 2^32 values for a single sample
2021-08-14 16:39:42	mohan43u-we	those values can be in +ve or in -ve
2021-08-14 16:39:57	mohan43u-we	then the next important hardware parameter
2021-08-14 16:40:00	mohan43u-we	the channels
2021-08-14 16:40:12	mohan43u-we	here there are two channels for this devices
2021-08-14 16:40:35	mohan43u-we	if we see two channels, that means, this device can do stereo playback
2021-08-14 16:40:57	mohan43u-we	stereo (left, right) playback requires two channels
2021-08-14 16:41:13	mohan43u-we	there are devices which provides 5 channel or 7 channels
2021-08-14 16:41:32	mohan43u-we	those are DTS specifications
2021-08-14 16:41:42	mohan43u-we	the next important parameter is rate
2021-08-14 16:41:48	mohan43u-we	very important parameter
2021-08-14 16:41:52	mohan43u-we	48000
2021-08-14 16:42:15	mohan43u-we	that means, this devices supports upto 48khz audio samplerate
2021-08-14 16:42:59	mohan43u-we	we usually use 44100Hz, or 44.1khz audio in our video/audio files
2021-08-14 16:43:21	mohan43u-we	but this device can support 48KHz
2021-08-14 16:43:47	mohan43u-we	so these three parameters is very important, format, channels and samplerate
2021-08-14 16:44:23	mohan43u-we	based on these parameters, we have to send the data to this devices
2021-08-14 16:44:49	mohan43u-we	 so when we write an audio application in linux, first we need to open the correct playback device
2021-08-14 16:45:01	mohan43u-we	read these important hardware parameters
2021-08-14 16:45:19	mohan43u-we	then send the audio data correctly to this device, so that it can play that data.
2021-08-14 16:45:40	mohan43u-we	one important thing about ALSA devices
2021-08-14 16:46:35	mohan43u-we	is when one application open the alsa device (let say /dev/snd/pcmC0D0p), than no other application will be able to open simulteniously
2021-08-14 16:47:02	mohan43u-we	that means, in ALSA, only one application can open the alsa device.
2021-08-14 16:47:29	mohan43u-we	but you can ask me how come in our linux, we are able to use two audio applications simulteniously?
2021-08-14 16:47:45	mohan43u-we	this is where mixers comes into place
2021-08-14 16:47:54	mohan43u-we	in other words , pulseaudio comes into place
2021-08-14 16:48:11	mohan43u-we	pulseaudio is a daemon which sits in between alsa and our audio application
2021-08-14 16:48:33	mohan43u-we	when the audio application wants to send data to and audio device, instead of opening the alsa device directly,
2021-08-14 16:48:48	mohan43u-we	it communicates to pulseaudio and asks pulseaudio to open the device
2021-08-14 16:49:12	mohan43u-we	so pulseaudio happily open that device and whatever the audio application gives, it sends to alsa
2021-08-14 16:49:30	mohan43u-we	in the mean time, another audio application also want to send data to the audio device
2021-08-14 16:50:03	mohan43u-we	that application also ask pulse instead of directly opening the alsa device, becuase that application know that the second time if it opens, then it will fail
2021-08-14 16:50:14	mohan43u-we	so it asks the pulseaudio to also send its data
2021-08-14 16:50:41	mohan43u-we	now pulseaudio get data from both audio applications, then it mixes those audio data into a single stream
2021-08-14 16:50:47	mohan43u-we	then send that data to alsa
2021-08-14 16:51:02	mohan43u-we	so this is how audio works in linux
2021-08-14 16:51:14	mohan43u-we	pulseaudio plays very important role here
2021-08-14 16:51:38	mohan43u-we	also pulseaudio manages all the audio devices through alsa
2021-08-14 16:52:08	mohan43u-we	there are lot of things to talk about the sound system in linux
2021-08-14 16:52:29	mohan43u-we	and ther are latest applications came up like pipewire to replace pulseaudio
2021-08-14 16:52:41	<--	dhanesh ([email protected]) has quit (Quit: Leaving)
2021-08-14 16:52:48	mohan43u-we	if you use archlinux, you can use pipewire instead of pulseaudio
2021-08-14 16:53:06	mohan43u-we	im finishing my talk here, any questions?
2021-08-14 16:54:38	@mbuf	mohan43u-we, very informative, thanks!
2021-08-14 16:55:00	mohan43u-we	I hope from this talk, all of you are able to understand where alsa sits, where pulseaudio sits and how audio applications communicate to the audio device.
2021-08-14 16:55:28	mohan43u-we	ALSA is a beast when it comes to audio, you can do wonders if you know how to play with it.
2021-08-14 16:56:17	mohan43u-we	unfortunately, because it is not very user friendly, people hesitated to use it directly, people wanted a simple way to do audio instead of doing the ALSA way.
2021-08-14 16:56:47	mohan43u-we	so linux community created layers and layers on top of ALSA to simplify the audio in linux.
2021-08-14 16:57:27	mohan43u-we	anyway, because of the layers, now we have easy going with audio
2021-08-14 16:57:57	mohan43u-we	thanks to pulseaudio for simplifying ALSA and taking care of mixing
2021-08-14 16:58:37	mohan43u-we	otherwise, new linux users would have left to deal with the beast called ALSA
2021-08-14 16:59:14	mohan43u-we	im finishing my talk.