Format of captured data from /dev/dsp?.?

[M.E.]

The following sequence captures audio from the microphone (if mixer has been configured to do so) and then playbacks to the speakers:

cat /dev/dsp0.0 > rawaudio
cat rawaudio > /dev/dsp0.0

I would like to understand which format is used. Is there any specific place where I could look for further information?

Note: this is not a practical question, so I am not looking for an audio command line utility such as sox. What I would like to understand is in which format FreeBSD delivers and receives data in dsp device files.

Thanks

 

[grahamperrin]

It might help to not visualise any one format in this context.

From <https://news.ycombinator.com/item?id=28055096> (2021-08-03), with added emphasis:

… enlightenment … you can just cat a wav file into /dev/audio (or /dev/dsp? forget the exact name). And that you can produce white noise by catting /dev/random to the same destination. With no barrier to the sound card like that, I was free to experiment with programming different noise algorithms and figuring out how digital audio works. I eventually did things the proper way with OSS and finally sndio on OpenBSD, but direct, universal interfaces like files invite that initial exploration …

audio - Playing, recording, and streaming sound with cat and /dev/dsp - Unix & Linux Stack Exchange (2021-08-22)

2010: any port use /dev/dsp directly? - Gary Kline - org.freebsd.freebsd-questions - MarkMail

---

More advanced (beyond me), from Goran Mekić:

<https://old.reddit.com/r/freebsd/comments/q7aari/-/> ▶ FreeBSD Audio (2021-10-12) ◀ <https://news.ycombinator.com/item?id=28850513>

PS welcome to FreeBSD Forums!

 

[M.E.]

Thanks grahamperrin, after posting my question I read Goran's post -which is very good-. I also found this one useful to have an idea on how audio generally works in Linux and FreeBSD -it covers more Linux than FreeBSD though-.

Goran actually kindly pointed me to the right place in FreeBSD source code where I can find some examples on how the device file /dev/dsp is opened and how to configure it via ioctl. A basic example can be found here.

I have started playing a bit with the examples -just because I am curious on interacting directly with the FreeBSD sound layer- but I still think I do not have all the documentation on how this is properly arranged.

For example, something I have noticed is that when you restart FreeBSD the /dev/dsp files are simply not there. If you load firefox and watch a video on youtube -so the system actually plays sound- then /dev/dsp0.0 appears. If I change the channel to the front jacks -I am testing with a desktop computer that has both rear and front audio connectors- with sysctl hw.snd.default_unit=1 and I open a youtube tab and play audio then /dev/dsp1.0 appears.

So it seems to me that at the application level you generate those /dev/dsp device files if they are not there, but I can not match that creation on the examples in the source code.

I wonder if somebody could either point to further documentation where this is explained or explain a bit in more detail how to create the /dev/dsp files -which is a question directly related to the post but not entirely the same-.

 

[oOiOo]

I wonder if somebody could either point to further documentation where this is explained or explain a bit in more detail how to create the /dev/dsp files -which is a question directly related to the post but not entirely the same-.

dsp devices are automatically created by kernel on use (if corresponding hardware exist). To convince you, these two simple examples

file /dev/dsp*
/dev/dsp0.0: character special (0/123)
/dev/dsp0.1: character special (0/129)
/dev/dsp1.0: character special (0/128)
/dev/dsp2.0: character special (0/130)

ls -l /dev/dsp*   
crw-rw-rw-  1 root  wheel  0x7b Jun 26 15:30 /dev/dsp0.0
crw-rw-rw-  1 root  wheel  0x81 Jun 26 15:30 /dev/dsp0.1
crw-rw-rw-  1 root  wheel  0x80 Jun 26 15:30 /dev/dsp1.0
crw-rw-rw-  1 root  wheel  0x82 Jun 26 15:30 /dev/dsp2.0

Before running either of these commands, the device nodes were not present under /dev.