Open Sound System

Hi forum...

just one totally newby question...
Would it be possible to read the actual level from an audio device from the command line?
I know I can easily "set" mixer settings trough ossmix
I know I can "read" mixer settings too that way...

But I wonder wether it would be possible to have a binary (I have read this is possible with ioctl and API, but I'm not an experienced programmer... )that would read the actual value and simply return a value...
That way I could, from userspace, verify remote machines are actually playing, detect silence, verify cabling, etc...

I have experience building from sources, so I think this has not to be much difficult for an expert on the matter (I have read the ossxmix source trying to get a clue... but is still too complex to me), maybe someone could put here the code to build such a binary...

Anyhow, thanks in advance for your patience !
Cheers!

Most audio devices to not have a mixer control that allows to read the current input level.

You have to use some tool like http://sox.sourceforge.net/sox.html to capture some data and compute its level (in the case of sox, with the stat command).

...Aha

Well, at least one clue to further investigate
At first seems it is gonna be harder than I thought...

Thank you very much!

cladisch wrote:Most audio devices to not have a mixer control that allows to read the current input level.

You have to use some tool like http://sox.sourceforge.net/sox.html to capture some data and compute its level (in the case of sox, with the stat command).

OSS Mixer GUI (ossxmix) displays the current input and output levels: vmix0-invu and vmix0-outvu. Right?
There is a source code...

Couldn't you just filter the output of ossmix using awk?

e.g.

seawright wrote:Couldn't you just filter the output of ossmix using awk?

e.g.

Code: Select all

ossmix vmix0-outvol |awk '{print $(NF-1)}'

ossmix shows the "mixer settings" and nothing more.
It does not show actual sound levels.

If you want to know the actual sound levels, you have to measure them with a kind of VU-meters https://en.wikipedia.org/wiki/VU_meter
You may find such VU-meters in Audacity http://wiki.audacityteam.org/wiki/VuMeter
The OSS4 virtual mixer (vmix) also has VU-meters: vmix0-invu and vmix0-outvu
You may find them in OSSXMIX.

Ah! I see what you mean.
Ok then, how about?

seawright wrote:Ah! I see what you mean.
Ok then, how about?

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 ossmix -D |grep outvu |awk -F "=" '{print $NF}'

It seems to work, although it prints the value in a "strange format" ( hex with the prefix 0x https://en.wikipedia.org/wiki/Hexadecim ... esentation ):



He may need to convert it to decimal number:



TEST: Conversion to the "strange format" and vice versa:

You rock man!

I though ossmix values were just settings... not actual vu values...
I have to test it tomorrow, but provide it work, I thing I could manage to have a very spartan, shell scrip based, silence detector...
I thing it would be even possible to write a munin plugin to display an average volume value graph to monitor unatended machines and trigger alarms!!!
Hope it works! wow!

Yeah it works!

I would have to be more acurate and realize we do have the nice -D option on ossmix that yelds the full card values...
OSS rocks!

I would put the thread as SOLVED.

Out of curiosity... Do you know guys what is the span for the values? is it fixed? varies with the sound card?... I guess 0 means obviously no sound, but I wonder what are the top levels, and wether a logarithmic scaling is applied to the values, and so...

Many thanks to everyone!!!

Edit (copy paste fro the API):
The value returned simply the largest sample value seen on the input since the previous time this call was made (the values will be set to zero after each call). The samples are scaled so that the maximum is 32767 and the minimum is 0. The scale is linear (NOT dB).

aolivan wrote:Yeah it works!
I would have to be more acurate and realize we do have the nice -D option on ossmix that yelds the full card values...

To be more precise, vmix is not a card, it is a kind of PulseAudio
http://manuals.opensound.com/usersguide/vmix.html

Some "professional soundcards" may have VU-meters.

vmix driver may not be loaded by default for "professional soundcards", because it is a kind of "virtual crap".
If you have a kind of LynxTWO, you may test it http://manuals.opensound.com/usersguide/lynxtwo.html

Tried using gstreamer audio test source with volume set to maximum:

Suspect maximum value is 32639, 0x7f7f or 127:127 value being highest possible positive signed integer that can be represented in an eight bit field.

seawright wrote:Tried using gstreamer audio test source with volume set to maximum:

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gst-launch audiotestsrc freq=880 volume=1.0 ! audioconvert ! osssink

Code: Select all

ossmix -D vmix0-outvu
Value of mixer control vmix0-outvu is currently set to 126:126

Suspect maximum value is 32639, 0x7f7f or 127:127 value being highest possible positive signed integer that can be represented in an eight bit field.

127 does not seem to be the highest value:



In a word, "ossmix vmix0-outvu" produces the same result:



There might be an "ultimate secret documentation" (i.e. source code).

Notice that vmix0-outvu is a "Stereo peak meter"

Ok so I'm wrong. Definitely eight bits per channel but must be unsigned which in theory would allow up to 0xffff or 255:255
Don't know how to test other than driving an external input with an audio signal generator. Suspect the A to D converter would saturate before that level was reached but don't want to risk damaging my sound card to find out.

seawright wrote:Ok so I'm wrong. Definitely eight bits per channel but must be unsigned which in theory would allow up to 0xffff or 255:255
Don't know how to test other than driving an external input with an audio signal generator. Suspect the A to D converter would saturate before that level was reached but don't want to risk damaging my sound card to find out.

ossmix says: max=144



max=144 seems to mean 144:144

Since one channel might be silent, one may need a stereo meter.
In this case, "ossmix vmix0-outvu" might be useful (e.g. awk)