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@@ -1,53 +1,102 @@ -This tool will help you to determine the volume scaling factors of your sound -card. If your sound card/chip manufacturer did not provide you with -documentation to which attenuation specific volume settings belong this tool -can help you to measure them. +WHAT IS THIS? -What you need: + This tool will help you to determine the volume scaling + factors of your sound card. If your sound card/chip + manufacturer did not provide you with documentation to which + attenuation specific volume settings belong this tool can help + you to measure them. -a) A sound card where input and output are synchronous +WHAT YOU NEED? -b) A feedback cable that allows you to feed sound card output back into the input + a) A sound card where input and output are synchronous -c) Some time + b) A feedback cable that allows you to feed sound card output + back into the input -In this is how you do it: + c) Some time -1) You plug in your feedback cable in the line out and line in of your soundcard +HOW TO USE IT? -2) You open the low-level ALSA mixer and mute/set to 0 everything you can -mute/set to 0, except what is necessary to get a PCM signal out of your machine -and back into the machine. The slider you want to measure you set the highest -possible value. The other sliders that are in our pipeline you set to some -'sensible' value, in the middle of its range somewhere. What does 'sensible' -mean? Something where neither clipping happens nor where the signal is too -faint. In 'alsamixer -c0' those points are usually near 0dB or a bit lower. If -you are not sure what to pick don't despair, our little tool will tell you if -you picked a good value. 'Inner' sliders should always be kept near to 0dB. + 1) You plug in your feedback cable in the line out and line in + of your soundcard -3) You run our little tool like this: + 2) You open the low-level ALSA mixer and mute/set to 0 + everything you can mute/set to 0, except what is necessary + to get a PCM signal out of your machine and back into the + machine. The slider you want to measure you set the highest + possible value. The other sliders that are in our pipeline + you set to some 'sensible' value, in the middle of its + range somewhere. What does 'sensible' mean? Something + where neither clipping happens nor where the signal is too + faint. In 'alsamixer -c0' those points are usually near 0dB + or a bit lower. If you are not sure what to pick don't + despair, our little tool will tell you if you picked a good + value. 'Inner' sliders should always be kept near to 0dB. - make && ./dbmeasure plughw:0 log.csv + 3) You run our little tool like this: -This will now measure a few things and then ask you to lower the volume one -step. Please comply and press return. It will now measure a little bit more, -and ask again and so on. When you went through all volume levels successfully -press C-D instead of return. + make && ./dbmeasure plughw:0 log.csv -If the volume levels mentioned in step 2 were not set up correctly the tool will -fail and ask you to adjust them a little. Then repeat step 3. + This will now measure a few things and then ask you to lower + the volume one step. Please comply and press return. It will + now measure a little bit more, and ask again and so on. When + you went through all volume levels successfully press C-D + instead of return. -The first argument to the tool is the device to use. It will be opened for both -capturing and playback. The second argument is the filename of a CSV file where -the tool will place its results. The first column of that file is the numer of -the volume step we are looking at. Starting with the highest one. The last -column is the attenuation in dB relative to the highest volume level (and hence -is always at 0dB on the first line). You can then load the CSV output into -Gnumeric to draw pretty graphs. + If the volume levels mentioned in step 2 were not set up + correctly the tool will fail and ask you to adjust them a + little. Then repeat step 3. -If you want to use the output of this tool in low-level drivers, keep in mind -that there volume level 0 is usally the lowest one, while in our CSV it is the -highest. You might hence want to invert the order of the CSV file before you -make use of it. + The first argument to the tool is the device to use. It will + be opened for both capturing and playback. The second argument + is the filename of a CSV file where the tool will place its + results. The first column of that file is the numer of the + volume step we are looking at. Starting with the highest + one. The last column is the attenuation in dB relative to the + highest volume level (and hence is always at 0dB on the first + line). You can then load the CSV output into Gnumeric to draw + pretty graphs. -Lennart Poettering, 2009 + If you want to use the output of this tool in low-level + drivers, keep in mind that there volume level 0 is usally the + lowest one, while in our CSV it is the highest. You might + hence want to invert the order of the CSV file before you make + use of it. + +WHAT DOES IT ACTUALLY DO? + + It's pretty simple. We basically just generate a sine wave @ + 440Hz which we output. Then we record that again and deduce + the amplitude of what we record. Then we ask you to change the + volume a bit and measure the amplitude again. + + To suppress the noise we play the sine wave for one + second. And then record the next second adding it to the + previously recorded second and so on. Dividing the resulting + summed signal by the number of seconds we did this will give us a + signal where the noise level is lower on each iteration. This + works correctly only if record and playback are perfectly in + sync. As last step we deduce the absolute maximum of our + summed samples and take that as the volume level for the + volume setting. + + Before the whole logic properly starts we do a run similar to + the above but with silence written to the device instead of + the actual sine signal. We then repeat that as often as the + resulting sum signal is stronger than -80 dB. This then gives + us a hint how many seconds/iterations we need to record and + play the summing game to push the noise level below -80 dB. + + This will only measure the volume dependant attenuation at + 440Hz. This is of course a drastic simplification of the + actual transfer function. + +WHY DO THIS? + + To do more advance volume control in PulseAudio we need the dB + values for our sound card volume controls. More specifically I + wanted them for my Thinkpad X60s hardware volume control. + +WHO WROTE THIS? + + Lennart Poettering, 2009 |