What scale is applied when reading audio files

[santigijon]

Hey,

I would like to know what sort of normalization/scale is applied to the sound files that are read using the SoundFile library, since they all are supossed to be within the (-1,1) amplitude range.

And I would also like to know why some audio samples are allowed to exceed this +1 limit?

[bastibe]

There are certain floating point file formats that may contain values greater than (-1,1). We are not applying any scaling to them but just return what is in the file.

Only integer formats such as PCM16 are scaled to (-1,1), with a possible rounding error of 2^-15 (in case of PCM16). But consequently, these formats cannot exceed (-1,1).

Can you upload a sample file, so we can have a look at it?

[santigijon]

Sure, I will attach two audio files.
sample audio files.zip

And regarding the first question, how is this (-1,1) range established? Does it depend on the voltage range of the ADC of the soundcard of my computer or something else?
Sorry if I say something that may sound strange, I am yet a newbie in this ADC audio conversion area.

[bastibe]

There is no ADC conversion happening in SoundFile. SoundFile merely reads audio files that are already stored on disk.

(ADC happens in your sound card, when you record audio. Your sound card has a reference gain that converts voltages to numbers. However, the ADC has a limited dynamic range (the range between most quiet and most loud sound that can be converted without error). This is usually set up such that the loudest possible sound maps to (-1,1), and everything above that will clip, i.e. will not be recorded correctly and represented as -1 or 1.)

[santigijon]

Great answers, thanks.

So then those values exceeding the (-1,1) range in the audio samples that I attached are due to some floating point file formats that do not estrictly map the maximum voltage to (-1,1)?

And a last question regarding what you explained in the parenthesis. It is then impossible (unless more information is provided) to compare the loudness of two audio files recorded from different devices, right? Since every different microphone will have a different voltage gain range and therefore a different scale.

[bastibe]

So then those values exceeding the (-1,1) range in the audio samples that I attached are due to some floating point file formats that do not estrictly map the maximum voltage to (-1,1)?

Exactly.

It is then impossible (unless more information is provided) to compare the loudness of two audio files recorded from different devices, right? Since every different microphone will have a different voltage gain range and therefore a different scale.

Yes, it is impossible to compare the voltages and loudnesses of the original recording. However, you can of course compare the loudness of the file if it were played. This is still subject to your current settings on your sound card, but as long as you don't change your sound card gain, the loudness of two files is comparable.

[santigijon]

Thanks a lot bastibe, you have been extremely helpful, thanks.

One very last question regarding the last thing you said about playing the sounds and comparing them.

If I play two sound files with my computer and one (L) sounds louder than the other one (NL). This does not mean that when the sound "L" was recorded, it was in fact louder than "NL". Because the device ("DNL") that recorded the sound "NL" might have had a greater voltage range than "DL", and therefore a digital amplitude value of 0.5 of the sound "NL" would mean higher voltage and thus higher pressure than a digital 0.5 for the sound "L" recorded with the "DL".
Right ?

L - loud (sound)
NL - not loud (sound)
DL - device loud (device with which the loud sound was recorded)
DNL - device not loud (device with which the not loud sound was recorded)

[bastibe]

Exactly. Comparisons like this are only possible if you calibrate your microphones (which is not particularly hard, but easy to get wrong).

[santigijon]

Sorry I forgot to reply to the last comment @bastibe , thanks again for the help, much appreciated.