gain~ to get dBFS/dBu to match
Still new to Max here, but am getting better at it every day and loving it!
I'm currently building a testing and measurement patch to characterize hardware (specifically compressors right now, but more later). But I'm running into a snag with the gain~ fader in that I need the dBFS to correlate with the dBu at the analog output.
Leaving the gain~ fader with default values and doing a 20log($f1/157) on the fader output value should yield dBFS, right? But when I do, say, a -6 dB change I don't get the corresponding dB change in measured voltage. On this setup I'm using an M-Audio Fast Track Pro USB interface which measures about 2.44 Vrms at Max output. Dropping to -6dBFS in the patch I'm getting 0.68 Vrms, yielding a about an 11 dB change.
Of course I can increase the resolution as the number of steps, but for now 158 should be good enough.
I suspect it has to do with the log increment and log scaling values for the gain~ fader set in the inspector. Default values are 1.071519 for the increment and 7.94321 for the scaling. What exactly are these values and where did they come from? I feel like I'm just not seeing the obvious math behind it.
Would someone be so kind as to help me with the reasoning and get the fader so that dBFS correlates with dBu? Thanks so much!
Jeff
Hi,
the [gain~] object, as it says in its help file, is using the same parametrization as the [linedrive] object. You can find the exact formula in the Reference Documentation of [linedrive] in the section 'Messages'. As you can see, it is not a dB formula, but you can easily scale it to normal decibels (in the help file it says that 'a change of 10 in the slider produces a 6 dB change in the output').
Hope that helps,
Ádám
I would recommend rolling your own slider and use [dbtoa] / [atodb] or the signal~ versions. that way you know exactly how to measure your reference signal.
ps. this website was really helpful for me to get my head round dbfs / dbu conversion: http://www.sengpielaudio.com/calculator-db-volt.htm