Update doc/DX7_output_level.md

doc/DX7_output_level.md

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+From dronus. Maybe this helps to get a better SNR?
+
 But polyphony is, using 16bit DAC. As 16 notes may playing simulttaneously, the peak sample value is actually 16 times that of a single oscillator. Hence the Dexed code scales the output down in some fashion, there is a explicit division by 16 at dexed.cpp:148 to cope for the 16x polyphony, and maybe some further attenuation depending on the algorithm to cope with the parallel carrier outputs.
 
 That means playing only one note, only 12 of 16bit amplitude would be used, this can be clearly seen if audio is recorded via USB and all volume and carrier output levels are at 100.