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To change the speed all you have to do is this:
TWBR = 12;
Just do Wire.begin once (in setup), not like this:
void Set_LED_NUM_PWM(byte LED, byte PWM)
Wire.begin(); // I2C-Start
I switched the i2c bus to 400kHz frquency ...
Doesn't seem to be much point if you have to throw in a 10 mS delay. But I suppose that's what the question is about.
In any case, isn't that over-egging the pudding? For a VU-meter, your eyes don't need the extra speed do they?
So it seems that the i2c bus transfer affects the a/d-converter although the Atmega doc says that i2c transfer should not affect the a/d-values!?
Well, it's not is it? You are really saying that if you send one number it works, and if you send a different number it doesn't? This sounds electrical to me. Like, the higher brightness is consuming too much power, and reducing the operation of something else.