Dither Plots
Reply #9 – 2009-05-01 16:23:22
rounding is only done correctly when this box is checked. Can you tell some more about correct and incorrect rounding ? On the Pro Sound Web forum someone listed a few audio applications that appear to apply the same "wrong?" rounding when truncating (without dither) to 16 bit: Logic 7, Logic 8, Pro Tools 7(native), Peak ProXT 6. I suppose (hope) that the rounding is disabled (or ineffective) when dithering is on. Well, to me, the only correct rounding is the one which rounds to the closest positive or negative integer, i.e. -0.5 becomes -1, 0.5 becomes 1, -0.49 and 0.49 become 0, and so on. The problem with David's plot from CEP 1.2 is that although the amplitude of the tone reaches less than 0.5 LSB (from a 16-bit perspective) around 9 seconds, is is not rounded to zero as in your izotope plot. Instead, either the positive or negative values are set to 1 or -1 LSB, respectively. This is what I get when not checking the above setting in Audition. The low-level tone is still resolved after conversion to 16 bit, but it doesn't decrease to zero amplitude . Apparently, this is due to an offset of -0.5 LSB being added before the rounding operation. If you're using dither, there should be no audible difference because a random value is added before rounding/truncating, so even for silence, the dithered output would toggle between 0 and +1 or -1. Still, there will be an additional offset of +0.5 or -0.5 LSB in the output signal, which is why I always use the "symmetric dither" option. Chris