ALSA: emu10k1: enable bit-exact playback, part 1: DSP attenuation

Fractional multiplication with the maximal value 2^31-1 causes some tiny distortion. Instead, we want to multiply with the full 2^31. The catch is of course that this cannot be represented in the DSP's signed 32 bit registers. One way to deal with this is to encode 1.0 as a negative number and special-case it. As a matter of fact, the SbLive! code path already contained such code, though the controls never actually exercised it. A more efficient approach is to use negative values, which actually extend to -2^31. Accordingly, for all the volume adjustments we now use the MAC1 instruction which negates the X operand. The range of the controls in highres mode is extended downwards, so -1 is the new zero/mute. At maximal excursion, real zero is not mute any more, but I don't think anyone will notice this behavior change. ;-) That also required making the min/max/values in the control structs signed. This technically changes the user space interface, but it seems implausible that someone would notice - the numbers were actually treated as if they were signed anyway (and in the actual mixer iface they _are_). And without this change, the min value didn't even make sense in the first place (and no-one noticed, because it was always 0).

OBOswald Buddenhagen committed 3 years ago