I'm trying to understand what kind of differences pulse-code modulation (PCM) and pulse-density modulation (PDM) cause when two audio signals are mixed together.
Mixing PCM signals seems to be a lot more common, and would typically be used for mixing two wav files to produce a new wav file that contains the combination of both sounds. Two wav files a.wav and b.wav are mixed together by taking corresponding samples from the two files and calculating the sum of the two values, essentially stacking them on top of each other. The result is played back at half volume to prevent each mixing step from making the result louder ad infinitum.
Mixing PDM signals seems less common, however it is used for mixing two dsd files together to produce a new dsd file that contains the combination of both sounds. Two dsd files a.dsd and b.dsd with the same bitrate are mixed together by taking one bit from each stream at a time producing a new bit stream containing bits from both streams. The resulting bit stream is played back at twice the speed to prevent each mixing step from making the result slower ad infinitum.
Is there some fundamental difference to the result achieved by these two alternative ways of mixing the audio?