NO Dolby Digital bitstream AC3 out

Hello at all , I've tested a lot of images but I can't obtain ac3 passthrough, out is only the downmixed stereo PCM (via HDMI and spdf) .I don't see any AC3 downmix settings under A/V setup for bitstream/pcm selection.Please help me to fix it by plugin or release of new image that fix them. In the specifics and manual of hypercube says that SPDF are used for dolby digital out but i can obtain only PCM stereo! How i fix them? Thanks at all.