I have found that format of the digital optical ouput changes if a channel such as BBC HD which uses AC3 sound is selected instead of an SD channel which uses MP2 sound.

Can anyone advise what format is output when AC3 sound channels are received, and comment on whether the HDR should allow you to select the format of the digital output as I understand some DVD players do.

For normal SD the Optical SPDIF outputs PCM. For BBC HD it outputs DD / AC3 as you say. I have not found a way of selecting PCM only from the optical SPDIF output.

The box will successfully convert everything to PCM via the HDMI output as selected by the EDID information it receives - this is the only way I have found to get PCM digital audio out of the box.

The internal menu option re DD on/off has no effect for the optical output - it may change the HDMI format, however, in my system the EDID takes precedence and this is how I control the Audio output format from the Humax (and then strip the PCM audio out of the HDMI stream using a separate box).