Originally Posted by grahamlthompson:
“Because the HD channels use a separate audio track for AD because the primary AC3 audio does not support Audio Description.
The menu option turns the AD on and off for lpcm sound used mainly by the SD channels (ITV1-HD Granada is also lpcm)”
There are no LPCM channels broadcast, full stop.
You're describing the two channel MPEG1 Layer 2 (MP2) soundtrack, as decoded and output as digital audio on one of the output connectors. Try not to confuse people with the wrong terminology.
As the Humax HDR with the .15 firmware can now be set to do AC3 mixdown, all audio channel formats can be output on S/PDIF or HDMI in LPCM format instead of AC3.
There's also no fundamental reason why the audio description soundtrack could not also be broadcast in AC3 format. In the case of BBC HD channels, a 192kbps 2.0 AC3 stream would save a little bandwidth compared to the 256kbps MP2 stream the BBC does use for AD.
In reference to the original post, the HDR often remembers which soundtrack you selected last time you watched a channel, independent of the menu settings. Use the speaker+questionmark button to change the soundtrack used.