The SHAC500 is the model number of a Technics DD/DTS audio processor. It will allow correct DD playback from the Panasonic TU-DST50/51 & PV-HD1000.
The general belief is that because Technics & Panasonic are both owned by the same company, that the equipment was tested together before release and proved compatible. All of it has been discontinued.
This problem is a compatibility issue between some equipment. It is not universal, as some DD decoders (for example Marantz) work fine with the Panasonic equipment.
It seemed like a cheap solution, at less than $250, if you wanted to use the Panasonic equipment to record HDTV and were unlucky enough to have a DD decoder that was not compatible.
If I remember correctly, this only happened during DVHS playback.
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