First, the signals are stereo as Dolby 2.0, so the conversion needed is to PCM 2.0.
Second, it is questionable whether these signals will benefit much (if at all) from the use of a good external DAC.
Third, if your cable box has HDMI output, you can get one of these: http://www.ebay.com/itm/HDMI-Dolby-DTS-PCM-5-1-7-1-To-Optical-Digital-Audio-Converter-Decoder-/230840436904
I wouldn't bother.
Second, it is questionable whether these signals will benefit much (if at all) from the use of a good external DAC.
Third, if your cable box has HDMI output, you can get one of these: http://www.ebay.com/itm/HDMI-Dolby-DTS-PCM-5-1-7-1-To-Optical-Digital-Audio-Converter-Decoder-/230840436904
I wouldn't bother.