I've got a comcast HDMI box that downmixes to PCM, controlled by the menu settings (its a Sci. Atlanta 8300HD?). I've also got one that doesn't downmix (8250HD?). Have you called Comcast to see if they have a box with both HDMI and PCM downmix in your market?
If not, you may be out of luck as far as cheap devices go. When I was in that boat, I was using a Theta Casablanca pre/pro to downmix, but that was cost-effective only because it was already sitting around. I did look at the time, and didn't find any "little black box" to do the trick. In part, its probably because: (i) the market for people downconverting from 5.1 to PCM is probably limited to a handful of A'gon contributors; and (ii) if you are going to build a downmixer, you might as well build a processor and make more money off it.
If you do find something, post where you found it... I'm settled for this gen. of boxes, but have a sneaking suspicion that I'll deal with this again in the future...
If not, you may be out of luck as far as cheap devices go. When I was in that boat, I was using a Theta Casablanca pre/pro to downmix, but that was cost-effective only because it was already sitting around. I did look at the time, and didn't find any "little black box" to do the trick. In part, its probably because: (i) the market for people downconverting from 5.1 to PCM is probably limited to a handful of A'gon contributors; and (ii) if you are going to build a downmixer, you might as well build a processor and make more money off it.
If you do find something, post where you found it... I'm settled for this gen. of boxes, but have a sneaking suspicion that I'll deal with this again in the future...