Digital cable is basically worthles as far as dolby digital goes.
I post this every time the issue comes up:
1) YOU DO NOT GET DOLBY 5.1 FROM YOUR DIGITAL CABLE BOX.
except POSSIBLY for cable HDTV, the audio you get is dolby digital 2.0.
2) YOU DO NOT GET DOLBY DIGITAL 2.0 FOR EVERY CHANNEL
You only get dolby digital for "digital only" channels.
I will expound.
First statement:
Dolby digital comes in many flavors, from 1.0 (mono), 2.0 (stereo), up to 5.1 (DVD). Read the FAQs on the dolby website for details.
Cable TV, and most satellite TV, is broadcast in dolby digital 2.0. Your dolby digital receiver will decode this and process it through a pro-logic receiver to get 4 channels. You can't tell the difference between this sound and the sound you get when you take the regular A/V stereo output and run it through a pro-logic receiver. The D/A converter in the cable box is just as good as that in your receiver.
Some satellite channels (STARZ!, for instance) and some satellite PPV is broadcast in DD 5.1. As far as I know, NO digital cable carries DD 5.1. Special channels like HDTV SHOWTIME and HDTV HBO might carry DD 5.1 -- I can't test this. Back when I first got digital cable (three years ago!) I was VERY disappointed to find this out -- but there is a reason they say "CD quality sound" in the advertisements and not "DVD quality sound". On the dolby site, there was a FAQ which suggested that the cable industry needed major infrastructure upgrades to go beyond DD 2.0.
Statement 2:
Another BIG disappointment for me was to find that digital cable wasn't really digital. Generally, cable companies use the same cable to carry both analog and digital services. To save bandwidth, if they send a channel in analog, they do not resend it in digital (except for premium channels). The digital box acts as an analog cable tuner for channels that are provided in regular service. So ESPN, TNT, USA, AMC, MTV, ABS, NBC, CBS, etc etc etc, were not digital, and did not have a digital audio signal.
If the cable you use ONLY carries digital broadcasts you'll get all digital channels.
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I guess what I am trying to point out is that for non-integrated receiver/PVR's, like TiVo, ReplayTV 1/2/3/4000 series, there isn't a big bonus to recording dolby digital. There isn't a difference between the digital audio and analog audio that you can detect by ear. Theses PVR's are inherently analog -- they record analog sound and video and store it digitally.
You start making gains when you store the digital A/V feed directly, and for that you still need DirecTiVo/UltimateTV/Dishplayer. Unfortunately (and the "announced" Motorola 52xx digital cable box/PVR aside) ReplayTV doesn't store the digital video feed directly -- the tradeoff it it is much more flexible than any of the integrated recorders.
Satellite television is starting to broadcast more 5.1 broadcasts, but cabe TV is not. Until it gets to the point where a significant number of broadcasts are 5.1, don't worry about dolby digital inputs on ReplayTVs.
Joe