[mythtv-users] HD3000 and High-Def

Cory Papenfuss papenfuss at juneau.me.vt.edu
Thu Sep 1 14:41:53 UTC 2005

> QAM is how the "signal" is delivered over cable and the HD content is
> either scrambled or unscrambled.

 	Think of QAM as AM vs. FM... basically the same thing.  It's how 
the digital stream is modulated on the analog signal.  It shapes how much 
RF spectrum is used by the signal (among other things). 
AM/FM/VSB/8VSB/QAM are all forms of modulation... the first two typically 
used for music, the third for regular TV broadcast, 4th uses the same 
bandwidth as regular TV broadcasts, but puts digital data on it (e.g. a HD 
stream).  The 5th (QAM) is typically used for digital signals, and it's 
what the cable companies have decided on for transmitting digital data 
over the cable wires.  The main reason is that it's easier to put more 
data (read: more channels/channel) on QAM.

 	The encryption just has to do with whether or not the digital data 
modulated has been encrypted.  That's all.  The music analogy continues if 
the music on FM has been garbled.  You can still *receive* the music, it 
just does you no good because the content is scrambled.... that's what the 
cable box is for.  It can both demodulate and get the garbled signal, but 
it also knows how to ungarble it.

OTA is a more a reference for HD content
> over the airwaves.

 	Actually, it just means "over the air,"... high-def or not.  In 
the U.S, OTA-HD is synonymous with 8VSB modulation, with unencrypted 
content.  The broadcast flag was supposed to be an "honor-system" 
enforcement of a "please don't copy me" bit in the bitstream... although 
it was to be backed up with legislation that would require compliance and 
mandate robust systems.

> If the content is unscrambled you will be able to watch HD content over
> cable using an HD2000 or HD3000. If the content is scrambled you'll need a
> cable box to watch the HD content but that content is then distributed via
> firewire to you Myth box.


> There is a difference between a PVR250 or PVR350 in that they only do SD
> the HD2000 or HD3000 does HD.

 	The HD2000/HD3000/Air2PC are modems without the "mo".  They 
demodulate an already MPEG-encoded bitstream off the analog RF signal and 
dump it to your HD.  The PVR-xxx demodulates and samples an *analog* video 
stream.  Then it encodes it to MPEG2 in pseudo real-time and dumps *that* 
to your HD.  Completely different processes.

> Am I understanding correctly?
>  p.s. I've never heard of a "Cablecard" who makes that?
 	Cablecard is the proposed standard to allow "generic" devices to 
decrypt encrypted digital content.  It is personal to you, but owned by 
your cable provider, and allows you to decrypt what you've paid for.



* Cory Papenfuss                                                        *
* Electrical Engineering candidate Ph.D. graduate student               *
* Virginia Polytechnic Institute and State University                   *

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