[mythtv-users] HDTV, Myth, HD-2000, HD-3000, questions galore (and hopefully answers from the smart people... )

[Brad Templeton]

On Tue, Jan 18, 2005 at 09:16:12AM -0800, Julian wrote:
> Hi,
> 
> Can someone explain then what the hopes for recording
> encrypted HD "digital cable" content with the HD-3000
> or a similar device?  I know there are smart cards and
> the like, which TiVo intends to support, but what hope
> is there that MythTv will be able to support such
> schemes?  For me, and prob. others, this is the root
> of the newbie's concern when brewing and investing
> into a PVR.

To answer this, and also to expand on the source question about analog
vs. digital cable.

Analog cable consists of signals modulated into channels, with a series
of carrier frequencies each so many mhz apart, just like over the air.

Most cable companies implement digital cable by taking some of these
frequencies, and instead of putting in an analog signal, they put in
a QAM digital signal (still modulated on a carrier in the same band.)

The digital cable box is able to decode the QAM signal.  Due to compression,
each old analog channel can hold several streams of MPEG for
SDTV.  So one old analog channel now carries I think up to 16 different
SD channels.  Quite efficient.   The analog channels are still there,
and this allows the same wire into the house to provide analog and digital
cable.

A new TV with a QAM tuner is able to do what the digital cable box does,
decode those QAM channels, demux them and view them.   Unlike the digital
cable box, it can't decrypt the ones that are encrypted.   However, the
cable card interface allows you to insert a card into the TV that can
do the decryption.

It turns out, at least where I live, but I think this is common, that if
you have analog cable all it means is that you don't have a digital cable
box.   I put a QAM tuner on my analog cable and was able to pick up all
the QAM signals, and watch the unencrypted ones (most, but not all local
OTA stations.)

As for the hopes of recording the encrypted content?   To do that they
would need to trust your system with the decryption keys, which they
don't want to do.   In theory one could build a box with a cablecard
slot to decrypt but they are unlikely to licence you the tech to do that.

In theory, one could imagine a PVR that records the encrypted stream and
plays it back later unmodified.  However, what I have heard is there
are timestamps in the stream so that the box can tell the difference
between live and delayed, and refuse to decrypt delayed.  Perhaps if
you fool the clock?