On Thu, 1 Sep 2005, Greg Woods wrote:
On Thu, 2005-09-01 at 10:41 -0400, Cory Papenfuss wrote:
The 5th (QAM) is typically used for digital signals
OK, I am now 100% confused. What do you call analog cable? I thought
that was QAM. The reason I thought that was because I can plug my analog
(not digital) cable into an HD3000, and it works just fine. This is NOT
a digital signal I am fairly sure, because it also plugs directly into
the antenna port on a TV and that works too.
What am I missing here?
Analog cable is exactly that... analog broadcast TV signal plumbed
right to your house. You could conceivably amplify it, put it on an
antenna on your roof, and beam it to your neighborhood. Anyone with a
regular TV could receive it. (Actually, the channel mappings between OTA
and cable TV are slightly different, but it doesn't change how it's
modulated). This regular analog TV signal is modulated as it has been for
50 years with a modified single-sideband AM modulation technique called
vestigial sideband or VSB.
*DIGITAL* cable (which comes over the same wire but is generally
on a different "channel" of RF space) generally uses the QAM to modulate
the digital bitstream into an analog signal to get upconverted to RF and
sent out the wire.
I think where you are getting confused is that the HD3000 is three
cards in one:
- 8VSB high-def receiver/demodulator. This will receive OTA HDTV signals
(which are by definition 8VSB in the US). The digital stream contains
MPEG2-data.
- QAM receiver/demodulator. This will receive digital cable bitstream,
which may or may not be encrypted. The digital stream contains MPEG2
data.
- VSB analog SDTV (i.e. NTSC) framegrabber card. This will capture
regular analog broadcast (or broadcast brought via a cable company),
digitally sample it, and output *UNCOMPRESSED* video. It is just like a
cheapo $20 CX88-based capture card. That stream will *NOT* be encoded to
MPEG2... the computer's CPU will have to do that.
-Cory
--
*************************************************************************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
*************************************************************************
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