[mythtv-users] Beginner Question

Brad Templeton brad+myth at templetons.com
Sun Dec 19 22:56:29 UTC 2004

On Sun, Dec 19, 2004 at 03:21:18PM -0600, Gerald S wrote:
> Anthony,
> Thank you for the info!  2 more questions for you:
> 1)  You mentioned you use both the PVR-250 and the pcHDTV.  So the
> pchdtv2000/3000 gets HD OTA, but does not replace the PVR-250 for standard
> TV?  To get both you need both cards?  Or are you using 2 so you can have 2
> tuners to have more simultaneous watch/record options?

The pchdtv cards as we use them are actually very simple cards,
computer-wise.  They might be better described as demodulators -- they
just tune an RF digital signal and convert it to a bitstream for your
computer.  No A to D (except at the most basic level) and not altering
the mpeg stream.   This section of the card can ONLY tune digital TV
channels from over the air -- nothing else.

However, there isa basic capture also inside the pcHDTV, which can
either use the tuner (same one that tunes the RF for the ATSC demodulator)
or composite/svideo inputs.   This section of the chip does A to D on an
analog signal.  That stream must be compressed in software.

There are a number of other capture cards which do the same thing.  They
take CPU to capture video.

The PVR-x50 contains a hardware encoder.  It tunes and brings in the
analog signal and using an onboard chip, spits out mpeg2 with minimal
effort on the part of the host computer.

> 2)  <<Component video is an analog colorspace signal format. Just a step up
> from SVideo really.>>  But svideo is still standard NTSC resolution, isn't
> component video HD resolution?  

Well, you would be pushing the cable's ability to carry a signal of such
frequency, but there is no "resolution" to either of these formats.  The
resolution is limited to how many mhz of signal you can put down each
wire without facing problems like loss, reflection and interference with
other wires.   So there is an upper limit.

The "resolution" of an analog TV signal is defined by the dot clock and
the refresh signals. 

Component video is, to all intents and purposes, very similar to VGA
(VGA puts all wires in one cable, and uses RGB instead of luminance-chromas,
but that's otherwise the same.)   VGA cables as you know don't have a
resolution, you use them at everything from 640x480 to higher than HD.
(As you can component video and to some extent DVI)

Analog streams are similar in many ways to DVI digital streams in that
they still use the sync concept etc.   Compressed streams of mpeg are
night and day different.

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