[mythtv-users] Various questions

[Dewey Smolka]

Just wanted to add a few things to earlier posts.


On 9/22/05, kteague at speakeasy.net <kteague at speakeasy.net> wrote:
> Good day Ladies and Gent's,
>
> 1. From what I've gathered, since the DirecTV receivers have tuners in them, I do not need a tuner for my MythTV box.  Is this correct?
>

You don't need a tuner per se, but you need a capture device. The
Hauppage cards have tuners in them, but they also capture over S-vid,
and RCA. I've never played with a DirectTV decoder, but I believe
those are the only two outs you can really use in Myth, besides the
coax.

Generally, you're not going to find a capture device without a tuner in it.

> 2. I would like to record HD stuff, however, it appears that the only way to do this is
>via encoding with CPU overhead (no hardware encoders), as there are
no video capture >cards (supported under Linux) that can perform
hardware HD encoding.  I did some
>reasearch on chipsets used on the Hauppauge PVR-500, and it appears
the maximum
>encoding resolution they support is SD.  Now this is where it becomes
very confusing.
> The chipset used on the card is a Conexant CX23416.  The product brief
>(http://www.conexant.com/servlets/DownloadServlet/102074B.pdf?FileId=996)
states
>that it supports hardware encoding of 720x480, yet, just below that,
it says it also
>supports "HDTV MPEG Capture".  So, does this card also perform
hardware encoding
>of 1280x720, or even 1920x1080?
>

This is where it gets a little tricky to understand and frustrating
when you learn that it's all irrelevant. The only way to get an HD
signal out of a decoder box is if the provider lets it through, and
you have a way to capture it. Theoretically, FCC rules require a cable
operator (don't know if this applies to satellite) to give you a
firewire-equipped decoder if you ask for it, which would allow you to
capture HD streams over firewire without needing a capture card.
However, they are only required to send OTA channels. You still can't
record cable HD programs in HD.

It would be possible, I suppose, if you had a decoder box that sent
DVI/HDMI/component outs and a capture device that supported one of
those, but I've never seen one.

At the moment, at least as far as I'm aware, the only HD you can
capture is OTA, or OTA channels through a digital cable decoder.

The chip on the Hauppage card might support 720p encoding, but you
need to give it a 720p signal to encode. That isn't possible with the
current inputs.

> 3. Are there any hardware encoding cards supported under Linux to encode the
>captured streams to MPEG4?  If so, does MythTV support this?  I know there's a
>Matrox USB device that can do this, but I would prefer something
internal to the PC.
>

Another poster noted one, but I'm not sure this is where you want to
go. Transcoding video takes a ton of system resources. Myth can
transcode recordings later (conveniently skipping commercials) if disc
space is an issue. Otherwise, there's not much point in trying to do
real-time MPEG-4.

Also, I don't know of many (any, really) success stories of people
getting USB devices to work.

> 4. Output -- does MythTV output through the capture card?... or through the video
>card?... or do I have my choice?  I would like to use DVI and keep
everything in the
>digital domain.
>

Either, if you have a PVR-350 with on-board decoder. But you're better
off going through the video card (and, yes, it should be an nvidia).
The DVI on the fx5200 works very well with minimal configuration.


> 5. With the front end/back end arch., where are the encoder cards installed?  In the >front or back end?
>

The BE manages capture cards and the master database for scheduling,
recordings, and serving content to FEs. The FEs play live TV and
recorded content delivered from the BE through Myth, play other media
mounted locally or on the network (MythVideo, MythMusic), and can set
up recording schedules.

If a box has a capture card in it, it has to be set up as a BE. If you
have more than one BE, then one has to be set up as the Master,
meaning that it controls the MySQL database that make the whole thing
run.

The easiest way to start is with everything in one box. Once you get
all the bits working together how you want, then you can start
worrying about adding FEs and slave BEs.

Keep in mind, though, that if you're using hardware-encoding cards,
you don't need a particularly powerful system to do recording. Even
with an HD card, since that basically just dumps an MPEG stream to a
very, very large file. The horsepower is needed to play back content,
particularly HD.


> 6. With the FE/BE arch., can the programs be recorded on the FE, then x-fer'd to the
>BE when the program is done recording?  I fear that a temporary
network failure could
>disrupt a program recording.  Having it record on a small FE box,
then x-fer'd later (or
>in chunks as the program is recording) to the BE, it can take
advantage of resuming a
>failed upload to the BE.  Streaming the data has its limits.
>

The recordings come from the BE where the capture card is and recorded
wherever you've told it to record. For recordings and live TV (which
in Myth is also a recording), the data is streamed from the BE to the
FE through an internal protocol; the FEs do the actual playing. Unless
you have multiple BEs, there's no need to have the recordings
directory mounted as a network share because FEs get to the recordings
through MySQL, not NFS.

The recordings become available to play back on a FE immediately (more
or less) after the recording starts. It will never be real time (even
live TV is delayed by 1-3 seconds), but why build a Myth box if you
want TV in real time?


> 7. With the video scaling capabilities, could I get MythTV to constantly upscale all
>signals to 1080p?  For example, lets say I'm watching SDTV (480i),
will it upscale that
>to 1080p?  Then, lets say I change to a channel that's outputting
HDTV (720p or
>1080i), will it automatically know to upscale those to 1080p?... or
will I have to fiddle
>with MythTV each time I switch channels that output different scan rates?
>

I suppose with a bit of work you could do this, but there's no point.
Upscaling would use every free bit of CPU, result in enormous files,
and give you a high-resolution image of exactly what you saw before.
Your picture won't be any better, but your HDD will be a whole lot
smaller. Just get it running in SD first.

Good luck