[mythtv-users] Various questions

kteague at speakeasy.net kteague at speakeasy.net
Mon Sep 26 16:47:32 UTC 2005

> Just wanted to add a few things to earlier posts.
> On 9/22/05, kteague at speakeasy.net <kteague at speakeasy.net> wrote:
> > 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.

Let me see if I'm understanding this correctly.  Firewire-equipped decoder boxes don't push out the pay channels through the firewire port?... only the OTA channels?

I could buy a DirecTV HD receiver modified with a firewire port (http://www.169time.com/).  They're expensive, but they're available.  If what I said above is true for cable providers, I don't think the same holds true (at least, based on the info from 169time.com) for DirecTV recievers modified with a firewire port... I hope!  This may be the route I go, but will I be able to use MythTV with this type of setup?

> 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.

The new DirecTiVo HD10-250 unit I recently purchased has HDMI and component out but, you're right, I've yet to find a capture card with any type of digital input.  Actually, I take that back.  There are a few cards out there (http://www.stream-video.com/drcstream.htm and http://www.canopus.us/us/products/dvrex-rt_professional/pt_dvrex-rt_professional.asp), but I don't think they're supported under Linux. :-(

> 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.

Understood.  This is all begining to make more sense now.

> > 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.

I wanted to do MPEG4 encoding to lessen the amount of disk space used by my recordings.  However, at the same time, I would also like to perform multiple recordings at once on the back end.  As you noted, the recordings are performed on the BE, and this will tax the CPU on the BE if the encoding isn't handled by hardware.  This is why I'd like an MPEG4 hardware encoder, if it's available.

Another gentleman by the name of Brian Steele responded to my post, also.  He noted:
You are correct, they are hardware tuners and they don't perform any hardware encoding.  The reason they don't need to do this is that ATSC video is already encoded and compressed into MPEG2 before it is broadcast.  The video recorded by the HD-3000 and Air2PC cards is just streamed over the bus directly to the hard disk with very little CPU involvement.  I have two HD-3000 cards and can record with both at the same time while using less than 5% of a 3Ghz Pentium4 CPU.  Playback on the other hand, requires quite a bit of CPU time.  I average about 85-90% CPU usage when playing back 1080i HDTV.

That said, why would anyone need a hardware encoder if the stream is passed through the PCI bus and dumped directly to the hard disk as an MPEG2 formatted file?  And where does ATSC come in to play?  I thought the signals we receive in the US are NTSC.  Please forgive my ignorance.

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

<shudder> I'm trying to stay away from USB. :-)

> > 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.

I was looking at some of the fx5200's.  Some have only an analog 15-pin D-sub VGA connection, while others have DVI.  The ones with DVI also have the analog VGA connection, but the back of the port is followed by a ribbon cable that is plugged in to the card.  Do these cards boast a *true* DVI connection?... or are they simply converting the analog output to DVI? 

> > 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.

I'm confused between this and what I quoted Brian on above.  He's saying the MPEG stream is passed through the PCI bus and dumped directly to disk.  The MythTV documentation states that a hardware encoder is necessary if you wish to free CPU cycles of software MPEG2 encoding.  The AirStar HD5000 looks tempting, but I don't think it would work with my setup, as I don't have coax output from my receiver.

> > 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.

The reason I'm looking at this option is to upscale SDTV to HDTV either on-the-fly in realtime or transcoded/deinterlaced later.  I'm curious about both possibilities here.

Thanks so much for the excellent information!

- Ken

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