[mythtv-users] Good system design documentation?

[Robert Tsai]

On Fri, Jun 03, 2005 at 03:44:29PM -0500, Steve Tate wrote:
> Is there a document available that goes through the various
> components of a mythtv system and talks about various system design
> tradeoffs?
> 
> I've been frustrated looking at the official documentation, which is
> in FAQ format, and seems to assume people understand the basic
> mythtv architecture going in.  The docs talk about frontends and
> backends as if everyone naturally knows that they are, but never
> actually explains this.
> 
> What I've gleaned is what defines a frontend is the ability to play
> things and what defines a backend is the ability to record.

Sounds good to me.

> But it sure would be nice if there were a document the clearly layed
> out the tasks performed by various systems so I can decide how to
> design my system.

The developers are probably the most qualified to write your desired
documentation, but are also by definition probably the most qualified
to fix bugs and add features.

Which would you rather have?

Anyway, if you haven't already seen it, try this page:

	http://www.mythtv.info/moin.cgi/HardWare

[Funny, this site comes up #3 on a google for "mythtv".]

> For instance, if I have a EPIA-based box hooked up to the TV with a
> tuner card in it, it can both play and record -- that makes it a
> frontend and a backend, right?

Yes.

> Can it be diskless?

Sure.

> In other words, could I have a "tunerless" backend, consisting of my
> main home network server, and a "diskless" frontend/backend at the
> TV? 

Sure.

But you're overloading the term "backend". A Myth "backend" is
basically a recorder. It has nothing to do with where the recorded
streams live (although it needs somewhere to put them and retrieve
them).

If you want, you can have the myth backend save the recordings over
NFS or something to some other file server.

> I could put the PVR-250 in the file server, of course, but does that
> mean that I'm consuming network bandwidth with a video stream just
> to watch live TV (which is why I'd want the tuner in the frontend
> box in the first place)?

Yes, but your diskless frontend/backend system will still consume
network bandwidth. Myth "live" TV actually records to disk to a
circular buffer. What you are watching is actually from that on-disk
buffer, a few seconds behind "live" TV (or more, if you've paused live
TV).

> And then I'd have problems using the remote control from the PVR-250
> as well...

The PVR-250 and other tuner cards with built-in remotes or whatever it
is (I don't know because I don't have one) assume a combined
frontend/backend system where the capturing system is close to the TV.
If you want to split the frontend/backend, you violate the design
assumption of the PVR-250, so you give up the ability to use the
remote to control the capture card.

I think the "standard" practice is to configure a remote to control
your frontend, which then tells the backend to do things over the
network. The remote never directly controls your capture card. Only
MythTV can do that.

> I'd really like a RAM-based ringbuffer for live TV, so that doesn't
> hit the network at all, and then the ability to stream pre-recorded
> video from the file server to the frontend.

You can configure a RAM disk on your frontend (or whatever the real
Linuxy way is to describe what it is I'm talking about).

> Is that something that can be done?

Yes, given sufficient programming skills, motivation, and time.

> Anyway, maybe some of those questions are confused, but it's mainly
> because I have a very poor mental image of the mythtv architecture,
> and can't find any documentation that explains this.  Does anyone
> know of something decent that's out there for me to read?

Keep thinking about it, keep googling ("mythtv" is a pretty good
search), and keep lurking.

--Rob
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