Require some technical clarification over MPEG encoding / decoding process.

[GarethP]

Hi all,

I am looking to set up a system at home but am having some difficulty in trying to guage and plan the hardware layout / requirements. This is mainly due to the fact that people everywhere seem to confuse the term encode and decode which to me are two very different things (in fact entirely opposite things!) and also because there does not seems to be any one place that clearly details this (I have tried searching google / forums / u name it).

Ok to summarise, I am looking to set up a server that receives 3 streams from a DVB-S feed. This server will have no monitor and be located in a cupboard so it has NO visual display requirements.

I then want about 3 clients connecting via the network to the server.

Ok, at this point you are probably thinking...yeah so? just do it!

My concern is about the decoding requirements for 3 streams and my question is really can someone please break down exactly what happens where in the process.

e.g. My view is...

Set up the server with 3 non-premium DVB-S cards. So no hardware encoder / decoder.

Streams arrive on satelite feeds ALREADY IN MPEG-2 format! So I dont really understand why people talk about having to encode these streams in to MPEG. (I am of course assuming that the software does not decode and recode them)

The server writes these streams in their raw MPEG-2 format to disk.

The clients connect and pulls the stream still as encoded MPEG and then uses a decoder (sw/hw/accelerated) to display them.

i.e. The server processing requirements should be minimal (other than some big fast disks and gigabit ethernet) but each client will need to be able to decode MPEG (displaying out graphics card on DVI connected to tellies / projector)

And nowhere should I need an MPEG encoder!

Is this right?

If the encoding/decoding is done on the server it means I would probably need a 3GHz+ proc and some serious hardware which I would like to avoid.

So, can someone please either correct my process flow above or explain step by step in a client / server architecture what bits of processing actually happen at which stage.


Also, final Q...Does the SageTV support the use of CI / CAM's (with a valid subscription!!!!!). This is a technical question, not a legal one. I have no intention of breaking any laws.

In this situation where does the decrypting take place then? I assume on the server also?

Any / all help appreciated! Many thanks... Gareth

[rickgillyon]

DVB-S does indeed come down as MPEG - however, it can be very tricky to do anything with this encrypted stream. Generally people are forced (required by the Sat Co) to take that input into a STB, which provides an Analogue output which needs encoding to MPEG then decoding for output. Crazy digital to analogue to digital, with the expected quality loss. The setup you speak of does require no encode, but in real life if you are to conform to the requirements of your Sat Co, is generally not possible.

DVB-S with CAM/CI interface is in testing and further development right now. I assume in that case it's the CAM/CI that does the decrypting, but I'm no expert...

[Mark SS]

Always worth adding your location to your profile so people have an idea what you're going to be receiving.

Originally Posted by GarethP

Ok to summarise, I am looking to set up a server that receives 3 streams from a DVB-S feed. This server will have no monitor and be located in a cupboard so it has NO visual display requirements.

Just to be sure about this... you have three feeds from LNB(s) and will have three DVB-S cards in the SageTV server each connected to one feed?

Streams arrive on satelite feeds ALREADY IN MPEG-2 format! So I dont really understand why people talk about having to encode these streams in to MPEG. (I am of course assuming that the software does not decode and recode them)

Not many SageTV users that are currently using DVB-S cards as the support is being added to the beta now - its almost complete, just channel scanning to finish. Lots of people are using S-Video out from STBs into video capture cards (e.g. PVR150), which is where the confusion may have arisen from.

The server writes these streams in their raw MPEG-2 format to disk.

Correct.

The clients connect and pulls the stream still as encoded MPEG and then uses a decoder (sw/hw/accelerated) to display them.

Generally yes. With the client you'll be pulling the same stream, if using the Placeshifter client there is the option to transcode so you can stream at lower bandwidths.

i.e. The server processing requirements should be minimal (other than some big fast disks and gigabit ethernet) but each client will need to be able to decode MPEG (displaying out graphics card on DVI connected to tellies / projector)

If you're just recording and streaming to clients then yes, pretty minimal. The server can do much more however such as the Placeshifter transcoding, converting Mpeg2 recordings to xvid etc so you may want a bit more oomph.

One option is to use a good quality HTPC case that doesn't look out of place in the living room and use that PC as the server and client for your primary display. If your looking to support HD content (now or in the future) you need a good video card, cpu and memory to get the best from a client, so it may pay to combine the two. If your client displays are all SD however, you might be better off looking at MediaMVPs instead of PCs and hiding the server away as you intially suggested.

Also, final Q...Does the SageTV support the use of CI / CAM's (with a valid subscription!!!!!). This is a technical question, not a legal one. I have no intention of breaking any laws. In this situation where does the decrypting take place then? I assume on the server also?

CI/CAM is being added at the same time as DVB-S which is currently in beta and under development. Decryption is handled by the DVB-S card and CAM so there is no impact on the server.

[GarethP]

Many thanks, this has cleared up much for me in understanding whats happening where.

Yes I am referring to 3 feeds from the LNB to 3 separate cards. I shall watch for the beta progressing and see how it pans out

[Necro]

Just to clarify this, the issue that I think I may be having (aside from the normal ATi craptastic driver issues) with my HD card is processor related where I didn't think it would be. So, recording the stream takes up a certain (small) portion of processing power. However, actually DISPLAYING the stream takes up a ton of resources.

This is what causes stutter/slow down. Theoretically, recording a show then watching it later while nothing is recording should work fine...

Right?

[heffe2001]

All depends on the specs of your PC, the video card you're using, memory, HD speed, etc. A post with your server/client specs would probably help out a bit to see why you're getting stuttering in HD playback.

I'm using a 2.53 P4, x1600 ATI, with 768m for one of my clients, and it plays HD almost flawlessly. I'll get an occasional stutter on playback, but I'm not 100% certain that it's not network-congestion related...

I plan on switching over my entire Sage network to a gb switch as soon as I can get time, and hopefully that will fix any remaining problems I have...

[Necro]

3400+ Athlon, 1gb RAM, GeForce 6200 (PCI version), and ATi HDTV Wonder.

I'm wondering if the MPEG decoding can be offloaded to the graphics card, and if so how can I make sure thats being done?

[Rogue9]

Originally Posted by Necro

3400+ Athlon, 1gb RAM, GeForce 6200 (PCI version), and ATi HDTV Wonder.

I'm wondering if the MPEG decoding can be offloaded to the graphics card, and if so how can I make sure thats being done?

What decoder are you using now?

Nvidia PureVideo Decoders are pretty good, and they allow Hardware acceleration.

Check your drivers/decoders for an option to enable DXVA.

[Necro]

I'm going to start a thread dedicated to this, but I spent the last 2 horus narrowing down what the issue is. I'm almost 100% sure it's MPEG2 playback in SageTV - I tried a DVD and that froze similarly to what is occuring with my HDTV card tv watching. (This is a DVD that plays fine in WMP, and tv that views fine using WatchHDTV and CPU usage on Sage prior to freezing never goes above 50%).