DVB-T Picture Quality

[moamoa]

What is the most important part to getting the best quality picture on screen?

I assume that the DVB-T tuner does little more than tune into the signal and decode the signal. Therefore, from a quality of picture aspect, there should be little difference between cards?

Assuming for example as in my case I am watching this on a 1360x768 display. There must be some mapping between the digital tv signal output from the card, and rendering this image on the screen? This would seem to be the most important part of getting a good picture.

Where does this functionality take place? Is it 1) still on the DVB card, 2) at application level (sage or codec), 3) on the graphics display card? where?

What things can be done on a HTPC, from a none change hardware point, to make the picture quality of a DVB-T as good as possible?

[StephaneM]

Originally Posted by moamoa

Where does this functionality take place? Is it 1) still on the DVB card, 2) at application level (sage or codec), 3) on the graphics display card? where?

The DVB-T card does nothing more than tuning to a transponder and saving the video stream to disk. So the only thing that may influence the video quality will be the tuner sensitivity : if it is bad, some packets will be missing and you'll miss some video informations. But this is generally not regarded as picture quality, either you are locked to the signal and all is right, either you are not so locked and image will stutter badly.

Then the video stream must be decoded into video that will be submitted to the video renderer of the graphic card. This is done by the application and the video decoder. The video decoder that will give you the best results are those that will use you graphic card hardware to help in the MPEG2 (or H.264) decoding. Many users are choosing either Nvidia DVD Decoder or Cyberlink DVD decoder (if you have a Nvidia card choose Nvidia DVD Decoder and if you have a Radeon card choose Cyberlink PowerDVD)

Then the video renderer will render the video to your display device and the video card you use will make the difference. A good DirectX9 graphic card with decent video memory is needed for best picture quality.

What things can be done on a HTPC, from a none change hardware point, to make the picture quality of a DVB-T as good as possible?

Basically :

* using a hardware accelerated video decoder like Cyberlink or Nvidia (of course your video graphic card must have hardware acceleration : all recent cards do).

* using VMR9 as the video renderer (and probably using Full Screen Exclusive mode : it seems that it is neccessary on Nvidia hardware to avoid tearing, but not so on Radeon Hardware)

* cablibrating your display for VMR9 (mainly adjusting brightness and contrast)

(and some other tweaks that applies to quirks on different hardware : like bad dynamic noise reduction on X1x00 Radeon for PAL content etc..)

Regards,
Stéphane.

[moamoa]

My plasma tv has an inbuilt freeview card. When using this the picture is excellent. If I plug an external Humax PVR freeview box in via SCART the picture is also excellent.

Using the PC with DVB-T is very disappointing indeed. The overall picture quality is not as good and seem more "pixelly". Also, it does not seem to be able to adapt to suit the aspect ration the way a dedicated box does, leaving black borders all over the place as the source channel varies.

Having just purchased a new PC just for the job, I'm gutted!

Does using a HDMI cable to connect to the Plasma have any effect, as opposed to using the standard monitor cable? Does that feed the digital signal in to the plasma in a more raw format so it can process it? My new ABIT motherboard has HDMI out but I have not got a able yet, as I did not see the point.

It never even crossed my mind that my new HTPC would not be able to perform as good as one of these of the shelf boxes.

Any tips on helping in bring this kit up to scratch would be much appreciated.

[StephaneM]

Originally Posted by moamoa

My plasma tv has an inbuilt freeview card. When using this the picture is excellent. If I plug an external Humax PVR freeview box in via SCART the picture is also excellent.

The video quality on your PC should be the same if not better. At least it is for me.

Using the PC with DVB-T is very disappointing indeed. The overall picture quality is not as good and seem more "pixelly".

Do you mean that you can see "blocks" in the picture? It seems that your problem is the video decoder you are using.

What is your video card and what is the MPEG2 decoder you use in SageTV?

Also, it does not seem to be able to adapt to suit the aspect ration the way a dedicated box does, leaving black borders all over the place as the source channel varies.

This is because normal TV does overscan : with a PC you see more of the image. So in SageTV you generall just have to overscan a little the display of the video (for instance I use 105% fill instead of 100%)

Having just purchased a new PC just for the job, I'm gutted!

I can assure you that I have a near DVD quality for DVB-T broadcast in 16:9 so you'll be able to do the same.

Does using a HDMI cable to connect to the Plasma have any effect, as opposed to using the standard monitor cable? Does that feed the digital signal in to the plasma in a more raw format so it can process it?

You'll have to try for yourself, it vary with the video card and the display. Some users says that a VGA (so it is analog) is better on their display than a DVI / HDMI (so it should be digital). Usually modern displays will process video signals coming from analog sources (for instance for deinterlacing the video or resizing it to the display resolution). If you use a digital connection there is a great chance that you'll feed the display exactly what it needs so the display will not make any transformation to the signal and this is what you want.

What I would suggest is first try to tweak the quality you can have by playing a DVD on your PC (and a good quality one like Star Wars Episode III). Play this DVD using SageTV and the first thing you'll have to do is choose within SageTV the video decoder to use.

If you think the quality is not so good, then you can try to improve it by changing the way the PC is connected to your display :

* either change the resolution / refresh rate to match the native resolution / refresh rate of your display

* either change the connection physically : try DVI/HDMI first, then VGA, then Components, then S-Video, then Composite (this is listed from best to worse connection order)

Regards,
Stéphane.

[moamoa]

Stéphane, thanks for your help... so far

I am using an ABIT mobile motherboard, with on board Intel 945GM express graphics, running at 1360x768. I'm sure the graphics are up to the job as I have watched some HD video files downloaded from the Microsoft site, and they were totally awesome! Both 720p and 1080i, amazingly good in fact.

Anyway, I have not installed any specific codecs at all. Just the standare ones that came either from installing; xp home, sagetv or the BDA drivers for the DBV-T card.

I was kind of thinking that the key thing here is the bit that maps the the 720x576 image to the 1360x768 screen. If this is they key part, what should I get/install to improve this?

This is a totally new setup. I only have XP + SageTV + BDA drivers.

Is it the same codec that does DVD rendering as renderes the DVB stream?

[StephaneM]

Originally Posted by moamoa

with on board Intel 945GM express graphics, running at 1360x768. I'm sure the graphics are up to the job as I have watched some HD video files downloaded from the Microsoft site, and they were totally awesome! Both 720p and 1080i, amazingly good in fact.

I have no experience of this chipset, though I'm sure that a Radeon will do a better job for upscaling the video (at least that's what they advertise).

I was kind of thinking that the key thing here is the bit that maps the the 720x576 image to the 1360x768 screen.

It could be but as you have not installed any DVD decoder and are actually using the SageTV MPEG2 decoder, I guess this is the key part here..

Is it the same codec that does DVD rendering as renderes the DVB stream?

Yes it is the same codec that does DVD rendering and DVB-T rendering (at least when DVB-T is broadcasted in MPEG2).

So what I recommend you is downloading the PowerDVD 7 trial and see if the quality is improving:

* First record some DVB-T broadcast and play it with PowerDVD, make sure you select the "Hardware acceleration" in PowerDVD properties.

* Then play the same video with SageTV (make sure you selected Cyberlink PowerDVD7 as your MPEG2 decoder in SageTV) and see if it improves things.

* Also try to use either the Overlay or VMR9 rendered in SageTV (renderer and MPEG2 decoder are options you'll find in the SageTV Setup Menu / Detailed Settings)

In my opinion PowerDVD is really nice for both picture quality and for audio playback versatility (for instance you'll be able to use the Dolby Prologic II Cyberlink decoder, so it is nice for expanding stereo to 5.1 if of course your PC is connected to a 5.1 speaker set using analog connections).

Regards,
Stéphane.

[Mark SS]

Originally Posted by moamoa

I was kind of thinking that the key thing here is the bit that maps the the 720x576 image to the 1360x768 screen. If this is they key part, what should I get/install to improve this?

You're forgetting that exactly the same thing is happening when you use the TVs internal freeview decoder. At the moment however, the TV is doing a better job of scaling the image than your PC is.

Try the PowerDVD codec as Stephane suggested but it would also be worthwhile comparing the Nvidia Purevideo codec as well. Purevideo is considered to be the best choice but compare them and see which you prefer.

[StephaneM]

Originally Posted by Mark SS

You're forgetting that exactly the same thing is happening when you use the TVs internal freeview decoder. At the moment however, the TV is doing a better job of scaling the image than your PC is.

I would say that it really depends on the TV and the PC video card.

A high end video card will do a better scaling job than any TV internal scaler (and you can always use FFDshow to do the scaling).

Regards,
Stéphane.