[mythtvnz] MythTV & ATI & Freeview HD

[Richard Watson]

Hi all,
 
I've been using MythTV happily for the last couple of years on Debian and recently decided to try upgrading my system to handle Freeview HD.  
 
This is still definitely a work in progress but thought I would share my current status with the list to see if anyone had any suggestions!
 
First of all I decided that I needed a dedicated graphics card as opposed the onboard Nvidia 6150.  I found a pretty cheap ATI HD3540 card that had a native HDMI output for around $100, no fan either so great for media PCs.   Getting this running under Linux was a bit of a struggle and I'm still 50/50 on whether it was worth it.  I'm using the latest 8.9 Catalyst drivers.
 
Pros:
- Doesn't use system memory
- Actually accepts my LCD TVs EDID modelines correctly (My onboard Nvidia 6150 was perfectly capable of doing 1080i on the component output, but refused to do anything above 720p on the DVI without EDID being disabled and a manual modeline added)
 
Cons:
- Bizarrely it only uses about 90% of the screen under X-Windows.  There is about a 1-2cm band all the way around the displayed image that is unused and left black.  The LCD TV says its getting a 1080i signal ok but unless my TV actually has more than 1920x1080 pixels then one of the them is lying.  The TV won't automatically expand the used space to the full panel either....
- Contrast seems to be permanently wrong.  "black" is just a medium grey.
 
The next part was installing a DVB-T tuner.  I purchased a Hauppauge WinTV-NovaTD USB stick for around $150-170.  This is a dual DVB-T tuner USB stick so I could record two channels simultaneously. 
 
The first challenge was getting the OS to register the USB tuner.  Apparently there were several types of Nova-TD and my one's USB ID was only listed in the 2.6.26 kernel. After upgrading, the tuners would register ok and load their firmware and I finally had /dev/dvb/adapter0 and /dev/dvb/adapter1 listed correctly.
 
For MythTV I've being using the latest SVN trunk (0.22).  Happily getting Myth to scan for channels and read the EIT EPG works fine.  The big problem now is the codecs used for Freeview HD.
 
With the default build I could sort of watch the 720p video from TVNZ, though the H.264 video was very heavily pixelated.  I couldn't listen to the audio thought as Myth can't play the HE-AACv2 codec, at least I think that is the problem.  I couldn't watch the 1080i signal from TV3 at all as FFmpeg doesn't support interlaced mode, but as some of there programs are broadcast in Dolby Digital - I could at least hear them.
 
After a bit of digging I found the CoreAVC project  (http://code.google.com/p/coreavc-for-linux) allows Myth to use the CoreAVC DirectShow codecs under Linux.  This codec does support interlaced H.264 and after patching Myth and after rebuilding I can finally watch both TVNZ's 720p an TV3 1080i video.  As I have an AMD64 CPU I have to use the pre-compiled IA32 binaries which are a bit out of date.
 
All this means that I can finally record and playback TV3 HD content.  However, now I find decoding 1080i in software is pushing my CPU to the limit so that it looks a little too jerky to be acceptable.  Running "top" shows that 720p with CoreAVC takes about 60-80% of my AMD X2 5000+ CPUs while 1080i seems to take 80-110%.  I assume >100% means it needs to use both CPUs and I think this is where the problem with the jerkiness comes from.
 
Questions -
- Has anyone else had more success with ATI under Linux?
- Does anyone have a better (i.e. less CPU intensive) way of decoding Freeview HD content?
 
Richard
 
 
 
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