UK Freeview HD New Encoder ?

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UK Freeview HD New Encoder ?

Post#1 » Wed Aug 22, 2012 11:12 am

Soon after the channel resolution on the UK Freeview HD MUX was increased from 1440x1080 to 1920x1080 (just before the Olympics), I started experiencing a brief intermittent stutter, as if a frame was dropped, typically when a live broadcast is panning (especially on BBC One HD). From a discussion over on AVForums, the concensus seems to be this looks like a new encoder has been introduced by the broadcasters and that the Microsoft H.264 video decoder can't handle its output very well. Maybe related to dymanic switching between 1080i50 and 1080p25 depending on content.

It does not seem to be a graphics card issue as both ATI and nVidia users are affected, and I don't think it's a lack of processing power as I use W7MC with a fairly high-end cpu and an ATI 6670 card myself.

Has anyone else noticed this stutter, or seen any comment on a Microsoft Forum?
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Post#2 » Wed Aug 22, 2012 7:47 pm

This sounds suspiciously similar to the "29/59 issue" that we experience here in the US when the frame rate changes between 29.97fps and 59.9401fps. You might try searching "29/59 issue" or "29/59 bug". There are some settings you can try changing to make it behave better... and you'll find posts about them on this site and also on the experts.windows.com site.
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Post#3 » Wed Aug 22, 2012 11:28 pm

I agree, this sounds like the 29/59 issue.

If the MPEG-4 specification is like the MPEG-2 specification, MPEG pictures should never dynamically switch from interlaced to progressive. The pictures being encoded can be a mixture of frame pictures and field pictures (as long as the MPEG sequence header doesn't have the progressive sequence flag set to 1), but there are flags that are supposed to be set in each picture header that cause the player to play everything back the same way....either all interlaced or all progressive.

And if Microsoft's MPEG-4 decoder works like the MPEG-2 decoder, it's not a problem of the decoder not being able to handle the content, it's a problem of the content being flagged incorrectly and the GPU not being able to react fast enough.
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Post#4 » Thu Aug 23, 2012 1:10 pm

richard1980 wrote:If the MPEG-4 specification is like the MPEG-2 specification, MPEG pictures should never dynamically switch from interlaced to progressive. The pictures being encoded can be a mixture of frame pictures and field pictures (as long as the MPEG sequence header doesn't have the progressive sequence flag set to 1), but there are flags that are supposed to be set in each picture header that cause the player to play everything back the same way....either all interlaced or all progressive.


Thanks for the helpful comments. Unfortunately, it seems the MPEG-4 spec is not like MPEG-2 in this regard, according to a BBC Technical blog I've found, that asserts "The Freeview HD spec has always included 1080p25 as an option". And it goes on to say "The HD encoder supporting BBC HD on Freeview HD has been set-up to automatically detect progressive material and change encoding mode appropriately. The encoded bitstream can only change at each GOP boundary to ensure decoders maintain a consistent display. This means that each coded video sequence either contains interlaced or progressive pic_struct values within the bitstream. The transitions between interlaced and progressive modes are entirely dependent upon how a programme has been made."

Source http://www.bbc.co.uk/blogs/researchandd ... d-on.shtml

Apparently though, this encoder change was introduced back in March 2012, so it doesn't correlate with my symptoms which only appeared this month. Anyone else in the UK seen this too?
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Post#5 » Thu Aug 23, 2012 1:40 pm

would this apply to freesat?
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Post#6 » Thu Aug 23, 2012 2:08 pm

bobbob wrote:would this apply to freesat?

I don't think it applies to FreeSat which I believe is 1080i only, but I haven't got a DVB-S receiver so I'm not certain.
Hopefully someone more knowledgeable will be along soon.
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Post#7 » Thu Aug 23, 2012 2:53 pm

In any case, it wouldn't hurt to search for the 29/59 issue and try to change some settings in your graphics adapter to see if they make the problem better or worse. The problems may be related.
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Post#8 » Thu Aug 23, 2012 5:25 pm

Here's a more in-depth explanation (it was one of the comments to the original article):

The following is a response to issues and questions raised by commenters, and is written by Phill Layton, the original blog post author and lead for this project:

Some of you have asked for more technical details on the change and whether the encoder really is encoding progressively shot material as pure progressive 1080p - just like Blu-ray.

The answer is that it is. This mode is signalled to the decoder by setting the video bitstream's "pic_struct" parameter to "0". "pic_struct" indicates whether a picture should be displayed as a frame or one or more fields.

A value of "0" disables all interlace coding modes and signals that the decoder should display the picture as a single progressive frame. So by switching your set-top box output to 1080p50, progressively shot material should remain in its native progressive format right through to the display. Interlaced material on the other hand, continues to be coded in MBAFF and PAFF modes which use "pic_struct" values of "1", "2" and "3". These indicate that the coded picture should be displayed as fields. So the receiver knows that it needs to de-interlace this content and up-convert it to match the 1080p50 output format.

The encoder automatically detects if material is interlaced or progressive and adapts the encoding mode accordingly. Both the H.264 and DVB standards restrict changes between interlaced and progressive coding modes to video sequence boundaries, signalled by an IDR frame, so the encoder adapts its GOP structure to ensure that this constraint is met.

Some of you have noticed that the coding mode can change part way through what appears to be a progressively shot programme. This can happen for a variety of reasons. Even if programmes are shot in 1080p, they are usually edited as if they were interlaced. Any cross-fades, dissolves or titles will most likely be rendered in 1080i format to produce smoother movement and transitions. So the encoder must switch to interlaced mode during the effect. Furthermore, if the speed of a clip is changed to realise slow/fast motion, or even to change the running time, interlaced fames will be created and once again the encoder must change modes. Lastly, it's not always possible to tell if a particular shot is part of a progressive or interlace sequence - for example a still image. This type of shot is therefore usually coded in interlace mode as it's the safest option.


So while not exactly the same as the 29/59 issue, the end result is pretty much the same. WMC should be sending on/off commands to the deinterlacer, which can cause problems when the deinterlacer can't perform fast enough. The difference is, your content actually matches the flags that are present.

Have you recently updated the GPU driver or made any changes to the GPU settings?
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Post#9 » Fri Aug 24, 2012 2:30 pm

No driver updates for me since Catalyst 11.12 last year. Maybe I'll try the new 12.8 release this weekend and report back if it makes a difference.
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Post#10 » Sat Aug 25, 2012 1:27 am

Before updating the driver, let's confirm that we're on the right track. While watching some content that stutters, press 4-1-1-Info on your remote, or 4-1-1-CTRL+D on your keyboard. This will bring up the debug menu. Press the right arrow to change screens within the debug menu. You'll want to go to the very last screen, which is "Debug: Presentation". Watch the value for frame rate, and see if it changes. If it does, and those changes coincide with the stuttering, then we're on the right track. Otherwise, there's something else wrong.

If the stuttering coincides with frame rate changes, then a driver update may resolve the issue. Or it might make the issue worse. Before attempting a driver update, play with the settings in the Catalyst software. Disable all post-processing options you can find, and see if that helps. If that doesn't work, try updating the driver.

If changing the driver doesn't resolve the problem, you may need a new GPU. There are only a handful of GPUs that can handle rapid switching between interlaced processing mode and progressive processing mode without stuttering, and you may just have a GPU that isn't well suited for this encoding method. Can you easily get your hands on some other GPUs for testing? There's a list of GPUs that are known to perform well with 29/59 content here. Since US frame rates are faster than UK frame rates, I would expect these GPUs to perform sufficiently well on 25/50 content.
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Post#11 » Sat Aug 25, 2012 11:01 am

This is very interesting as I've had this judder ever since I dropped a Blackgold BGT3620 DVB-T\T2 card into my system that also contains a TBS 6981 DVB-S\S2 card, but only with the HD T2 transmissions.
I tried the 411 info as suggested, firstly using the BGT card and I noticed that there's a very quick fluctuation with both the bit rate and frame at the time of the judder, but with the TBS on the same channel, there is NO fluctuation at all. Could it be a weak signal problem?

I've also just updated my AMD 5450 to Cats 12.8 and same judder. All processing options are off.
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Post#12 » Fri Sep 07, 2012 12:00 pm

I'm trying to catch a judder with 411(info) enabled, but it seems limited to live HD broadcasts, typically during panning or other camera movements, and so far my observations are inconclusive. In the past I've tried ATI HD 5000 and 6000 series GPUs and both were affected. I've also read on other forums that using the LAV decoders in place of the MS decoders eliminates the problem, although of course this is not possible in Media Centre.

But as richard says, it's important to confirm this judder coincides with framerate changes.
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Post#13 » Fri Oct 05, 2012 12:59 pm

Hi

I don't suppose anyone has had any success with finding out what causes the stutter on Freeview HD channels? It's particularly noticeable when watching football and it's annoying the heck out of me

Oli
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Post#14 » Fri Oct 05, 2012 1:50 pm

well it was suggested that it was due to the 29/59Hz bug and if that's the case you're only hope is a different video card.
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Post#15 » Fri Oct 05, 2012 3:57 pm

PAL isn't effected by the 29bug, the obvious things to check are signal quality, cable connections, tv card quality, hard drive speed and space, is your system overheating, is your PC powerful enough
Do you get HD on any other non-pc decoder, what's its quality like
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Post#16 » Fri Oct 05, 2012 4:02 pm

^

We do have the UK equivalent 25/50hz "bug" though.

I've seen it myself during the Olympics on Freeview HD.

Freesat HD isn't affected.
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Post#17 » Fri Oct 05, 2012 11:51 pm

Yeah, from what I understand certain content in the UK is purposely encoded with the switching frame rate. The solution really depends on the symptoms that you see. If it's stuttering, the only thing you can do is disable all post-processing on the GPU to see if that helps. Also, if you are running an NVIDIA GPU, you have to be careful what driver you are running because lately their drivers have been causing all sorts of problems with 29/59 (25/50) content. If those things don't help, your only option is to get a different GPU. There's a list of known good GPUs you can reference. However, keep in mind that UK frame rates are slower than US frame rates, which means GPUs have more time to do all the work. This means a GPU might stutter with 29/59 content, but might play just fine with 25/50 content.
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Post#18 » Sat Oct 06, 2012 8:53 am

oliroe wrote:I don't suppose anyone has had any success with finding out what causes the stutter on Freeview HD channels? It's particularly noticeable when watching football and it's annoying the heck out of me

There are two problems that are more common than they should be: stuttering and flickering. Flickering is when the screen momentarily lightens (as though a flashgun had discharged).

I have not encountered stuttering (yet!), but I have encountered flickering, and that occurred only on Freeview HD; standard-definition Freeview was not affected; see this thread:

http://www.thegreenbutton.tv/forums/viewtopic.php?f=28&t=2371

In my case, the solution was to use a newer version of the Nvidia drivers. But be aware that newer does not always mean better -- sometimes newer versions re-introduce bugs that were fixed in earlier versions. So I would suggest starting with the most recent drivers, and then trying successively older versions to see if one of them fixes your problem. It is a zero-cost solution, so worth trying before investing in different hardware.

-- from CyberSimian in the UK
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Post#19 » Tue Jan 22, 2013 4:49 pm

Just stumbled across this thread. The frame dropping is the problem I'm encountering on my system, I'll add my experiences, just for the record.
I thought it was to do with refresh rate mismatch, but now having read the above, and watched the debug screen, I can confirm that HD channels are indeed switching randomly between 25 and 50 Hz, usually on cuts, but occasionally mid shot. The frame rate display will usually flicker on cuts, before settling to one rate or the other, but what is it in the signal that causes the codec to switch mid shot? As someone already noted, pans can cause it, but I've seen it happen on static backgrounds with end credits rolling over them. Watching 'Dispatches' on C4hd last night, occasionally, a couple of frames after some cuts there would be a dropped frame.
My system is an AMD F1A75 motherboard with an A6-3500 APU, I've tried an Nvidea graphics card I had, it shows similar frame stuttering (but was showing more aliasing on up-rezzed sd channels).

I suppose the question is if the AMD processor has the grunt, or should I go for a faster one?

Sorry for the ramble

Huw
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Post#20 » Tue Jan 22, 2013 10:01 pm

Horwig wrote:but what is it in the signal that causes the codec to switch mid shot?

The content actually contains a mix of both interlaced and progressive content.

Horwig wrote:As someone already noted, pans can cause it, but I've seen it happen on static backgrounds with end credits rolling over them.

Panning doesn't cause anything. The effects are more noticeable when there is a lot of motion, and panning has a lot of motion (as do rolling credits).

Horwig wrote:I suppose the question is if the AMD processor has the grunt, or should I go for a faster one?

There is a list of GPUs that are known to play this kind of content smoothly, although the list is actually compiled based on US frame rates, so I don't know how those GPUs will perform with 25/50 material. You can find the list here. Note that the absence of a specific GPU from the list does not necessarily mean that GPU can't play the content smoothly.
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