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.
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.
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
Horwig wrote:but what is it in the signal that causes the codec to switch mid shot?
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.
Horwig wrote:I suppose the question is if the AMD processor has the grunt, or should I go for a faster one?
Users browsing this forum: No registered users and 1 guest