How good is HD supposed to be?

[Simon Morgan]

Thanks for your helpful explanation. Demystified a few points for me. I have
a new Panasonic Viera (is this "high end"?) and I find the 25fps from my GiP
downloads more than adequate - perhaps because I know no better!
RGds
Simon Morgan

-----Original Message-----
From: get_iplayer [mailto:get_iplayer-bounces at lists.infradead.org] On Behalf
Of Christopher Woods
Sent: 29 April 2016 20:42
To: Dave Lambley; Dave Liquorice
Cc: Get_iplayer List
Subject: Re: How good is HD supposed to be?

"Upscaling" is a misnomer in this context. That implies a change of picture
resolution, when there's no resizing going on. What iPlayer does for the 50p
streams is double frame rate deinterlacing, using what looks like a bob
deinterlace technique.

If you watch content originated in 25i, you will (once the stream steps up
to 720p50) see the deinterlaced 50p content and you'll notice immediately
how fluidic motion is - "just like TV", because that's exactly what CRTs
used to do.

25 interlaced frames per second yields 50 interlaced fields per second (due
to odd and even line scanning), with the resultant persistence of vision
effect inducing a pseudo 50 frames per second on viewing as each field's
worth of capture by the camera sensor 'sees' a slightly different point in
time. This renders as smoother motion, with a slight loss of sharpness due
to the low overall temporal resolution, but as it overall appears more
lifelike the eye prefers it.

25psf (progressive segmented frames) is the "filmic" look, where there's
only 25 distinct 'captures' of motion per second; the video simply
'transported' as interlaced. Each field 'sees' its half of the same source
frame. When decoded properly, you get 100% progressive output. However as
this gives you half the temporal resolution, motion is visibly less fluid.

Modern flat panels all deinterlace all interlaced source material to display
a progressive image, but only the higher end panels do quality deinterlacing
(Yadif or similar) Cheaper screens will usually bob deinterlace
(computationally less demanding) and porbably convert to 60p as their panels
and processing will be running internally at 60Hz.

You can even spot some cheap screens doing this as they'll add or duplicate
frames periodically to equal 60 fps from 50 fps source material, or they'll
do weird interpolation which can result in jumpy credits or news ticker
scrolling artifacts.

For an example of 720p50 iPlayer content, watch any episode of EastEnders
(be sure to enable HD), full screen it and wait for the bandwidth to step up
to max - you'll need 5 megabits per second minimum to stream (... Or just dl
it with gip).

For an example of progressive scan material, just about any documentary
(e.g. Horizon) or episode of Click will do. The latest Horizon about dating
is 25 PsF: you can see the deinterlacing 'interline twitter' on high
contrast edges during the programme - an example being the leaves of the pot
plant moving on the windowsill at around 22 minute mark.


On 29 April 2016 12:44:31 p.m. Dave Lambley <dave at lambley.me.uk> wrote:

> On 29 April 2016 at 00:57, Dave Liquorice <allsorts at howhill.com> wrote:
>> On Thu, 28 Apr 2016 07:07:33 -0500, artisticforge . wrote:
>>
>>
>>>>> NB hvfhd DOES NOT OFFER HIGHER RESOLUTION (CLARITY), only doubled 
>>>>> framerate (25FPS x2), which results in smoother scenes where 
>>>>> motion is involved!
>>>>
>>>> How does repeating frames improve smoothness of movement? Or does 
>>>> this encode upscale each field(*) and encode that to increase the 
>>>> temporal resolution?
>>>
>>> It is the frames per second that provide the human eye with the 
>>> persistence of vision, the illusion of motion.
>>
>> I could show you 100 fps but if there where only 4 different images 
>> displayed the illusion of motion would be no smoother than 25 fps. 
>> You only get smoother movement by increasing the number of different 
>> images displayed.
>>
>> So if this hvfhd only repeats each frame to get a higher frame there 
>> is no increase in smoothness. How ever if they take each field, 
>> upscale it and encode as a frame that would inrease the smoothness.
>
> I believe the frame repeating idea is a red herring. Real 50 frame/s 
> computer video is a thing which exists. If your browser's up to it you 
> can see for yourself here, https://www.youtube.com/watch?v=PmNapQdWFKg
>
> You'll need to choose one of the "p50" resolutions on the Quality menu.
>
> Dave
>
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