Indeed, half the vertical resolution per field but they're composited to
make one whole frame. Yes, the field resolution is half a frame - It's
basically analogue compression. And although 625 lines per frame, there's
timing, blanking and audio lines too - picture content gets 576 lines, 288
lines per field.
The fact analogue TVs essentially gave you some free motion interpolation
from interlacing is a fortunate, quirky byproduct of the old mechanical
process. Even with the quick scanning process (both on capture and
reproduction) you were always capturing that microsecond in time for each
line, then the electron gun scanned at the same rate so you actually see
more of a 'fluid' representation of the light as captured. In practice your
eyes can't keep up with microsecond shifts in picture position between
lines, but they can easily see 25 vs 50 pictures per second.
The deinterlacing algorithm is doing no resizing - it's interpolating
between the frames and then 'printing' that to 50 progressive frames. The
resulting image will have slightly lower definition due to the bob
artifacts as it's reconstructing the frame from two sequentially interlaced
fields, but it hasn't changed resolution.
Downsizing to 720p50 gives you a resolution saving which can be put into
the higher fps with less of a net cost on bandwidth and disk space. Losing
absolute 1080 pixel definition but getting smoother motion is a win for me!
25i analogue video never has quite the per-frame definition of 25p video,
but it wins for motion accuracy and suited the constrained bandwidth
environment of analogue telly.
The bandwidth savings translate to the digital domain, so it "costs" a
little less to transmit interlaced. IMO it's a pain to work with and is
computationally more complex to decode and foremost.
Why they didn't just ditch interlaced encoding in the DVB-T standard and
let the boxes interlace for old TVs is a bone of contention for me! I think
it almost was the case that interlacing was almost dropped from the DVB
spec until fairly late in the day too... Trying to recall a discussion with
a fellow engineer from many months ago but struggling now...
On 30 April 2016 21:19:35 "Dave Liquorice" <[email protected]> wrote:
On Fri, 29 Apr 2016 20:42:06 +0100, Christopher Woods wrote:
"Upscaling" is a misnomer in this context. That implies a change of
picture resolution, ...
Agreed.
... when there's no resizing going on.
Not convinced.
25 interlaced frames per second yields 50 interlaced fields per second
(due to odd and even line scanning),
But does a frame have the same number of lines as a field? It doesn't in the
analog world. 625 line frame, comprising two 312.5 line fields. Frame rate
25 per second, field rate 50 per second but half the vertical resolution.
If a digital field only has half the vertical resolution, which I think it
must have or you can't interlace them, then to create true 50 *frames* per
second each field needs to be upscaled and the missing lines interpolated
from the existing ones. If you just construct a frame from the two fields
you have to repeat that frame or playback at double speed...
I missing something but don't know what.
--
Cheers
Dave.
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