A field is transmitted in 1/50 sec, two combined make a single 1/25 sec frame (each field has only half the vertical resolution of the complete frame). A crt scans odd and even lines in 1/50 sec for each alternate field. Persistance of the screen phosphors build the appearance of a single frame. 1080i Is downscaled to 720 x 576 for SD only connections (RGB, S-video or cvbs) the same format as a uhf analogue TV picture PAL at 25fps.

Well I'm really not sure about this now, in DVBViewer, I am watching ITV1 HD and if I look at the EVR renderer data, it says:

average frame rate achieved=50

If I play back an iplayer HD clip (downloaded with get_iplayer before this was blocked), I get

average frame rate achieved=25

They are not combined into a frame on a crt as your explanation describes.

On a crt The fields are displayed sequentially and convey the motion between them.

As yousay it's only the "appearance" of a single frame.

Absolute rubbish, a crt has two scanning coils one generates the horizontal raster with a left to right scan and a rapid flyback to scan the next line which is the next but one using the vertical scan coils. After the complete field is scanned to the screen the vertical scan flybacks to to line 2 and fills in the blanks in the same way. In the PAL system each alternate line scanned is reversed in phase (PAL is phase alternate line). No CRT of any age can generate 50 frames/second

Read up how it really works

http://en.wikipedia.org/wiki/PAL

A progressive display simply scans line 1 then 2 then 3 etc but there are very few crt TV's that can display a progressive signal and those that can require a component input.

Might I suggest a cool down? You both seem to actually be saying the same thing just in different words.
1 field every 1/50 sec != 1 full frame every 1/25 sec.

It does and you do (or you lose spatial resolution).

That's why the small (IMO) reduction in spatial resolution of 720p makes it better than 1080i, it can transmit a genuine 50 frames a second. Full resolution, spatial (at 720) and temporal (at 50): no compromises (if you are happy with the jump from 576 to 720)..

"A television, however, does not deal with video in terms of frames. Instead, it displays a video using half-frames, called fields. Each frame contains exactly two fields. One field is made up of the odd horizontal lines in a frame. This is called the odd field or the top field since it contains the top line of the image. The other field is made up of the even horizontal lines in a frame. This is called the even field or bottom field. Here is an illustration:



Since there are two fields in every frame, a television (CRT) actually updates the display at 60 fields per second (or 50 fields per second for PAL/SECAM video).

Each field is displayed 1/60th of a second after the preceding field (or 1/50th of a second for PAL/SECAM video). "

Well, not according to my post above- it shows 50fps framerate achieved on all channels, including 1080i HD, 25fps is only rendered on iplayer content.

LOL, thanks for the laugh, I've not seen a post quite as comical as that for quite a while on these boards.
You actually think a Full HD LCD display has 6,220,800 separate wires to the display along with circuitry that is capable of altering the voltage on every one of those wires instantaneously

To be more accurate, the second field is displayed while the previous one is still showing - giving 25 frames per second from the 50 fields.

The persistance of the phosphur on a CRT is designed specifically for the correct decay period to achieve this.

LCD/Plasma duplicate it by combining both fields in a single frame and displaying them as one frame.

[quote=d'@ve;41168168]It does and you do (or you lose spatial resolution).

QUOTE]

It depends on how the deinterlacing is performed.

Here's a full analysis of a BBC-HD 1080i transmission

General
ID : 802
Complete name : E:\BBC HD Preview_20100501_1403.ts
Format : BDAV
Format/Info : BluRay Video
File size : 69.7 MiB
Duration : 54s 512ms
Overall bit rate : 10.7 Mbps

Video
ID : 5500 (0x157C)
Menu ID : 6940 (0x1B1C)
Format : AVC
Format/Info : Advanced Video Codec
Format profile : High@L4.0
Format settings, CABAC : Yes
Format settings, ReFrames : 4 frames
Duration : 54s 512ms
Bit rate : 9 643 Kbps
Width : 1 440 pixels
Height : 1 080 pixels
Display aspect ratio : 16/9
Frame rate : 25.000 fps
Resolution : 24 bits
Colorimetry : 4:2:0
Scan type : Interlaced
Scan order : Top Field First
Bits/(Pixel*Frame) : 0.248
Stream size : 62.7 MiB (90%)

Audio #1
ID : 5501 (0x157D)
Menu ID : 6940 (0x1B1C)
Format : AC-3
Format/Info : Audio Coding 3
Duration : 54s 528ms
Bit rate mode : Constant
Bit rate : 384 Kbps
Channel(s) : 6 channels
Channel positions : Front: L C R, Surround: L R, LFE
Sampling rate : 48.0 KHz
Video delay : -1s 129ms
Stream size : 2.50 MiB (4%)
Language : English

Audio #2
ID : 5502 (0x157E)
Menu ID : 6940 (0x1B1C)
Format : MPEG Audio
Format version : Version 1
Format profile : Layer 2
Duration : 54s 480ms
Bit rate mode : Constant
Bit rate : 256 Kbps
Channel(s) : 2 channels
Sampling rate : 48.0 KHz
Resolution : 16 bits
Video delay : -1s 49ms
Stream size : 1.66 MiB (2%)
Language : nar

Text
ID : 5504 (0x1580)
Menu ID : 6940 (0x1B1C)
Format : DVB Subtitles
Language : English

Menu
ID : 258 (0x102)
Menu ID : 6940 (0x1B1C)
Format : AVC / AC-3 / MPEG Audio / DVB Subtitles
List : 5500 (0x157C) (AVC) / 5501 (0x157D) (AC-3, eng) / 5502 (0x157E) (MPEG Audio, NAR) / 5504 (0x1580) (DVB Subtitles, eng)
Language : / eng / NAR / eng

That's why I said "or you lose spatial resolution". You can't have 50 fps and 1080 vertical resolution at the same time and on average, blending each field-pair together into 25 progressive fps looks the best on moving TV pictures, but the blending loses temporal resolution so, some motion blur or jerking or both.

"In the second type of recorded video, called interlaced video, each field represents a unique instance in time. More specifically, each field of video represents a point in time 1/60th (1/50th) of a second later than the preceding field."

OK, so if the fields are displayed at the same time why is interlaced always used for sport due to it's better motion portrayal?

If you combine (weave) two interlaced fields together and display them as one progressive frame at the same time on a plasma/lcd, you lose the temporal resolution advantage of 50i.

I can assure you the tenns or the football I am watching does not have the motion judder of a 25psf drama or film source.

Then you are either insensitive to it (some people are) or you are losing sharpness during the fast moving scenes or pans (try pausing an action frame to compare).

And don't forget, you can only compare motion judder on live sports with UK drama recorded at 25 fps - US drama recorded at 30 fps will usually have dreadful judder when converted for UK TV.

An active matrix display has a transistor for each and every on screen pixel connected to the memory buffer. They can all be turned on at the same time, Each transistor delivers 24 bits of video info (8 for each of red green and blue making around 64 miliion possible colour variations). This transfer happens as near instantaneous as is possible. When compared to the 1/25 sec the frame will be displayed it's instant.

Now who's laughing . They are not connected by wires but built in the same way as a microprocessor chio can have millions of transistors

Surely that is not really accurate since interlaced content has fields not frames? My question was how this is rendered by displays- looking at the info from the Microsoft EVR renderer it is stating I am achieving 50fps on all TV, including 1080i, it only drops to 23.97/25/29.97 on progressive material, including iplayer. Presumably if I could receive the german HD channels I would get real 50fps (720p).

HD TV allows two transmission standards 1080i (up to 1920 horizontal res and 1080 vertical delivered interlaced at 25 frames/sec and 720P which is better for fast action, this reduces the resolution to 1280 to 720 and increases the frame rate to 50fps. 720P as the P indicates is progressive, 50fps is possible within similar bandwidth restrictions as 1080i because there is less pixels to transmit. 720P is better for motion because there is no time difference of 1/50 sec between alternate rows of pixels which gives interlaced a slightly jagged look. No broadcaster afaik uses 720P apart from video on demand like BBCi player

Most would say that the best picture you can currently get is a 1920 x 1080 progressive picture displayed at it's native 24 frames/second from a bluray player. It has virtually unlimited bitrate capability.

The motion artefacts you describe are nothing to do with the frame rate but created by an inadequate bitrate causing visible artefacts when the mpeg compressed signal is decoded. Mpeg works by sending 1 full frame and series of partial frames with difference info. THe encoder rebuilds the video by adding the difference info to the full frame (I frame) an inadequate sample rate produces very visible artefacts. Some channels have woefull bitrates giving rubbish pictures.

We aren't talking about 720 50p.

None of the above answers my questions above:-

OK, so if the fields are displayed at the same time why is interlaced always used for sport due to it's better motion portrayal?

If you look through the BBCHD commissioning requirements it says to use 50i not 25 psf for sport because "sport requires smooth motion".

As I said the 50i advantages for motion portrayal are not lost after deinterlacing on my plasma. How would that be the case if the two 50i fields were simply combined into one 25frame\sec?

AFAIK most of Europe uses 720p/50 rather than 1080i that we use in the UK.

I just tested an NTSC DVD of 480i TV content- surprise, surprise the "average framerate rendered" is 59.94 not 29.97! This suggests that progressive displays do indeed in some way show interlaced content at either 50 or 60 fps, but probably not as smoothly as 720p/50/60 does.

1080i is the only system used by any UK broadcaster not just for sport, the only alternative is 720P50. If you read my posting i said apart from electronic jiggery pokery. It's entirely possible that the TV processes every other field to attempt to remove motion jaggies in exactly the same way that a 100Hz TV doubles the frame rate to 50fps.

Just one of a million posts on the net

http://www.zdistrict.com/2007/01/10/720p-vs-1080i/

By describing it as that you are suggesting it is unusual- I don't think it is, I think 50fps simulation must be a standard function of deinterlacers whether in STBs, TVs or HTPCs. When you watch any interlaced content on a progressive display you are not watching 25fps or 29.97fps as TagMclaren has stated, it seems you are watching 50fps or 59.94fps, even if it is in someway simulated (and there is no way to turn it off unless you turn off deinterlacing completely- this is possible on some PC decoders, you then see the rendered framerate drop to 25 or 29.97).

And Sky, Virgin, Freesat etc. Only broadcast in 720p.

Yes the transmission method is 1080 50 i but the source material is very often 25 PSF. Nearly always used for drama and has been inapropriately used on things like Electric proms and others like Michael Macintyre IIRC. It suffers from motion judder, but is very easy to deinterlace as you can simply stitch the two fields back together.

My question to you is if the sport (for example) produced at 50i because it has smoother motion, is simply deinterlaced into one frame 25 fps, why wouldn't it's motion suddenly suffer from the judder that afflicts film/25 psf?

Can you see it's temporal resolution would be lost?

That really doesn't appear to be the case on my non-frame interpolating plasma.

It depends how the picture is de-interlaced.

There are no 720P transmissions it's all 1080i, either 1440 x 1080 or 1920 x 1080