Autor: Redaktion



When thinking about how the moving image is perceived by the brain, it’s important to consider how our eyes detect movement. 16 to 18 frames per second is perceived as a moving sequence. However, a flicker is perceived at frame rates of below 48-50 frames or fields per second. So when a film is recorded at 24 frames per second, it is played back through a projector displaying each image twice (ie 48 frames per second) to reduce flicker.
In television production two different capture and broadcast mechanism are used: Interlaced and Progressive.

Interlaced Video

When standard definition television was invented a transmission rate of 50 full image frames at the desired resolution per second was practically impossible. The necessary transmission bandwidth required to broadcast this resolution was not available. To achieve a flicker-free display with 50 images per second at the desired physical resolution, a compromise had to be reached. That compromise was to transmit two offset half resolution fields per frame. This approach is called interlacing.

Each frame is sequentially scanned on alternate lines, i.e. one pass on lines 1,3,5 and the second pass on lines 2,4,6 etc. The two half-resolution fields combined give a full image of lines across the frame. This produces 50 flickers at half the vertical resolution across the 25 frames. The fields are known as the Odd and Even fields for obvious reasons.
The fields are captured, stored and transmitted sequentially.

Early display devices (tubed televisions) mimicked this process on replay. The perceived image was flicker free and full resolution. With the development of flat panel televisions the images have to be re-combined for the display. (Displays today typically work in progressive mode only).  Depending on the internal processing of the display, jagged/blurred edged can sometimes be seen. This is due to the temporal sequence of the two fields and the poor de-interlacing algorithms used to remove the temporal movement.

Progressive Scanning

When using a progressive scanning method, the image is scanned in full and in a single pass from the first to the last line. The full frame images are displayed sequentially. Today, there are various progressive image combinations:
24 frames per second is used for film material but played back twice per frame.

For television, 25 full frames per second can be used. 50 frames per second is possible, however broadcasts of 1920 x 1080 pixels at 50p are currently difficult to achieve because of the bandwidth required to broadcast this amount of information. To keep the broadcast bandwidth within reasonable limits a couple of compromise options are used:
Option one retains the pixel resolution and works at a lower frame rate, e.g. 25 frames per second (1080p / 25), or
Option two changes to a lower pixel resolution of 1280 x 720 pixels but keep the higher 50 frames per second (720p / 50). For high-end drama productions, recording at 25 frames per second progressive is sometimes used to create a filmic look.

The slower frame rates can sometimes be too low for the human eye to feel comfortable on certain types of material. Fast movements (e.g. football or fast panning) displayed as a  1080p/25 image can create a "stuttering" feeling in the video.

Progressive Segmented Frames

Progressive segmented frames (PsF) were designed to allow the manipulation and transportation of progressive scanned material through equipment designed for interlace working without apparent loss. A progressive picture of 1080p/25 has 25 pictures of sequential scanned lines. On equipment designed for manipulation of interlaced images of 1080i/25 it causes problems due to the expected field/frame timing.

In PsF the captured image is in full line sequence per frame (25) but then manipulated and transmitted as odd and even line fields but of the same temporal image. Unlike interlaced scanning, both fields are of the same movement phase, therefore, they can be reconstructed completely and without loss from the partial images transmitted.
Content signaling for PSF is important for television displays. The television set needs to understand that the content is broadcast in PsF mode and that there is no temporal change between the frames.

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