In the HDTV broadcasting sector, two alternative formats are used unlike SDTV. Data can be broadcast in full frames (= progressive) at a resolution of 1280 x 720 pixels, or in half frames (= interlaced) at a resolution of 1920 x 540 pixels. The frame rate is 50 Hz or 60 Hz respectively depending on the location viewed.
People like to insist that the progressive data transfer produces the better image in the end. In fact, these over-simplifications are incorrect. To simplify this, we have looked at the situation in the PAL sector below; this can in principle also be transferred to NTSC.
If the source was film (recording in 24 full frames per second), PAL speedup is applied. The material is sped up to 25 full frames per second. If material is broadcast in 1080i50, the material is then segmented into half frames. For broadcasting in 720p50, every full frame must be repeated once. This redundancy drives the bandwidth requirement up unnecessarily, but it is needed in order to achieve a 50 Hz signal.
In both cases, two images with the same time index are present. This is known as a 2:2 cadence. In the case of 1080i, although a half frame only has a resolution of 1920 x 540 pixels, the original full frames (with 1920 x 1080 pixels) can be reproduced with no loss once a deinterlacer is available. The half frames with the same time index must simply be placed one over the other to achieve this. The result is a signal with a full 1920 x 1080 pixels and 25 full frames per second. Thus, broadcasting in 1080i50 is more suitable for films.
Where SDTV with recording in 50 half frames per second is in use (=video), the situation is different. Many shows, documentaries or sports programmes serve as examples. In the case of the latter alone, the resolution is twice as high, since playback with 24 or 25 frames per second would not be fluid enough.
Thus, if recording is in 50 half frames per second for broadcasting in 1080i50, there are no longer half frames with the same time index. After the deinterlacing that is obligatory for flat screens, 50 full frames per second are created. However, even good adaptive algorithms (which analyse neighbouring half frames in order to achieve the best possible reconstruction of the full frame) never work perfectly, and less so where motion is concerned (further information in our deinterlacing article).
Placing half frames on top of each other causes unpleasant "comb" effects for moving pictures. Simple implementations scale every half frame to a full frame. The effective resolution in this is case is actually only 1920 x 540 pixels - and this causes distracting flickering.
Broadcasting in 720p50 can have visible advantages here if recording is in 50 full frames per second. The deinterlacing needed for interlaced broadcasting is no longer needed. Each time, we receive the full quality of the 1280 x 720 pixels. The timed resolution is identical to the case described previously.
Ultimately, then, the source material decides whether it makes more sense to broadcast in 720p50 or in 1080i50.
1080p – the solution?
Certainly not for broadcasting. Here, what is important is the smallest possible bandwidth requirement. In addition, there is the aforementioned redundancy in film material, which has a negative effect on this factor.