In The Frame, Barry Bassett VMI Managing Director
The intricacies of frame-rate conversion for international co-productions.
In an international HD co-production, you will encounter the problem of differing frame rate compatibility between the USA/Japan and Europe. In Europe, programmes are delivered in 25 fps (aka 25P/50i) and in the USA/Japan, they are delivered and broadcast in a form of 30 fps, called 29.97P or 59.94i. For film projects, delivery is always at 24 fps. Acquisition and post is easy at 25 frames per second, so how do we solve this problem? Happily, programmes posted and mastered in 25P can be reversioned to these new frame rates easily and without compromise. In order to do this, the programmes need to be either slowed down, speeded up or cross-converted. But there can be some ramifications to consider, since some combinations work well and others not so well.
The conversion process from 25 frames to 30 frames (or 29.97 frames) involves slowing down the images by 4% to create 24P and then cross-converting to the correct frame rate - an automatic function of HDCAM VTRs. In doing so however, this introduces a ‘shuttering effect’ to the 29.97fps images which is acceptable ONLY if the programme was shot in 25P progressive mode.
The distinction between shooting in interlaced (50i) or progressive (25P) is important since the shuttering effect is more noticeable if the programmes were shot in 50i interlaced, since a bonus field is introduced every 4 changing pictures instead of every 2 frames in progressive mode. This repeat of every 4th field creates a very unnatural ‘shuttered’ look which is very noticeable.
Progressive images also contain the same repeat fields but because of the 25P acquisition mode, the 29.97P converted images contain a repeat field every second frame . As a result, although the ‘shuttering effect’ is visible, it is less obvious than with interlaced images which have every 4th image affected in interlaced acquisition.
Thus, most programmes which are sold Internationally on HD tend to be shot in progressive - either 24P or 25P, which is the solution that VMI always recommends.
Interestingly, programmes shot at the US speed of 29.97P/59.94i are not convertible to 25P without this same effect being introduced which explains why most US productions are shot at 23.98 or 24 fps for international TV delivery.
