WO2001078388A1

WO2001078388A1 - Method of conversion from an interlaced format to a progressive format having a lower frame rate

Info

Publication number: WO2001078388A1
Application number: PCT/GB2001/001258
Authority: WO
Inventors: Andrew Major; James Shepherd
Original assignee: Snell & Wilcox Limited
Priority date: 2000-04-07
Filing date: 2001-03-22
Publication date: 2001-10-18
Also published as: AU4430401A; GB2361134A; GB0008652D0

Abstract

In standards conversion, a 60 Hz interlaced video signal is de-interlaced, producing a 60 Hz progressive signal, which is then synchronised with a 24 Hz frame rate, producing a 24 Hz progressive signal, with the unwanted frames being discarded.

Description

METHOD OF CONVERSION FROM AN INTERLACED FORMAT TO A PROGRESSIVE FORMAT HAVING A LOWER FRAME RATE

VIDEO SIGNAL PROCESSING

This invention relates to video signal processing, and particularly to standards conversion. A particular aspect is directed to the conversion of higher frame rate video material to lower frame rate material.

There are a wide variety of known methods of standards conversion; these methods are employed for a variety of different conversions, from a variety of different signal sources.

However, known methods of standards conversions can have certain disadvantages, particularly when converting from higher, video-type frame rates to lower frame rates such as that of film. Artefacts are often produced in the output material, and the interpolation involved in the conversion can produce frames at the film rate which are of lower quality, particularly if viewed as stills. Known standard conversion methods are also often complex, leading to the need for extra processing power and expense.

It is therefore an object of this invention to address these problems, to provide better quality film-rate output material and to provide simpler and more efficient conversion methods.

Accordingly, the present invention consists in one aspect in a method of converting 60 Hz interlaced video signals to 24 Hz progressive video signals comprising the steps of: de-interlacing the 60 Hz interlaced video signal to produce a 60 Hz progressive signal; synchronising the 60 Hz progressive signal with a 24 Hz frame rate; and discarding the unwanted frames.

It is an advantage of this embodiment of the invention that information is only transferred across unit input temporal sample periods. Multiple image artefacts which can be produced by fractional temporal interpolation are thus not introduced. Though some temporal distortion is inevitable with the off-setting (temporal perturbation) of the output frames, this is negligible in comparison to the usual artefacts, such as judder, associated with film and other low sampling rate signals, and can therefore be tolerated. It is a further advantage that the steps involved are more simple than those used in conventional standards converters, thus saving processing power and expense.

In another aspect, the inventio n consists in a method of standards conversion, comprising the steps of: employing a motion compensated field doubling process on an input video signal; and employing a linear standards conversion process on the output of the field doubling process to produce an output video signal.

It is an advantage of this aspect of the invention that artefacts such as judder and blurring can be removed, or at least reduced in comparison with known converters. It is a further advantage that the architecture is again, less complex than conventional methods.

The invention will now be described by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagram showing a method of conversion of a video signal according to an embodiment of the invention;

Figure 2 is a block diagram of a standards converter according to another embodiment of the invention; and

Figure 3 is a block diagram showing a method of standards conversion according to another embodiment of the invention.

Referring to Figure 1: a 60 Hz interlaced video input is first converted to progressive video by a de-interlacer. The 60 Hz progressive signal is then synchronised with the 24 Hz frame rate. In this case, the first of each pair of output frames is in the same temporal location as one of the input fields, and the second frame of the pair is phase shifted with respect to the input fields in a synchroniser, the shift being equivalent to half the time between input fields. The result is a progressive 24 Hz signal. In a modification, this shift of half the time between fields is "shared" between all output fields, so that each is shifted no more than a quarter of the time between input fields. ln alternative embodiments, the de-interlacer need not produce every frame of the 60Hz progressive signal shown in Figure 1, as only a portion are required for the 24 Hz signal. Figure 2 illustrates a block diagram of a standards converter according to another embodiment of the invention. An input, interlaced video signal (20), which is accompanied by forward and backward motion vectors from a motion estimator (not shown), is fed to a store (21) capable of storing at least four video fields and their associated motion vectors. The store has three outputs for reading data (22) (23) (24) and is controlled by a controller (25), which organises the read and write addressing of the store.

The controller (25) receives synchronising signals (26) (27) which define the input and output field rates for the required conversion. The input syncs (26) define the frame rate and field phasing of the video input (20), for example 60 Hz interlaced. The output syncs (27) define the required output frame rate, for example 24 Hz progressive.

When the controller (25) receives an output frame synchronising signal (27) it causes the three most recent input fields and their motion vectors to be read from the store and input to a de-interlacer (28). This creates a progressive output frame from the three consecutive input frames in accordance with the known art of motion compensated de-interlacing.

The resulting de-interlaced video signal (29) represents a regular sequence of progressive frames having a fixed phase relationship with the output sync signal (27). There is thus no further temporal processing performed after the de- interlacing step in order to produce the desired output frame rate, merely a synchronisation. This produces better quality picture material, particularly in each output frame. For film-type (24Hz) output, this produces more aesthetically pleasing picture sequences, and higher quality stills.

In alternative embodiments, the de-interlacer may receive only two fields from the store, or may receive four or more. The number of fields chosen to be read may also be flexible, in dependence upon, for example, the complexity of the material being converted.

Figure 3 illustrates a method of standards conversion according to another embodiment. The input signal, for example, 60 Hz video, is first passed to the motion compensated field doubler (30). This produces an intermediate signal, with twice the field rate. This signal is then passed to a linear standards converter (31), which produces the required output.

The field doubler is only required to produce new intermediate fields at a single temporal phase with respect to the input fields. Compared with the fully flexible field rate converters required by known standards converters, the processing complexity is therefore greatly reduced.

The second step of standards conversion is linear, rather than involving motion compensation, and therefore also represents a considerable saving in complexity.

The combination of these two simpler steps also helps to reduce the severity of certain artefacts. The field doubler provides twice as many samples to the input to the linear standards converter. This allows the converter to make more accurate estimates of required output fields, and this greater accuracy results in reduction of judder and blurring.

It will be appreciated by those skilled in the art that the specific arrangements have been described by way of example only and a wide variety of alternative approaches can be adopted. For example, in the converter of Figure 2, all the input fields could be de-interlaced before being stored and the required output pictures read directly from the store. The output progressive frames could be "packaged" in the format of interlaced signals with alternate, co-timed lines being transmitted at different times on succeeding "fields"; these "fields" could even be arranged in a "3:2" format so as to give a "field rate" close to 60 Hz although the true frame rate is 24 Hz. Also, the processes shown here could be combined with line standards conversion. It will be understood that the term 60Hz is used to refer not only to exactly 60 Hz, but also to field rates close to 60Hz such as the standard rate of 59.95Hz.

Claims

1. A method of converting 60 Hz interlaced video signals to 24 Hz progressive video signals comprising the steps of: de-interlacing the 60 Hz interlaced video signal to produce a 60 Hz progressive signal; synchronising the 60 Hz progressive signal with a 24 Hz frame rate by selecting frames close to the desired output frame position, retiming those frames and discarding the unwanted frames.

2. A method of motion compensated television standards conversion from a higher to a lower temporal sampling rate wherein motion is only compensated from one input field to the immediately preceding input field, or to the immediately succeeding input field.

3. A method of television standards conversion from an interlaced input format to a progressive output format, wherein the output has a lower temporal sampling rate which is not necessarily an integral multiple of the input temporal sampling rate, comprising the steps of creating progressive frames at positions on the input temporal sampling structure and aligning the progressive frames with the output temporal sampling structure without further temporal processing.

4. A method of standards conversion, comprising the steps of: employing a motion compensated field doubling process on an input video signal; and employing a linear standards conversion process on the output of the field doubling process to produce an output video signal.