GB2250655A

GB2250655A - Digital television standards conversion

Info

Publication number: GB2250655A
Application number: GB9020154A
Authority: GB
Inventors: David Lyon
Original assignee: Snell and Wilcox Ltd
Current assignee: Snell Advanced Media Ltd
Priority date: 1990-09-14
Filing date: 1990-09-14
Publication date: 1992-06-10
Also published as: GB9020154D0

Abstract

Video material in the PAL standard which originates from cinematographic film is converted to the NTSC standard by the duplication of fields in a manner analogous to the known procedure for producing NTSC video direct from cinematographic film. A slight difference in the frames per second can be compensated by varying the recording video tape speed. <IMAGE>

Description

DIGITAL TELEVISION STANDARDS CONVERSION This invention relates to digital television standards conversion and is concerned for example, with apparatus and processes for converting a PAL 625 line - 50 fields per second video signal to the NTSC 525 line 60 fields per second standard.

Techniques exist for conducting the necessary field and line interpolation to convert from one standard to the other and these techniques operate well with "true" video signals. A difficulty arises, however, with video signals that derive from film material.

It is common practice in countries using the PAL television standard to transfer film shot at the standard cinema speed of 24 frames per second (or 25 fps) to video tape on the PAL standard. Film is replayed at 25 fps to achieve a one to one relationship with the video signal even if the original film was shot at 24 fps. Each film frame is scanned twice to produce 25 video frames per second each consisting of two interlaced fields called "odd and even".

If material produced in this way is passed through a conventional standards converter to produce a 525/60 signal, the output is degraded as a result of temporal distortions since the video odd and even fields are from the same point in time, unlike material shot with a video camera. The degree of perceptible degradation will vary according to the visual subject matter of the film and will in many cases be significant.

It is one object of this invention to provide a television standards conversion process for use with a film-originating video signal which overcomes or considerably reduces the importance of this difficulty.

Accordingly, the present invention consists in a television standard conversion process for converting a film originating video signal in a standard having interlaced fields at n frames per second to a standard having m frames per second, where m > n, comprising the steps of duplicating approximately 2 (m-n) fields in a specific field sequence of n frames of the input video signed and re-assigning fields to produce approximately m fields of video output signal.

Suitably, video tapes of the output signal are created using a tape speed differing from that at which the tape is intended to be replayed by that amount necessary to produce exactly m fields per second in the replay.

In an important example, the invention operates to create from a PAL signal additional fields which are duplicates of previous fields to construct the field sequence identical to that generated by an NTSC telecine machine. This is known as the 3.2 sequence. Thus, in this invention, from 25 input PAL frames, 31.25 frames are created - for every 2 original film frames. 5 fields are created on the new standard.

Line interpolation is conducted as necessary, in accordance with known line interpolation techniques, to produce 525 lines from the 625 line input signal. It will be understood that the 31.25 fps/525 line signal does not conform to the standard NTSC format and the non-standard signal is recorded on an adapted VIR running 4 % faster than standard. On replay at the normal speed, the conventional 525/60 (525/59.24) signal is reproduced. If the original film had been shot at 24 fps, this process would reporduce the images at the correct speed.

The invention will be more particularly described by way of example with reference to the accompanying drawing which represents schematically the process according to the present invention.

The sequence repeats after 4 input PAL frames consisting of eight fields. From PAL frame 1 the odd and even fields are line interpolated to create two 525 line fields called frame "A". From PAL frame two, the odd and even fields are interpolated to create 525 line frame "B" and so on up to 525 line frame "D". These new 525 line frames are at the PAL rate of 25 frames per second.

This must be increased to 31.25 frames per second by field repetition in the following sequence. 525 line frame "A" produces odd and even fields of NTSC frame 1 plus an additional odd field of NTSC frame 2. The even field of frame 2 and the odd field of frame come from 525 line frame "B". 525 line frame "C" provides the even field of frame 3 plus the odd and even fields of frame 4. Finally frame "D" provides the odd and even fields of frame 5 completing the sequence of providing 10 output fields from 8 input fields.

The signal so produced conforms in every way to conventional NTSC signals generated from film via an American 3:2 pull-down telecine channel with the exception that all signal rates are 4% higher. As mentioned previously, this can be accommodated by generating video tapes on a recorder adapted to run 4% faster than standard.

While the invention has been described using the example of the PAL 625 line format being reproduced in NTSC 525 line, the principle applies to any conversion from a 25 frame per second frame rate to a 30 frame per second rate, for instance 1250 line 50 field per second converted to 1125 line 60 field per second, both High Definition formats. Indeed, with an appropriate modification to the 3:2 sequence in which fields are duplicated and re-ordered, the invention would have applications in other standard conversions involving an increase in frames per second.

Claims

CLAIMS:

1. A television standards conversion process for converting a film originating video signal in a standard having interlaced fields at n frames per second to a standard having m frames per second, where m > n, comprising the steps of duplicating approximately 2 (m-n) fields in a specific field sequence of n frames of the input video signal and re-assigning fields to produce approximately m frames of video output signal.

2. A process according to Claim 1, wherein video tapes of the output signal are created using a tape speed different from that at which the tape is intended to be replayed by that amount necessary to produce exactly m frames per second in the replay.

3. A process according to Claim 1, for converting a film originating video signal in a PAL standard, wherein fields are duplicated to produce a video output signal at 31.25 frames per second.