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Home , Film frame, Telecine

/24 to Tape Transfer by David Strachan Director of Sales, Evertz Microsystems Ltd.

Common Image Format As the transition to HDTV gathers momentum, the shortage of high quality source material, becomes increasingly apparent. Even in Europe, where no serious consideration is yet being given to the transmission of HDTV, there is still a need for wide screen video material with a quality at least compatible with the capabilities of ITU-R 601. Fortunately, the world has a huge stock of film material, which is both wide screen and high quality and which simply needs to be converted to video. But in which format? It is clearly not practical to employ expensive bays each time a video is required in 525, 625, 720 or 1080 line formats. What is needed, is a high quality Common Image Format (CIF) from which all other formats may be derived. 1080p/24 is this new format.

1080p/24 represents a video format containing 1920 pixels per line and 1080 active lines per picture, scanned progressively at 24 Frames per second (Fps). This was chosen to match the 24 Fps film exposure rate, which has been standard in the motion picture industry around the world, almost since movies were first screened. If we examine the current process of transferring 24 Fps film to 30 Fps NTSC, we realize quite quickly that there must be a better way. In order to increase the frame rate, the telecine machine, employs a 3:2 pull-down scheme, which results in every other being used to generate 3 video fields, instead of the usual two. Nowadays, digital compression schemes are used to minimize the amount of bandwidth, or hard drive space, occupied by the video, so the first thing an MPEG-2 systems does is to identify these extra fields and discard them. During the decompression stage, the MPEG-2 system recreates the additional fields, which are needed to comply with the NTSC frame rate of 30 Fps. As the images were initially scanned at only 24 Fps, it is clearly counterproductive to increase the frame rate by adding images early in the transmission chain. Far better to defer this process, ideally until the video signal arrives at its destination. (In the film world, the frame rate is doubled to 48 Fps in the movie theatre, by projecting each frame twice. This reduces the flicker to an acceptable level).

24 film frames will occupy 60 NTSC fields The ratio is 24:60 (2:5)

Fig 1 - 3:2 pull down for NTSC

1080p/24 offers the cleanest solution for film to tape transfer, with the added bonus of increasing tape or disk storage capacity by more than 25%, because we are only storing 24 Fps instead of 30. It is the ideal format for video mastering and from this master, copies may easily be made in whichever display format is needed The Film to Tape Transfer Process In the production of a movie, shots requiring being filmed at a particular a particular location, are generally taken at the same time. This is preferable to returning to the same site and having to re-assemble all the equipment. The different film clips are later physically cut and joined in the order dictated by the script. In the early days, re-editing the film was often necessary and was far from convenient, so nowadays the video “” are converted to video and edited using Non-Linear Editing (NLE) techniques. This process generates an Edit Decision List (EDL), which can be simply rearranged as required by the director. It is not until the EDL is finalized, that the actual film is cut and re-assembled in the required sequence

Fig 2 - The Film to Tape Transfer Process

During the filming process, the initial reference point for sound to picture synchronization, is the electronic clapper board (Fig 2). The sees the precise moment when the clapper closes and the microphone picks up the “click” at the same instant. Time code, recorded with the audio on the Digital Audio Tape (DAT) recorder, is used as a reference for the start time. When the film is loaded onto the telecine, the starting frame is positioned into the gate and the Evertz film reading head reads the optical film edge number. The Telecine Edit Controller ensures that, from this point on, the telecine and the DAT machine track each other in time. The Film Encoder accepts the KeyKode, bi-phase, frame pulse and video from the telecine and the LTC from the DAT machine and generates 3 line VITC (Vertical Interval Time code) to insert on the NTSC or PAL signal. An internal character generator also supplies an analog video signal with “burned” in Time code and KeyKode numbers. Finally, the Windows based KeyLog Tracker software in the logs all of these parameters, together with dates, times, scene numbers and other program data. In NTSC or PAL, the 3 line VITC contains video Time code on line 14 of the VBI (Vertical Blanking Interval), film edge numbers on line 15 and audio Time code on line 16. When the analog video tape has been made, the video is transferred to an Avid, or other NLE device, for editing. The NLE uses the 3 line VITC to track which piece of film and which audio from the DAT machine, was used to generate a particular video clip. Digital HDTV formats do not support 3 line VITC. The required audio and video Time code and the film edge numbers, are recorded in the VANC (Vertical Ancillary) data area of the HDTV bit stream. (Early HDTV VTRs did not have this capability).

Film to 1080p/24 Transfer

Fig 3 - 1080p/24 Film to Tape Transfer

Fig. 3 shows how an Evertz HDTV Film Footage Encoder and an Afterburner are used to generate an NTSC video clip with 3 line VITC, which can again be used by a standard Non-Linear Editor. The HD9025TR accepts all of the usual telecine pulses and the audio time code (at 30 Fps) from the DAT recorder and, for 1080p/24 applications, generates the require time code at 24Fps*. This video Time code, the audio Time code and the film KeyKode, are then inserted into the VANC area of the HDTV bit stream. The HDTV signal, carrying the VANC data is then passed on to the HD9150 “Afterburner” which performs the following tasks. The 1080p/24 (or /60) video is converted to /60 digital video as well as to NTSC, both with 3 line VITC information. The time code and KeyKode data is also “burnt” into the NTSC monitor for reference during the film to tape transfer process. As the frame rate must be increased from the film rate of 24 Fps to the NTSC frame rate of 30 Fps, the Afterburner also applies a 3:2 pull down, adding the additional fields.

24sF So where does the 24sF (segmented Frame) format fit into the 1080p/24 picture? Well, for many years telecine machines have been scanning the film progressively and then converting the progressively scanned image into an interlaced image compatible with the NTSC or PAL format. This is exactly what happens in the 24sF format. All in one frame, are progressively scanned in sequence from 1 to 1080. The complete frame is scanned in a 24th of a second. All the odd lines are stored in one buffer (in sequence, 1,3,5, etc) and the even lines in another (2,4,6, etc). These two fields are then successively dispatched down the video cable at 48 fields per second (fps), in the same way as in an interlaced television signal. But there is one major difference. Line 2 in a conventional interlaced image would not be scanned until after all the odd lines of 1 have been scanned. This means that any movement in the top end of the picture will be captured successively by lines 1 and 3, but then line 2 will not capture the interim movement until much later in the cycle. In a progressively scanned system (and in the 24 sF system) the movement on line 2 is captured in the correct sequence, between line 1 and line 3. In the 24sF system, it would be quite feasible to incorporate a buffer in the display monitor to display the lines in the correct sequence at 24 Frames per second, but even if the picture is displayed as an interlaced picture (lines 1,3,5….2,4,6, etc) at 48 fields per second, the movement on line 2 will still be correctly positioned, because it was originally scanned before line 3. The higher picture rate of 48 fps is also much more compatible with the human persistence of vision than 24 pictures per second, which is not acceptable. 24sF is a convenient way of utilizing circuitry designed for interlace field rates at the same time as reconstructing a progressively scanned image for display at a faster picture rate. (In reality, the display rate will usually be increased to the more acceptable 30 pictures per second). All of the products referred to as compatible, are also 24sF compatible.

Supporting Products The NTSC tape machine and the DAT recorder must operate at 30 Fps (or 29.97 Fps). However the 1080p/24 video operates at 24 Fps. Therefore there is a need for a reference generator which can be genlocked to NTSC, but can provide a 24 Fps HDTV reference. Such a reference is available from the Evertz 7750SRG Slave Reference Generator, which also offers tri-level sync for 1080i/60 and 24 Fps PAL (“Slow PAL”). This card conveniently plugs into the same frame used by the 7750TG-HD HDTV test generator, the 7700DA-HD distribution amplifiers and the 7710MD monitoring down converters, which convert HDTV to 601 SDI and NTSC. There are also two more new products in the 7700 series. These are the 7730DAC which converts 1.5Gb/s HDTV to SVGA or RGB and the 7732PFT which adds 3:2 pull down to a 24p signal to generate the more common 60 fps. This module also adds the correct amount of delay to the audio to compensate for the additional video fields added. To round of the range of accessories, Evertz also offers a Slate Generator and a Graticule Generator for masking and correction.

Fig 4 - Evertz 7700FR frame with supporting products

Conclusions As HDTV proliferates, it is reassuring to know that 1080p/24 is poised to provide the world with a video Common Image Format in much the same way as film has been used as the international image exchange format for most of the last century.

* Note: The expression 24p, is used for convenience. HDTV material produced for the NTSC world is generally at 23.98 frames per second. This simplifies conversion to the NTSC rate of 59.94 fields per second.