Background

Sony's HVR-Z1U and HDR-FX1 HDV camcorders include a range of unique features beyond standard HDV that enable filmmakers and videographers to produce startlingly good images from such an inexpensive camera. In particular, Sony's CineFrame^TM modes offer progressive image acquisition, greatly increasing the flexibility of the camera. CineForm is enthused to announce that all its products - Connect HD, Aspect HD, and Prospect HD - now directly support Sony's CineFrame modes, enabling an entire progressive workflow throughout post production.

Sony's HVR-Z1 plus the US version of Sony's HDR-FX1 offer the CineFrame 24 mode for image acquisition. This capture mode, which is set within the camera's picture profile settings, will deliver 24 frame-per-second progressive pictures through a 60Hz interlaced output, in much the same way that 24p film material has "3:2 pulldown" added to create a 60i interlaced output for television broadcast. Consequently the "look" of CineFrame 24 is similar to traditional film that is broadcast over television.  Material with 3:2 pulldown added is commonplace for delivery, but it is awkward for editing and post production as we'll show below.

 

When this sequence of rolling balls is edited as a standard 60i project, you can see that some frames, which are composed of two interlaced fields, appear interlaced while others appear to be progressive.  This is the result of the "cadence" created when converting the 24p progressive source material into 60 interlaced fields.  On a frame-by-frame basis, the sequence above is what is seen by the video editor. And all have seen the interlaced "fingers" present when playing interlaced material on progressive computer monitors (see on right). This is fine for simple productions that use few effects and transitions, but much better results can be obtained by editing the progressive frames directly. The question is, how do we recover the original 24 fps sequence without the extra interlaced fields?

CineForm solution for CineFrame 24

Because CineForm products include our visually lossless CineForm Intermediate™ format which replaces the camera-compressed source, we have a unique opportunity at the time of capture to create a new video sequence that contains only the 24p progressive source data. Consequently, during the capture process CineForm's software can automatically reverse 3:2 pulldown, the result of which is a 24p progressive AVI file that is compatible with all common Windows video tools. After capturing the above sequence with the "Remove 3:2 Pulldown (CF24 mode)" enabled we get the following results:

 

Notice the results are totally void of interlaced "fingers".  The progressive result can be edited on a 24p timeline, adding 24p effects and transitions just the same as if the material was originally shot on film. However, the Sony Z1 and European Sony FX can do one better by shooting in CineFrame 25 mode as we describe below.

There are a couple of reasons to choose CineFrame 25 over CineFrame 24. Perhaps most importantly, the CineFrame 25 mode doesn't introduce any motion stutter that can be visible in some sequences shot with CF24. CF24 stutter is due to the characteristics of the algorithm Sony uses to create the 24p frames inside the camera, and their resulting multiplex into a 60i stream.  The progressive 24p sequence is created internally by a repeating sequence of first sampling frames at an interval of 1/20th of a second, then at 1/30th of second. The resulting output is 24 frames per second. For many video sequences this non-uniform temporal sampling is hard to perceive. Yet for advanced operations like motion tracking, or sequences containing a lot of motion this can be a problem. Comparatively, CineFrame 25 samples video at the very predictable 1/25th of a second, so no motion stutter is introduced.

The second reason to select CineFrame 25 is its greater flexibility in adjusting shutter speed. When using CF24, the camera is limited to a shutter speed of 1/60th of second (or faster), whereas CF25 offers 1/25th, 1/50th, 1/60th and faster. To maintain a filmic look 1/50th is recommended (this emulates the typical 180 degree exposure of 35mm film).

 

Here is how the motion looks when using CineFrame 25 (it will look the same as true 24p).

24p vs 25p Editing

CineForm offers support for both 24p and 25p editing, so the choice is up to the editor. Even when shooting using CF25, CineForm's capture software allows you to optionally perform the 25p to 24p conversion. When you change frame rates like this it is necessary to stretch the audio 4% to match the video -- yet this is done without changing the audio's pitch. This capture mode allows 25p shooters to work on a 24p timeline. The 4% shift is in audio is not perceptible, and is the identical the method used to transfer PAL video projects to film. The speed change is great for mixing elements from film with your HDV camera. CineForm's capture offers a range of frame rate conversions including 25p to 23.976p and 30p to 24p (for subtle slow motion effects), all while maintaining audio sync and pitch.

Once you have completed your 24p production, some may wish to archive back to HDV tape. The CineForm M2T export with Aspect HD and Prospect HD will support 24p encoding into a 60i stream, by simply adding the 3:2 pulldown back.

The high quality of HDV cameras will encourage producers to shoot HDV solely, and in some cases to use HDV in conjunction with other HD sources. To enable tighter integration, CineForm's Prospect HD enables HDV up-conversion to 1920x1080 10bit (from HDV's non-square-pixel 1440x1080 8bit.) This enables all HD and HDV elements to have the same pixel count and pixel aspect ratio, which improves editing performance and simplifies complex compositing projects.  The left image shows HDV 1440 x 1080 (non-square pixel) displayed on a square-pixel monitor without aspect ratio correction.  The right image shows the same material up-converted to 1920 x 1080 (square pixel) as seen on a square-pixel monitor.

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