Avid 24p - The Basics

by Alan Stewart

What does 24p Mean?

The "24" refers to the frame rate which is 24 frames per second.
The "p" means progressive image storage, as opposed to interlaced fields.

Therefore, 24p benefits projects where image material originates in a progressive form at 24 fps, or possibly 25fps (i.e., film or 24p video). Any system with the Universal option is a frame based nonlinear system for television, where the final product mastered directly out of the Avid is intended for NTSC or PAL. The Universal option also allows creation of multiformat EDLs for HDTV online mastering, and the creation of true 24fps film Cut Lists and Change Lists for 16mm, 35mm 2-perf, 35mm 4-perf, 35mm 3-perf, 35mm 8-perf, 65mm 5-perf, 65mm 8-perf, 65mm 10-perf, 65mm 12-perf, and 65mm 15-perf.

Telecine and Digtizing

Telecine for a 24p project is the same as it has always been for both NTSC and PAL shows. When capturing the NTSC 24p images, an ordinary telecine (film transfer) tape is used, with 2-3 pulldown. The Avid software removes the extra fields and reconstructs the original film frame in its entirety. So the image that is captured is a "snapshot" of each entire film frame. There are no fields to drop when digitizing in a PAL project, otherwise the frame capture is the same as in NTSC (the appropriate fields are combined into a single progressive frame).

Image Display

Due to the throughput limitations of the current hardware, the image displayed in the edit monitor is at 1/2 the full data rate in order to achieve normal playback speed. The image can be displayed in its entirety by switching off Fast Frame Display under the Special Menu. This allows the application of effects while viewing the entire image from the progressive media file. When playing out at speed, fast Frame Display is turned on and the full image is viewed on the NTSC or PAL client monitor (interlaced). Normal playback speeds utilizing the full progressive frames will be possible as bus speeds increase and other hardware improvements are implemented.

Interlace v. Progressive Images

The most common form of video image display is interlaced (i). NTSC is 30i and PAL is 25i. An NTSC video image consists of 525 horizontal lines of information (483 active picture). The electron gun scans top to bottom, left to right, odd numbered lines first, then the even numbered lines. Each full scan of even numbered lines, or odd numbered lines constitutes a "field". An alternative method of capturing and/or displaying a video image is progressive scan where the image is drawn in one pass, top to bottom (no interlacing of fields). Avid systems with the Universal option (24p) capture and store each frame of image without interlacing. While these systems do not actually output the image progressively, they do display a higher quality image and gain several other advantages from the 24p process, which I will detail a little later.

A Ball Moving Left to Right

HDTV - Brief Overview

With the advent of High Definition Television (HDTV) several new frame sizes, aspect ratios and display methods are possible. An understanding of the basic principals and terminology is required to appreciate the value of other technology offerings.

Aspect Ratio

HDTV set will be capable of displaying todays 4x3 images, but with black sidebars because the screens will be 16x9. Some 4x3 shows will be stretched, enlarged and/or cropped to fill the 16x9 screen, while others, which won't resize well to 16x9, will be shown at 14x9.


These tables show the aspect ratios, pixel dimensions, frame rates and scan options for HDTV
and SDTV (digital Standard Definition Television).

HDTV

Aspect

Active Picture

FPS / Scan

16x9 (1.78:1)

1920x1080

30p, 29.97p, 24p, 23.976p,
30i, 29.97i

16x9 (1.78:1)

1280x720

60p, 59.94p, 30p, 29.97p, 24p, 23.976p

SDTV

Aspect

Active Picture

FPS / Scan

16x9 (1.78:1)
4x3 (1.33:1)

720x483

60p, 59.94p, 30p, 29.97p, 24p, 23.976p,
30i, 29.97i

4x3 (1.33:1)

640x480

60p, 59.94p, 30p, 29.97p, 24p, 23.976p,
30i, 29.97i

Obviously the higher the pixel density the sharper the image. Tests have also shown that a progressive image has significantly higher perceived image quality when compared to interlaced images. A 720p image is perceived as 30 percent sharper and clearer than 1080i, while requiring roughly the same bandwidth to transmit. However, broadcasters are arguing for the interlaced format because it requires less bandwidth to transmit and allows the interleaving of other data with the image. Progressive transmission requires that each frame be transmitted as a whole, so no other data can be interleaved with the image.

How to Master?

The most practical way to master a show for all these various delivery formats is 1080p/24. This because it is easy to then derive all other dubs from that master element at the highest possible quality. Due to the nature of MPEG encoding, any pulldown in a show will need consistent cadence from the top of the program. Making a 30i dub from 24p is easy. Making a 24p dub from 30i would be problematic. There are separate issues for shows originating on either film or video, when mastered at 30i. Going from 30i to anything else requires one to deal with artifacts arising from interlacing and frame blending to go from 30 to 24. 24p is virtually universal... hence Avid's 24p Universal option for some models.

What Avid is Doing?

Avid 24p systems, other than the Avid|DS HD, are not HD devices and are displaying current broadcast standards (720x486/NTSC and 720x576/PAL) with varying degrees of compression, ranging from 35:1 to 1:1 (uncompressed). The Avid Symphony with the Universal option is capable of working at 24 frames per second using progressive media, at several resolutions, including 1:1. Avid Media Composers with the Universal option are 24p offline systems working at 14:1, 28:1 and 35:1.The principal advantages that arise from this methodology are...