Aspect Ratio Confusion In HD

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Aspect Ratio Confusion In HD

16x9 HD video is often referred to in one of two ways:

• "Squeezed" (or anamorphic or partial frame)
• "Full Raster" (or full frame or un-squeezed)

Although all HD video is displayed as 16x9, most formats do not actually record a true, full raster 16x9 image. There are only two square pixel, full raster 16x9 frame sizes:

• 1920x1080
• 1280x720

Instead, most HD images are usually recorded to tape in an anamorphic or "squeezed" mode, which has become a common way among manufacturers of HD equipment to lower the bandwidth of the video- a smaller frame equates to less data to process and transmit. As we saw when looking at standard definition 16x9, the video is recorded on to a more square-like frame, and is stretched out on display. This is the same overall concept with most HD (although the amount of "squeezing" is different with the various HD formats).
 

Let's examine a few 1080 and 720 HD formats to get an idea of what's going on with different formats.

From the chart above, you can see that HDCAM SR records a true, full raster 16x9 image natively. Also, 720p HDV records a true 16x9 image. Most other above formats are employing some type of anamorphic (squeezed) recording. In other words, they use non-square pixels to record the image, although each format is using a slightly different pixel aspect ratio.

When you work with video uncompressed, it is implied you are working at the full, square pixel 16x9 dimensions (without any squeezing in the horizontal direction). When working "natively" with compressed 16x9 video, you are working with the video at its native recorded, non-square pixel frame size (squeezed), yet its displayed in the editing system or to a monitor as 16x9 (its un-squeezed for display).

A couple new formats that are also listed above include Panasonic's AVC-Intra and Sony's XDCAM EX. Both of these formats allow for true 16x9 recording. Both Panasonic and Sony have two flavors of these new formats. Choosing the higher quality format from Panasonic or Sony records full 16x9 images, either 1920x1080 or 1280x720. In addition, there are even consumer level HD cameras that record a true, full raster frame size.

Important Note: In Final Cut Pro, we refer to the anamorphic option specifically only when working with standard definition 16x9 video. When working with HD video in Final Cut, the aspect ratio is handled automatically and you should never choose the anamorphic option in your audio/video settings. Instead Final Cut uses something called Pixel Aspect Ratio to interpret these formats. Uncompressed or true 16x9 HD uses square pixels, but these squeezed formats use a non square pixel aspect (not the same as SD non square pixels).

So, although a true 16x9 frame size would be 1280x720, we can see that the DVCPRO HD format shown above is using a frame size of 960x720. It’s the Pixel Aspect Ratio that ensures that this video is interpreted/displayed as 16x9.

Quality And Squeezed HD

Because the frame is compressed horizontally, the end result is less resolution and detail in the horizontal direction than when working with true 16x9 video.