Bit Depth

Bit Depth

When working with digital video, another big factor in quality is bit depth. Its a way of specifying the resolution in an image by measuring the number of bits devoted to each component of the pixels in the image. There are two main flavors:

• 8-bit
• 10-bit

8-bit

The number of levels in an 8-bit video signal is 256 levels per channel. The scale that's used spans from 0-255. The "legal" range that is used for broadcast is from 16-235, 16 for black and 235 for white.

Many video formats utilize 8-bit encoding including:

• D1
• DV (Mini DV, DVCAM)
• DVCPRO (25 and 50)
• HDV
• DVCPRO HD
• HDCAM (not HDCAM SR)

Data rates of 8-bit video are substantially lower than 10-bit video, which may be a consideration when determining a post production workflow.

Pitfalls of 8-bit video:

• Not as good for applications like chroma-keying.
• Gradient effects may suffer from banding (contouring) where noticeable steps can be seen from one shade to another.

The above 8-bit gradient is producing visible "steps" from one shade to another, referred to as banding.

10-bit

The number of levels in a 10-bit video signal is 1024 levels per channel, 4 times the precision of 8-bit video. The scale that's used spans from 0-1023. The "legal" range that is used for broadcast is from 64-940, 64 for black and 940 for white.

Some of the video formats that utilize 10-bit encoding include:

• Digital BETACAM
• D5 (and HD-D5)
• HDCAM SR

Additionally, when film is digitized at a high resolution (2K or 4K) for a digital intermediate (before eventually printing out to film), the files are often created at 10-bit to maintain the full range of exposure values.

10-bit video is superior in many ways to 8-bit. Advantages of 10-bit video come in the form of better gradient effects with less possibility of creating banding artifacts. 10-bit images can also be pushed further in post production without easily producing artifacts- for example: pushing images to be much higher contrast than the original photography, as well as more successful chroma keying.

We won't be able to show you a good screen shot here on the web representing a 10-bit gradient in all its glory, but when you're producing images for TV and film you'll be better able to see the full benefits.

Bandwidth Differences

You would think that if 10-bit video is a superior resolution to work at in post production that we would never opt to work at 8-bit (even if the video originates in 8-bit). But data rates (bit rates) of the video will rise with 10-bit over 8-bit, requiring faster and bigger hard drives for storage. Below are a couple of examples.

8-bit SD uncompressed video will have the following specs:

• Data rate is about 21 MByte/sec.
• Storage is about 1.25 Gigabytes per minute.

10-bit SD uncompressed video will have the following specs:

• Data rate is about 28 MByte/sec.
• Storage is about 1.7 Gigabytes per minute.

8-bit HD video (1080i60) will have the following specs:

• Data rate is about 125 MByte/sec.
• Storage is about 7.5 Gigabytes per minute.

10-bit HD video (1080i60) will have the following specs:

• Data rate is about 165 MByte/sec.
• Storage is about 10 Gigabytes per minute.

32-Bit Floating Point

Although current video formats are limited to 8 or 10-bit depth, many post production applications can process the data of these formats at higher bit depths. Although the final output will still be 8-bit or 10-bit, it may help when integrating visual effects to process at a higher bit depth. For effects that require heavy image manipulation like chroma keying and other image stressing operations, or when layering multiple effects, processing (rendering) at a higher bit depth can possibly create more precise results.

32-bit Floating Point processing refers to processing data using floating point math, which preserves fractional values during processing. Although very precise and high quality, the processing demands will cause longer render times. So not all processing needs to be done at the best bit depth possible. 10-bit video with effects will most often look pristine using 10-bit processing.