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18
May
posted by - Wednesday, 18 May 2011

It occurred to us when creating the Camera Mounted Recorders comparison chart that there is often confusion around different HD formats. In 2009, I wrote up a blog titled Making Sense of HD Formats that covered the different HD formats used today. But what I didn’t go into was those other words and numbers that we often see associated with a video compression format: bit rate and bit depth. Don’t stop reading just yet, I promise to keep this simple and you’ll see that these numbers may actually mean something when it comes to your next production.

Most professional HD video cameras today have an HD-SDI (High Definition – Serial Digital Interface) output, so this will be a good place to start the discussion. An example of a typical HD-SDI video output would be a 1080 60i Uncompressed 10-bit signal. If you are scratching your head already, let’s break it down a bit with a couple definitions.

Bit Rate:

Sometimes also referred to as Data Rate, this is a term used to describe the amount of digital information (bits) that is conveyed or recorded per unit of time. In our world, this is typically expressed as an amount of bits per second (bit/s) that make up the digital video signal or recording. The higher the bit rate, the greater the amount of information being transmitted, and generally speaking the higher quality the video signal. When talking about a compressed video format such as DVCPRO HD or XDCAM, the bit rate refers to the amount of data recorded in a second. As the bit rate of a given compression increases, so does the amount of data recorded in a second.

For example: the XDCAM EX format in its highest quality mode has a data rate of 35 Mbit/s (35 Million bits per second), which translates to about 50 minutes of footage per 16 GB SxS card.

Bit Depth:

Though the word ‘bit’ is also used in this term, Bit Depth actually describes something completely different. Bit Depth, aka Color Depth, describes the amount of information stored in each pixel of data. As you increase bit depth, you also increase the number of colors that can be represented. In the case of an 8-bit RGB image, each pixel has 8-bits of data per color (RGB), so for each color channel the pixel has 28 = 256  possible variations. In the case of a 10-bit RGB image, each color channel would have 210 = 1024 variations.

That wasn’t so bad, so now let’s go back to the original example of the 1080 60i Uncompressed 10-bit video stream:

1080 60i – This is the video format, which has a resolution of 1920 x 1080 and a frame rate of 60 interlaced fields per second (59.94 to be exact).

Uncompressed - This refers to the fact that there is no compression being applied to the video signal, but it also refers to the Bit Rate of the video signal. An uncompressed video signal has a bit rate of 1.485 Gbit/s, which is a whole lot of data. In fact, it would fill up a 256 GB hard drive in just under 22 minutes.

10-bit – This is the bit depth of the signal, so each pixel has 210 = 1024 variation per channel.

Another example would be the XDCAM EX codec, which in 24p we would describe as a 1080 24p 35Mbit/s 8-bit compression.

1080 24p – This is the video format, which has a resolution of 1920 x 1080 and a frame rate of 24 (23.98) progressive images a second.

35 Mbit/s – This is the bit rate of the video format, which is greatly compressed from the uncompressed video signal. The compression makes the video much smaller to store on a hard drive; on a 256 gb hard drive I could store up to 800 minutes of footage in XDCAM EX format. Many people believe that increasing bit rate also means increasing quality, however the quality of the image is greatly dependent on the compression type, which can often deliver excellent results at much lower data rates. This is the case with XDCAM EX or the AVC HD / AVCCAM formats available in many cameras today.

8-bit – This is the bit depth of the signal, so each pixel has 28 = 256 variations per channel.  While an 8-bit type of compression reduces the number of color variations, XDCAM EX cameras, as well as other camera’s with 8-bit compression, are designed to minimize any effects that are associated with an 8-bit color space. The good news is that many cameras with 8-bit recording formats also output a 10-bit HD-SDI signal. Combined with a camera mounted recorder, you can greatly increase the overall quality of your recording.

I hope this guide has helped clear up any mysteries about bit rate and bit depth. In the future we will also cover chroma sampling and color space.

 

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