Editing Digital video (section)

==Overview ==
Digital video comprises a series of [[digital image]]s displayed in rapid succession. In the context of video, these images are called [[film frame|frames]].{{efn|In fact the still images correspond to frames only in the case of progressive scan video. In interlaced video, they correspond to fields. See {{slink||Interlacing}} for clarification.}} The rate at which frames are displayed is known as the [[frame rate]] and is measured in [[frames per second]]. Every frame is a digital image and so comprises a formation of [[pixel]]s. The color of a pixel is represented by a fixed number of bits of that color where the information of the color is stored within the image.<ref>{{Cite web |last=Winkelman |first=Roy |date=2018 |title=TechEase, What is bit depth? |url=https://etc.usf.edu/techease/win/images/what-is-bit-depth/ |access-date=2022-04-18 |language=en-US}}</ref> For example, 8-bit captures 256 levels per channel, and 10-bit captures 1,024 levels per channel.'''<ref>{{Cite web |last=Steiner |first=Shawn |date=12 December 2018 |title=B&H, 8-Bit, 10-Bit, What Does It All Mean for Your Videos? |url=https://www.bhphotovideo.com/explora/video/tips-and-solutions/8-bit-10-bit-what-does-it-all-mean-your-videos}}</ref>''' The more bits, the more subtle variations of colors can be reproduced. This is called the [[color depth]], or bit depth, of the video.

===Interlacing===
In [[interlaced video]] each ''frame'' is composed of two halves of an image. The first half contains only the odd-numbered lines of a full frame. The second half contains only the even-numbered lines. These halves are referred to individually as ''fields''. Two consecutive fields compose a full frame. If an interlaced video has a frame rate of 30 frames per second the field rate is 60 fields per second, though both part of interlaced video, frames per second and fields per second are separate numbers.
[[File:Pavek_Museum_-_TV_Camera.jpg|thumb|A broadcast television camera at the Pavek Museum in Minnesota.]]

===Bit rate and BPP===
By definition, [[bit rate]] is a measurement of the rate of information content from the digital video stream. In the case of uncompressed video, bit rate corresponds directly to the quality of the video because bit rate is proportional to every property that affects the [[video quality]]. Bit rate is an important property when transmitting video because the transmission link must be capable of supporting that bit rate. Bit rate is also important when dealing with the storage of video because, as shown above, the video size is proportional to the bit rate and the duration. Video compression is used to greatly reduce the bit rate while having little effect on quality.<ref>{{Cite book |last=Acharya |first=Tinku |url=https://www.worldcat.org/oclc/57585202 |title=JPEG2000 standard for image compression: concepts, algorithms and VLSI architectures |date=2005 |publisher=Wiley-Interscience |others=Ping-Sing Tsai |isbn=0-471-65375-6 |location=Hoboken, N.J. |oclc=57585202}}</ref>

Bits per pixel (BPP) is a measure of the efficiency of compression. A true-color video with no compression at all may have a BPP of 24 bits/pixel. [[Chroma subsampling]] can reduce the BPP to 16 or 12 bits/pixel. Applying [[JPEG]] compression on every frame can reduce the BPP to 8 or even 1&nbsp;bits/pixel. Applying video compression algorithms like [[MPEG1]], [[MPEG2]] or [[MPEG4]] allows for fractional BPP values to exist.

====Constant bit rate versus variable bit rate====
BPP represents the ''average'' bits per pixel. There are compression algorithms that keep the BPP almost constant throughout the entire duration of the video. In this case, we also get video output with a [[constant bitrate]] (CBR). This CBR video is suitable for real-time, non-buffered, fixed bandwidth video streaming (e.g. in videoconferencing). Since not all frames can be compressed at the same level, because quality is more severely impacted for scenes of high complexity, some algorithms try to constantly adjust the BPP. They keep the BPP high while compressing complex scenes and low for less demanding scenes.<ref>{{Cite book |last=Weise |first=Marcus |url=https://www.worldcat.org/oclc/1295602475 |title=How video works |date=2013 |others=Diana Weynand |isbn=978-1-136-06982-6 |edition=2nd |location=New York |oclc=1295602475}}</ref> This way, it provides the best quality at the smallest average bit rate (and the smallest file size, accordingly). This method produces a [[variable bitrate]] because it tracks the variations of the BPP.