The six most important specifications to know about a codec are: codec type, resolution, compression, GOP, bits and color sampling.
Type of codec:
here come the little names of maras. H.264, MPEG-4, MPEG-2, H.265 … will give us an estimated indication of the efficiency of the codec, although as I indicated above, be careful because it can be misleading. Nothing like comparing the material directly. Within each one, the rest of the specifications are defined below, there being generally different variants in each.
Resolution:
number of vertical and horizontal lines. Mind you, it is another one of those deceptive factors, the real resolution that a camera gives has little to do with the resolution of the codec, nor does any of this have to do with the sharpness. We will expand this in another chapter.
Compression:
In Mbps or Mbit / s, it indicates the information contained per second
GOP:
Group of Pictures, specifies the order in which images are stored. It can be Intra, where each image is independent, or employ various methods where an independent reference image is used and others are stored next to it containing information regarding movement compensation. That is, it stores an image, and the differences in a certain group of the following, until you have a complete image again. Example: GOP12, if we record at 24fps, it will contain two complete images at half-second intervals, and 11 will start from it, only saving the differences from the whole image.
Bit Depth:
the more bits, the more information we will have available, allowing us a more aggressive grading and thus avoiding banding.
Color sampling:
Broadly speaking, it indicates the way in which the chrominance is compressed. We will make a chapter dedicated to this, since it is a complex and important factor.
Codec Standards
Each codec has a series of variants within it, and sometimes these variants are used without specific names or certain new variants. There are some codecs however whose specifications are already fully predefined. For example, there are the well-known Apple Prores, used in assembly regardless of the codec with which we record (some professional cameras and external recorders have it incorporated), or the most used in AVCHD and XAVC S consumer cameras.
However, we must bear in mind that even with a predefined specification there can be huge differences from camera to camera. And there are many other factors to consider, since the internal processing of the video will be crucial for the subsequent compression process.
The (little) importance of bitrate
Bitrate defines the data that will be saved per unit of time, usually expressed in Mbps (megabits per second, not to be confused with megabytes). It may seem a priori that a greater amount of data per second means higher quality, but here comes the codec efficiency factor, and the truth is that we must compare the material directly (and without extra compression from YouTube or Vimeo) to Really see the differences.
An example can be seen in the Panasonic GH4. This camera has many bitrates to which we can record and also several resolutions. Something curious, is that if we record at Full HD, we can use a bitrate of 200 Mbps, while if we record in 4k, the bitrate is 100 Mbps. This can lead us to think that if the material is going to be broadcast in Full HD, it will be better to use this resolution and the higher bitrate. However, the reality is that it is better to rescale the 4K and use its lower bitrate: we will get better quality. It can be easily observed even with the compression of Vimeo in the video by Andrew Reid.
Another example is the Canon C100, a camera with very striking specifications and really good results. It uses the AVCHD codec (MPEG-4 AVC 25Mbps 4: 2: 0), which has never had a good reputation. Its quality is such that the differences with its older sister the C300 (MPEG-4 50Mbps and 4: 2: 2 color sampling) are negligible. Even using an external recorder like the Ninja Atomos, the differences are almost nil, even if it uses Prores and the output is 4: 2: 2. What’s more, broadcasting in Full HD its sharpness is superior to a GH4 with 4K rescaling. It is the magic of video.
