Sensors, codecs and bitrates

Biting into bitrates

With a need to clear up the confusion of differing recording standards, and the need to ensure footage maintains a high enough level of quality through the encoding and transmission chain from the station to the viewer, broadcasters started to demand not just HD resolutions, but also higher bit rates and colour spaces. They have also started to set minimum standards for camera sensor size.

In the SD broadcast world, for a long time the standard news camera sensor was 2/3 inch and the standard video formats usually Sony’s DVCAM or Panasonic’s DVCPro. But the move to HD and to solid state cameras produced much more variety, prompting broadcasters to be more specific about what they need. This led to frequently cited “rules”, such as the BBC’s requirement for 50Mpbs, 4:2:2 colour, and a camera with at least three, 1/2 inch sensors. Bitrate, in particular, is considered important for maintaining video quality during transmission; a wider colour space is beneficial for grading and effects work, such as chromakey.

This requirement is still stated by the BBC, and followed by many other broadcasters. But in practice, most broadcasters’ requirements are rather more varied.

There are plenty of cases where producers and commissioning executives have been willing to accept material that is technically filmed at a lower standard, especially 35Mbps footage from cameras such as Sony’s EX1 and EX3, which became popular because of their portability as well as their lower cost.

Workrounds, such as recording 50Mbps to an external recorder, also helped overcome the bitrate limitations. The BBC has tested cameras such as Panasonic’s micro four thirds AF101, and allows its use, with external recorders, and more programmes are making use of footage from action or POV cameras, such as GoPros.

But producers who wanted “native” 50mbps found their choices limited to high-end cameras such as Sony’s PMW-500, or Panasonic cameras recording 100Mbps intra-frame. Then Canon entered the 50Mbps market with its XF100/105 single sensor, and XF300/305 three-sensor cameras. These cameras proved popular, and the BBC bought a quantity of XF305s camcorders in 2010. But, on the surface at least, the XF305, with its 1/3 inch sensor, fell short of the BBC’s technical requirements, despite the all-important 50Mbps bitrate.