Sensors, codecs and bitrates

Technical standards and tiering

Inside track: an APS-C camcorder sensor, here from the consumer NEX-VG10. Picture: Sony

The answer lies in what the EBU describes as tiering. Although 50Mbps, 4:2:2 and three ½ inch sensors is the ideal for the BBC (and Sky likes 2/3 inch sensors), different programmes can use different standards. For news, 35Mbps is acceptable – and in some cases it can be preferred, because file sizes for 35Mbps footage are that much smaller. Long-form documentaries, though, require 50Mbps. In the BBC regions, 35Mbps is also acceptable, and regional news crews do use the Sony PMW-320, a 3x 1/2 inch, 35Mbps shoulder-mount camera.

To help clarify the situation, and provide some consistency for both broadcasters and programme makers, in January 2012 the EBU published a document, R118, which sets out equipment in “tiers”, that programme makers should use for working out which cameras to use for different types of production.

The tiers range from single sensor, large format cameras, through to professional shoulder-mount cameras, 50Mbps camcorders, and even consumer cameras. R118 also sets out the bitrates for cameras: 50Mbps for inter-frame and 100Mbps for intra-frame for general use, and 35Mbps/50Mbps for news.

Specialist cameras, though, such as action cameras or cameras for covert filming, can have a minimum sensor count of 3x 1/4inch and 720p output.

So for most mainstream HD broadcasting, the requirement is now fixed at 50Mbps and 4:2:2, with 35Mbps 4:2:0 for news or video journalism.

Camera makers have responded to this. Sony has joined Canon in offering 50Mbps recording with the PMW-100, PMW-150 and PMW-200 now filling out Sony’s range of 50Mbps camcorders, in addition to the PMW-500 and the optical-disc based XDCAM range.

Panasonic has its AVC Intra based line, such as the AG-HPX250 recording at 100Mbps. JVC, for the moment at least, is sticking with the ENG or videojournalism tier, with 35Mpbs recording on its GY-HM600 camcorder.

Bitrates, though, are not the only consideration. As well as colour space, sensor sizes can be an issue. The minimum standard for sensors for news or journalism under R118 is three, 1/3 inch sensors and for general HD programming, three ½ inch sensors. This rules out, on paper at least, single-sensor cameras such as Canon’s XF105 and Sony’s PMW-100, even though they meet the bitrate requirement. The BBC’s HD requirements do, though, allow for single sensors of 1 inch or above. This does suggest that for most projects, it is best to play safe with a 3x 1/2inch sensor block or a large-format, single sensor camera.

Non-broadcast formats – and non-broadcast work

Does this mean, though, that a “broadcast” specification camera is always needed? The answer, of course, is no – and that even applies to network broadcasting.

As well as 35Mbps MPEG-2, set out in its EBU R-132 document for acquisition codecs, the EBU explains that other codecs such as AVCHD might be used. According to the paper, “additionally, AVCHD above 35 Mbit/s 4:2:0 may be acceptable provided all post processing is carried out in the native camera codec”.

Currently, there are few cameras that can record AVCHD at such high bitrates, as the AVCHD standard only goes up to 28Mbps, and many cameras still use the lower, 24Mbps setting. For news and journalism, the EBU does say that 24Mbps, 4:2:0 may be acceptable.