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Fri 19 Mar 2010 04:00 AM

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Choosing the right codec

As quality lost at the beginning of a production chain can never be recovered, end users should always opt for the best possible codec, says Ron Veel from Grassvalley.

Choosing the right codec
Although in an ideal world, it would be wonderful to set a standard in a camcorder and never transcode or rewrap until the moment of delivery, we have to acknowledge that the acquisition codec, the editing codec and the transmission codec are going to be different and optimised for each area.
Choosing the right codec
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As quality lost at the beginning of a production chain can never be recovered, end users should always opt for the best possible codec, says Ron Veel from Grassvalley.

When we talk about cameras in the digital world, what we actually need to consider is the file format. We’ve had digital technology, in the form of SDI, for 25 years now. What is transforming our world is the ability to handle content as digital files, where material can be moved around standard IT networks, moved faster than real time, and stored on inexpensive server arrays.

The challenge is that, while analogue video and even SDI were fully standardised and uniform, there are many, many ways to compress digital content and package it as a file. One leading asset management system has more than 180 different flavours in its library, and that is certainly not all the possible permutations of video format, codec, bitrate and wrapper.

Some of these variations are designed for different applications. If we take a self-contained Quicktime sequence, for example, it has three variants. MXF uses the frame wrapper, which as its name suggests contains a video frame with its associated audio, then the next video frame and audio, and so on. That makes it ideal for playout where you might want to start and stop at any point within the sequence, not at the beginning and end.

But it is challenging to write in a low power device, so many ENG cameras use the clip wrapper, which writes all the video frames in a shot followed by the audio tracks. This is also supported by MXF.

The third is a compromise version, the mixed wrapper, which has a block of video frames followed by the audio, then the next block of video frames and so on. Final Cut Pro usually produces Quicktime in this form.

Certainly standards would help in this area. MXF was drawn so widely that files generated by one device could be “MXF compliant” and utterly incomprehensible to another vendor’s “MXF compliant” system. Turner Broadcasting has taken important steps to move MXF towards a more standardised format, but it is still not universally recognised.

In an ideal world, it would be wonderful to set a standard in the camcorder and never transcode or rewrap until the moment of delivery. For now, we have to acknowledge that the acquisition codec, the editing codec and the transmission codec are going to be different and optimised for each area.

It is an obvious point but worth repeating here: quality lost at the beginning of a production chain can never be recovered. So we should acquire using the best possible codec, with the best budget we can afford. We should not be forced into quality compromises just because we are recording a file rather than to linear video tape.The television standard has been set for some time now as 10 bit 4:2:2 digital sampling and its equivalent in HD. Digital cinematography applications may go even higher, but this is a good standard for television.

10 bit 4:2:2, in high definition, can be captured using AVC Intra or JPEG2000, and for premium work, I would suggest that these are the codecs to be considered. Eight bit quantisation, or 4:2:0 or 4:1:1 colour sub-sampling, will produce visibly inferior image quality in most productions, and will cause big problems in green screen work.

The difference between AVC Intra and JPEG2000 is that the first is an MPEG-type codec which splits the picture into blocks, whereas JPEG2000 uses a different mathematical compression process, known as wavelets. The AVC Intra standard also encourages the use of temporal compression: using information from one video frame as the basis of other frames in a group of pictures (Gop). JPEG2000, on the other hand, is always an intraframe compression scheme with each individual frame complete in itself. This relieves the pressure on the editing system, as well as maintaining quality.

The other consideration to be made is that higher quality codecs use a higher bitrate, and higher bitrates mean bigger files. Just as quality should not be compromised because we have moved to file-based acquisition, we should also look at convenience. If we are used to recording 90 minutes onto a tape, then we should be able to record 90 minutes of files onto whatever recording medium we select.

Some applications, of course, will not require very high quality. News footage is unlikely to need sophisticated post production much beyond cuts only editing, and so lower grade video formats and codecs may well be appropriate, running at lower bitrates and thereby increasing recording time.

As well as recording capacity, the choice of bitrate will also be driven by constraints in the post production system. A one hour drama production with a 20:1 shooting ratio will generate around a terabyte of top quality raw footage, but with today’s commodity Raid arrays that is not an issue for a standalone editor.

On the other hand, news production means moving quickly, and so there is pressure to ingest the rushes into the production network and transfer the content to multiple editors as quickly as possible. That might lead you to choose a lower bitrate codec, to trade quality for convenience.

The important message, though, is that in planning for file-based environments the acquisition format or formats have to be the primary consideration. This sets the quality standard for the rest of the network, and it should be maintained as far along the pipeline as possible. With today’s technology, that bar can be set very high indeed, with 10 bit 4:2:2 high definition and excellent compression algorithms, now readily available in practical and affordable camcorders.

Ronny Van Geel is the camera products manager at Grass Valley.