Technology for slow-motion is evolving fast. Adrian Pennington gets up to speed on a trend toward ever-increasing frame rates that is enhancing sports production at a breakneck pace
Replays are an essential component of sports coverage. They are increasingly used to deliver the best possible angle on the action. A typical Premiership match might feature five slow-motion positions, while Sky Sports will field 11 for each of the Ashes cricket matches this summer. But not all slo-mo cameras are created equal and the limits of what is technically possible - or practical - vary between applications and manufacturers.
“Slow-motion replays help a director tell a better story, show the action in more detail, help with analysis and can bring the emotion of participants or spectators closer to home,” says SIS LIVE engineering manager Adrian Kingston.
Standard high-def slo-mo captures video at 150 and 180 frames per second (fps) - three times the 50 and 60fps rate of normal HD cameras. Both the Sony Super Slo Motion HDC-3300 (introduced for Sky’s coverage of the Ryder Cup in 2006), and Grass Valley’s LDK 8300, of which 50 were deployed during the Beijing Olympics, record at this rate and have become fixtures on the sporting circuit.
The BBC’s Wimbledon coverage, produced by SIS LIVE, featured eight Sony 3300s including two each on Centre Court and Court One, providing a wide shot of the whole court and a close-up angle at one end. A third was positioned opposite the umpire’s chair on Centre Court to catch ‘personality’ shots of the players at rest.
“If you play back material at three times speed, you are getting a perfect rendition of motion,” says Kingston.
Nonetheless, since it can take three seconds to replay one second of slo-mo material, it is sometimes advantageous to capture at twice speed.
“Although you are losing some effect of the super-motion, you are saving time getting the clip to air, which matters if a director wants to insert a replay (such as between points in a tennis match) before the next action begins,” says Kingston.
Fractions of time such as this are what slo-mo technology is built on. A 3x recording is considered optimal for most sports, not only because it delivers a smooth image but because the time it takes to replay the sequence fits snugly into pauses in the live action.
“A 4x speed product is technically possible but visually it would add nothing new,” says Paul De Bresser, product manager broadcast cameras, Grass Valley. “Because of the way our eyes and brain receive visual information, its quality would be worse.”
Handily, when not used for slo-mo, triple speed cameras can be switched to shoot normally and operate like any conventional OB camera. However, greater versatility is being called for.
“Ideally, the camera chain would be agnostic so that each camera could be switched to shoot 1x, 2x or 3x, and all at an economical cost per camera chain,” says Visions contract director Brian Clark. “That would be a massive advance. Instead of fixing a position for the entire game, directors would have the ability to swap around which positions they wanted to record in slo-mo at half time or, indeed, at any stage of sporting event coverage.”
Slo-mo cameras do not come cheap and their use is dictated by the broadcaster’s budget. However, the greater part of the additional cost is absorbed by a hard-disc recorder. In most cases, this means a six-channel EVS machine, bearing a £100,000 price tag. Each 3x camera would occupy three channels, tying up multiple EVS machines per event.
Ironically, specialist cameras that capture at far higher frame rates take up less space on an EVS server, because they record to a RAM unit built into the camera itself. However, as they are a niche technology, those cameras cost considerably more and are usually hired.
The most familiar is the ARRI Hi-Motion, in regular use at Premiership matches, the TT motorcycle race (North One for ITV), flagship equestrian events such as Hickstead and Royal Ascot (Sunset + Vine for the BBC) and on court for the second week of Wimbledon. Although only 17 exist worldwide, eight are in the UK.
“It’s a different animal to the super slo-mos and will be positioned differently,” explains Kingston. “It is used less frequently because the imagery takes longer to play back but the clarity of detail it gives often astonishes athletes. It shows them technique and muscle movement that can’t be caught with the naked eye.”
ARRI’s Hi-Motion records at 12fps-300fps and 600fps. At its top speed, every second generated takes 24 seconds to replay, making it unsuitable for snappy instant replays, but ideal for a replay that can be teed up for longer breaks in play.
“On sports such as football, rugby and boxing (deployed by Sky Sports weekly), the cameras are operated up to 300fps, whereas on sports such as cricket or golf, which have longer pauses in play, they typically run at 600fps,” says ARRI Media digital high-speed manager Andy Hayford.
Since April, the camera has been more tightly integrated with the EVS server, widening its OB application.
“Previously, an EVS operator had to operate across two different control panels, one for the Hi-Motion record/replay and one for the EVS to clip up and play into the live transmission,” says Hayford. “With the release of Multicam 10, the operator can work on one EVS panel, which speeds up the workflow.”
The ARRIs are robust, but aren’t the last word in ultra-motion photography. Belgium firm i-Movix develops SprintCam V3 HD, which records 500-1,000fps, corresponding to a replay speed 20 to 400 times slower than usual speed (a one second shot at 1,000fps takes 40 seconds to replay). An SD version of the camera can capture at 500-10,000fps. Its systems have been used by NBC during the recent Olympics, Sky Sports’ darts coverage and extensively by Canal+ for its major football OBs.
Then there’s Vision Research’s Phantom camera, which has until recently proved more suitable for action sequences (such as explosions) in feature films or commercials than live sports environment. Its 1,920x1,080 pixel sensor matches a 35mm film frame so it works perfectly with film lenses and other accessories.
The most recent iteration (version 12.1) features two HD-HDI ports, which can be used together for 4:4:4 video out or independently at 4:2:2, a set-up that lets operators simultaneously feed replays to the broadcast truck while still viewing and capturing new footage at the start of each play. Replays can be captured at 6,933fps, the camera’s top speed at 720p resolution. With a reduced resolution, shots have been broadcast at more than 65,000fps. The maximum rate is claimed to top one-million fps.
The Phantom has been modified by US firm Inertia Unlimited to work more fluidly in OBs. NBC used a number of its ‘X-Mos’ on the last Super Bowl while OB firm CTV is currently deploying the world’s first radio-link (cable-free) unit for coverage of the European Golf Tour. It is also likely to feature during CTV’s Ashes production for Sky alongside the ARRIs, Sony super-slos and a number of infra-red and stump cameras.
The two issues holding back ultramotion cameras are the time needed to process the images for playback to air (something likely to be improved as data rates increase between camera head and server), and light.
Since the available light per second is being chopped into hundreds of frames, the cameras’ operation is limited to daylight or extremely well-lit indoor venues.
“These cameras are hungry for light and they can begin to show artefacts [errors] at high speed,” notes Clark. “However, development on them is very rapid considering they have only become available in the last couple of years.”
Tech Tricks: Flicker-free images
When events are held under artificial lighting, such as floodlights, the level of lighting generally varies with the sine wave of the mains power. A problem arises when the television frame rate is not the same as the power frequency, as it is when shooting at the normal rate of 3x.
“Each frame is now integrating only a part of the sine wave, and the light level in each will be different,” explains Paul De Bresser, product manager broadcast cameras, Grass Valley. “When it is played back as slow-motion, the flicker is clearly visible.”
The solution Grass Valley integrated into its LDK8300’s is to automatically detect where a certain area in the frame appears to flicker, and apply compensation to the whole frame so that it creates a perceived flicker in parts of the frame that did not originally flicker.
“The technology also lets the camera take care of different lighting sources and areas with different flicker behaviour,” he says. Sony has also integrated an anti-flicker algorithm to its cameras.