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What Frame Rate?
Synch pulses
Camera noise
Audio Tracks

HD Doesn't Sound As Good As It Looks

by David Fox

Timecode differences and camera noise give sound recordists problems with High Definition on set.

What Frame Rate?

While High Definition video is essentially a single worldwide production format, the "frame rate gymnastics" needed to make it work in PAL and NTSC have considerable implications for audio, according to freelance sound recordist (and executive committee member of the Institute of Broadcast Sound), Simon Bishop, who works mainly in TV drama, but also movies, documentaries and commercials.

Cameras can shoot at a number of different frame rates. "If we don't know what the camera is doing, and what the post production is doing, we don't know how to record our sound." This means sound recordists "must keep a very close eye on the timecode," he says.

Timecode in HD is mainly the same for shooting as in other video, where the problems arise is in how it is dealt with in post-production. With file-based recording, it is easy to change timecode or the sample rate of something, "so, if you get it wrong on the set, it is not the end of the world." But, you should know what frame rate the camera is shooting at and what frame rate the edit will be done at (often not the same), and what frame rate the finished programme will be shown at. There is usually no difference, but Bishop has worked on programmes where all three have been different.

In Europe, everything is usually done at 25 frames per second, but a feature may be shot at 24 fps, edited on Avid at 25 fps (as the Media Composer doesn't do 24p), and shown in cinemas at 24 fps. A US feature can be more complex. The picture may be shot at 23.98 fps, edited at 24 fps and shown at 24, while the sound is shot at 29.97 fps, edited at 29.97, then pulled up from 59.94 to 60Hz to give 24 fps for presentation.

For multicamera work, the only reliable method at present is to use time-of-day timecode, either from an external timecode generator, or centrally distribute ToD or continuously-running timecode. He shot a number of concerts with 10 or 12 cameras running off central timecode. There was a lot of cabling, but it was reliable.

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Synch pulses

The BBC's recent HD drama, Rockface, was shot on location in the mountains of Scotland. The boom operator was roped to the rock and didn't want cabling. HD cameras use tri-level synch and can't see standard synch pulses. Although a cigarette packet-sized, Ambient ACL 202T tri-level box is now available, it was too late for Rockface. So, recordist Stuart Moser recorded an old lockit box onto one audio channel on the HDCAM and a guide track on to the other. There was an edit suite on location where, as the HD rushes were down-converted to PAL (DV) and Avid (and burnt-in VHS), the audio DAT chased the HD and the sound dub was made, so the editor had the DAT sound and the final EDL still related to the DAT tape. The guide tracks on the camera tape were never needed (although they were clean).

On Devil's Tattoo, shot and edited at 25 fps for potential theatrical release, recordist Brian Milken used timecode radioed from the camera to the audio recorder. In return, one channel of guide audio was radioed to the camera from the mix position.

On Star Wars Episode II: Attack Of The Clones, Bishop and Brian Simmons were the sound recordists for shots done in the UK. It used two HDCAM camcorders almost all the time, one usually on a Technocrane for speed of set-up. They did 29 set-ups on the two cameras on the first day (compared to three or four on a similarly effects-heavy shoot he did - Dinotopia), which really put the sound under pressure.

The central engineering station generated timecode at two different rates (23.97 for the HDCAM and 29.98 for audio and the video assists). Bishop sent in two channels of mixed audio from four on his Zaxcom Deva II digital recorder, then split and fed to the camera's rack-mounted back-up recorders (two for each camera - giving three master tapes for each), as well as to a downconverter. This introduced five frames of video delay, so that the audio had to be delayed also, for use by the non-linear video assist on the set. They used three frame rates for post: 29.97, 23.976 and the end result, 24 fps. However, all the extra work saved more than £1 million in stock costs alone, and he recalls one producer calculating that HD would have saved twice that on Band Of Brothers.

A Robbie Williams concert shoot Bishop did last year, with 12 HD cameras, recorded at 25 fps. Tri-level synchs plus timecode (generated by the audio truck) were sent by cables to all cameras, but if he did the same now he'd use Ambient lockit boxes and save on the cabling.

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Camera noise

Besides timecode, the other main difference in shooting for HD is noise - most of which comes from the camera. "The HD cameras we are working with at this time are noisy." Most contain one or more fans running "at inconvenient frequencies" - 12 kHz and 17kHz. "If you are in a small room doing intimate stuff with the Hi-Def camera, you are going to get noise," says Bishop. Most HD monitors also have fans in them.

On Devil's Tattoo, Milken also found the fans in the camera to be "a constant problem". He tried putting covers over the camera, but discovered that the HDCAM had a louder, second-level fan which switched on.

The "video village" that usually accompanies the shoot contains more undesirable fans and other noises. On Star Wars, there were two large buggies containing equipment - about 20 rackmount units, four rackmount HD recorders, four 24-inch HD monitors, all with there own fans, like a machine room for a large edit suite.

"It was possibly the loudest set I've worked on," says Bishop. "If it had been a dialogue-based drama, it would have been impossible to record usable dialogue." The central engineering station was particularly noisy. There was also a huge amount of cabling, including an "extraordinarily heavy" arm-width cable to each camera. "It is not nimble," he says.

To make a sound recordists life easier, Bishop would recommend having the camera head on tri-ax to a portacabin (or similar), housing the recorder and all the other equipment. The very last shoot he did on Star Wars worked this way and it reduced the noise considerably.

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Audio Tracks

These problems aren't helped by the fact that producers now dictate what the sound is recorded on. In the past, everyone used a Nagra, so equipment choice wasn't an issue. "Now it is common that sound recordists will find themselves mastering onto a camera. If they want to do a back up it's up to them and they are not paid extra for the equipment." He fears the consequences.

The choices are: to record onto the camera; onto something else; or both. With going direct to the camera, "the most important thing is to get audio recorded that is usable." However, with cables snaking across the set, possibly gathering interference from HMI leads, etc., as they go, the only way to know is to have a cable return. But if there is interference, is it on the send or the return?

There are only two easily available recording tracks on the camera (there are tracks three and four, but that needs another SDI unit that fits between the camera and the battery, and is unlikely to be available).

The alternative is to use a separate recorder and put guide tracks on the camera (so there is something useful for the editor), which is "a very much safer method of recording." However, it doesn't get rid of all the wiring, as send and return cables are still needed, "but they are no longer critical." There is also the implication of needing to synch up the sound later, but in return you get more creative possibilities during track laying and mixing by being able to record more than two tracks - if you have the equipment to do it.

Recorder choice is limited at present, says Bishop. If you want to record one or two tracks, there are: DAT, Nagra V, Zaxcom Deva II or the Fostex DV-40. DAT is linear and non-linear digital recorders are preferable, but they are only becoming available now. If you want more than two tracks, only the Deva II (which has four tracks) is currently in use as a portable machine (the DV-40 four and eight track machines are mains only). However, there are several four, six and even eight track units being introduced in the coming months.

July 2002

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David Fox