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Mastering HD Audio Problems In Post
by David Fox
Theoretically, High Definition video should be easier to work with in mastering than Standard Definition, as you can generate just one master for all the formats you need from a single telecine run, whereas SD needed two (Pal and NTSC), "but it's not quite like that on the audio side," which "has caused us a lot more headaches than we initially envisaged," says Michael Ashley, head of facilities at 4MC, London, part of Liberty Livewire. About 10% of its work is in HD, which has grown quickly over the last few months, especially as production companies switch from film.
Sorting out the audio
"Video is fairly flexible, but they've neglected the audio side," says Taksim Salih, 4MC's restoration and HD business development manager. "Audio seems to have been an afterthought, but it's just as important as the picture."
"They seem to think that 'the audio guys can sort that out'," adds Chris Brent, sound technical support, 4MC.
Technical differences between early and current models of Panasonic's HD-D5 VCR mean that tapes created on one can be unusable on the other. Early models recorded four audio channels, but later versions also record eight. However, four-channel tapes won't playback audio or video on a machine set up for eight channels and eight-channel sound won't playback on early machines.
Some clients, such as CBS, prefer to use four-channel audio because of the mix of old and new machines, so staying with four tracks is a safe option, but others want 5.1 surround sound, which requires delivery on eight-channel tapes. "One of the first questions we ask when jobs come in is 'how do you want the audio?'," says Salih.
The first model also only did 50i and 60i frame rates, while the new model has the full range, from 24p. Although both HD-D5 and Sony's HDCAM can play out video at different frame rates, they don't automatically pitch correct the audio. This is not as much of a problem for viewers as being out of synch, but can be noticeable, especially for music.
"Traditionally, when you transferred 24 to 25 frames material from film you didn't do pitch correction, but now we have the technology to do it, so people want it," says Salih, but that does mean a visit to the audio suite.
4MC is heavily involved in reformatting, so does a lot of frame rate conversions. All the video tools are in place for this, but audio has been left behind, says Ashley. Its Snell & Wilcox convertors resample the audio and make sure it is synchronised, but don't pitch correct.
Of course, "pitch correction out of the VCR would be dangerous," admits Brent (pictured left). As long as synchronisation is correct, they can always do any pitch correction necessary to suit the programme. However, this adds another stage to the process, at extra cost for the client, so it has to be considered whether it is really necessary, which varies from programme to programme.
Ashley believes manufacturers haven't addressed the problem. "If the clients want to pay for it, that's fine, but most clients expect it to be sorted," he says.
Getting the audio in synch
Only a minority notice if the pitch is wrong, "but you've got to get the synch right," says Salih. "That's the really hard part," adds Brent, because the extra video processing HD needs causes video delay, pushing it out of synch with the audio.
On the HD-D5, setting up the audio for an insert edit is not as simple as merely getting it in synch and pressing record. Do this and you'll get an eight frame delay, because there is no automatic audio delay to compensate. As Panasonic hadn't notified users of the problem, it took a lot of trial and error to get it back in synch. "What should have been a four second insert took two hours. We had to do it blind until it worked," says Brent. If you are doing eight channels of audio with a 5.1 surround mix, each channel has to be checked. It doesn't help that the HD-D5's digital jog isn't good at scrub audio, making it difficult to tell if something is out of synch.
Then there is the problem of what to view the HD material on: plasma, projector or monitor. "All of them have different problems," says Ashley. An HD monitor is very expensive (about £15,000) and not very big, but doesn't add delay. Projectors not only add delay, but some don't work with 24fps, and there is no true-HD projector at a reasonable cost. That leaves plasma. "They are probably the best option. They are large, reasonably cost effective, but have delays of one to two frames." Added to the VCR delay, about ten frames must be added to the speaker output in its audio suites.
These are not significant factors in traditional sound dubbing suites (of which 4MC has three), but do cause problems in sound mastering (it has two suites for resynching, reversioning, pitch correction, restoration and re-editing).
Like many facilities, 4MC uses obsolete Adams Smith machine controllers in its audio suites, because of its abilities to cope with different timecodes and lock up machines doing varispeed playout. "It's a very useful piece of kit. Nothing else does it as well," says Ashley.
Problems with the HD-D5 timecode output make it difficult to control frame accurately. The Adams Smiths were designed for SD and have problems running with 24fps mastering timecode. The HD-D5 can do timecode interpolation from 24p, via its Longitudinal Timecode interface rather than the 9-pin output. Although the Adams Smith has an LTC reader, as it tries to edit it gets contradictory timecode coming out of the 9-pin port it needs for VCR control.
However, Panasonic has recognised the problem and written a software upgrade which sends the correct timecode via the 9-pin port.
4MC also has an upgraded Sony BVE 9000 edit controller, which can do 24fps, but can't do vari-lock.
The HD-D5 has a further glitch when it converts 24fps timecode to 30fps. It has to generate a virtual key number. "The timecode you look at and use as a reference isn't on tape. So, if that key number changes, all your offsets change," says Brent. Unfortunately, this happens. "It has suddenly jumped 12 frames when we've done nothing to the machine," possibly due to a software crash.
"I don't think [Panasonic] envisaged the machines being used in this way," he adds. HDCAM works differently, as its key number is always zero.
One way to overcome the problems would be to move to a disk system for processing, but that takes more time and money and Ashley would rather not do so for what should be a simple job.
To synch up and record six reels of film to HD takes about twice the runtime of the film, but using disks can double this again, so they tend only to use it if there is a lot of editing, restoration or repairs needed.
"There are quite a few headaches. Each job is a bit of a learning curve a the moment," says Ashley. "You've got to get the manufacturers working with you."