Solving compression, conversion issues

Post, July, 2004 by Marc Loftus

SAN FRANCISCO -- Post pros doing format conversion or time expansion and compression will often notice that their soundtracks have been negatively affected. The most common problem comes from adverse pitch changes.

In 2002, Dolby introduced the Model 585 Time Scaling Processor, a realtime audio scaler designed to work with multichannel soundtracks, providing high-quality pitch correction. The unit accepts multiple channels of PCM or up to eight channels of decoded Dolby E or Dolby Digital and metadata. According to the manufacturer, audio tracks can be pitch shifted up or down by as much as 15 percent. In the time scale mode, they can be made 15 percent longer or shorter while maintaining the proper pitch.

Gary Fradkin, director of engineering for LA's SSI Advanced Post (www.advancedpost.com), first sought out a solution for pitch changes while working on the PAL conform for the first Lord of the Rings DVD. Fradkin says the NTSC to PAL conversion presented a 4.1 percent change in speed, and brought out a noticeable pitch change in the Rings soundtrack.

"When you are dealing with heavy orchestral and dialogue at the same time, it is very noticeable," notes Fradkin. "You get an 'underwater' kind of effect. We had to do a pitch change and were having trouble with the existing equipment. So we ended up going to a Pro Tools plug-in, which took a long time. You had to bring in all of the audio and that took one pass in realtime, then process the audio and output the audio again."

The studio bought its first Model 585 soon thereafter and today has three units that it regularly uses.

While SSI was once exclusively an audio post house handling theatrical trailers, it's since expanded into telecine services, performing color correction, transfers and downconversion. The facility is performing D-5 high definition downconversions for Sony/Columbia Pictures and is calling on its Model 585s to maintain sound quality.

"From a D-5 you can make an NTSC and a PAL [downconversion]," says Fradkin, "so for the PALs that we were making for Columbia, we were having to pitch change those also. That's when we bought more 585s, because the applications were becoming greater."

Fradkin says the studio works from the 5.1 final mix/master. The Model 585 works well on multi-channel assignments, he notes, because it is optimized for 5.1 output.

Ryan Guardino, engineer/technical supervisor at Santa Monica's 2G Digital Post, says the studio regularly uses its Model 585 to correct pitch changes incurred through time compression. The studio edits features for broadcast and airline use. And with broadcast projects in particular, timing has to be precise, so the studio will use its Model 585 on the audio of projects being time compressed in order to correct resulting pitch changes. According to Guardino, pitch shifts are most noticeable with deep voices and in scenes that feature both music and dialogue.

2D Digital Post regularly works with Fox, Sony and MGM. Prior to acquiring their Model 585, 2G Digital (www.2gdigital.com) would send troubled soundtracks out of house, on Digi Betas, and would lay them back once they returned. In the year since the studio took delivery of the Model 585, it's used it on approximately 50 titles, both in HD and SD.

Man on Fire, for example, underwent a three percent change in timing through compression while Teaching Mrs. Tingle was compressed by 2.9 percent.