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5.1 Surround Sound editing using Avid’s Liquid 7.1 (Part 6)

Nov 21, 2006 12:31 PM, Steve Mullen

In the SD world, the high-quality audio option is stereo audio. While many HD productions continue to employ stereo, the norm is rapidly becoming 5.1 surround sound. Currently, there are several ways to create a 5.1 mix. Some editing paths enable you to build the surround mix in the DVD creation application. Other paths put the creation point earlier in the production stream, at the point where an audio editing application is used.

In both cases, the NLE is used to build a set of tracks that carry discrete audio. These tracks do not yet have spatial information — with the exception of stereo tracks. (More about this later.) However, Avid’s Liquid and Sony’s Vegas are able to create a six-channel spatial mix directly in a timeline.

For consumers, Dolby Digital (DD), AC-3, and 5.1 surround sound are all the same. During production, however, it is important to understand that these are not synonymous. Until 5.1 has been encoded using AC-3 — audio is carried by six discrete channels. In both Liquid and Vegas, even after spatial information has been added, the 5.1 soundtrack has not yet been AC-3 encoded.

Liquid forces the AC-3 encode to be done only during the production of a NTSC or PAL DVD. This limitation means that one cannot export HD MPEG-2 with an AC-3 bit-stream. Therefore, no simple path exists to either Blu-ray or HD DVD. (Nevertheless, using a “shareware” tool the AC-3 audio can be obtained.) Vegas can, however, output MPEG-2 video and AC-3 audio as separate files.

Spatial Issues

In the early '60s, I worked for a small record label where I took pride in stereo recordings that had excellent ambience. My model was the Living Presence series produced by Mercury Records (www.mercurylivingpresence.com). These classical records were produced using only three Telefunken U-47 tube-condenser mics that fed a three-channel, 1/2in. Ampex tape recorder. Naturally, I was disappointed when the recording industry moved to multi-track recorders with a mic stuck to, or shoved into, every instrument.

The advantage of multi-channel recording for the producer is that each track is an “element” with little or no ambience. This makes any slice of sound interchangeable with another slice. Since ambience in a 5.1 mix is typically created by tools rather than by nature, the introduction of “real-world” ambience conveyed by stereo recording can be a problem. (In fact, even stereo converted to mono has more ambience than desired.) I found that for music, stereo seemed to work only when placed in only the rear channels. Here the ambience worked to the advantage of the mix. All other mix elements should be captured as mono with little or no ambience. That suggests lav mics rather than boom mics, for example.

Building Stereo Tracks

For my tests, the first step was to create a set of tracks that would be used for stereo sound. I created three tracks: Sync A 2-ch, Sync B 2-ch, and Aux Stereo 2-ch. The latter held a 1kHz tone (with bars) plus music created by Liquid’s built-in SmartSound application. The A and B tracks were used when I needed to overlap sync audio.

After placing and trimming all audio clips, the next step was to adjust all audio levels. To do so, one has to switch to Liquid’s Audio Editor. It functions almost like a separate application — which may be how it was developed.

Since you must enter and leave Audio Edit mode, this is one of the few instances in which Liquid has a modal interface. (One of the reasons often advanced as a benefit of a “modal” interface is that by restricting certain operations to specific modes, accidents are prevented. This is generally not the case with Liquid. If one accidentally slips moving a node up or down, one can very quickly move an entire clip to another track, doing untold damage.)

After all audio levels were set, I went through and used the pan controls to spatially create a stereo mix. As when adjusting levels, you can use rubber bands in the timeline or keyframes set by the Audio Edit interface. As shown below, there are both level sliders and a pan control for each stereo track — plus a Master Stereo Level slider.

All sources have level meters, as shown below.

Building 5.1 Tracks

Now that audio levels were established on the stereo tracks, I made a copy of each stereo track using Liquid’s handy Duplicate Track command. I named these tracks: Sync A 5.1-ch, Sync B 5.1-ch, and Aux 5.1-ch. I then made a copy of Aux 5.1-ch and named it, and its original, Aux 5.1-ch (L) and Aux 5.1-ch (R).

Once again, I entered Audio Edit mode. You will see a new set of controls. Each track has a level slider, a two-dimension panning control, and the Low Frequency Effects channel level slider. LFE is the ".1" in 5.1 audio and carries low-frequency sound from the other five channels.

Monitoring

To monitor surround sound during a mix, your PC will need a six-channel audio sound card or a USB six-channel audio box for your laptop. Because the audio has not yet been encoded to AC-3, even if your computer has an SP/DIF port, it will be of no value. Liquid and Vegas should support the AC3Filter (http://ac3filter.net) that software-encodes six channels to AC-3 for output via S/PDIF.

Export

When you are ready to export your production, simply enable either the “2-ch” or “5.1-ch” audio tracks. Using this procedure, I was able to export a stereo HDV videotape, a stereo D-VHS videotape, plus a DVD with AC-3 5.1 surround sound.

The only problem I encountered in creating these media was Liquid’s all-too-frequent breakdowns during playback in Audio Edit mode, where during playback, all control over the application is lost. The only truly fatal Liquid crash came while I was in the audio editor. (One of the features I love most about Liquid is that it auto-stores every action you make. Therefore, if there is a crash — which is very, very rare when editing video — your project is restored to your last action.)

One of Liquid’s most useful capabilities is the ability to create either “auto-play” or “menu-driven” DVDs right from the Timeline. I made a simple test DVD and burned it directly from Liquid.

You can see the three markers placed into auto-created DVD track. It’s also possible to export an HD-resolution MPEG-2 file, which means it’s possible to create high-definition DVDs. (In March 2007, DVC will release an e-book on “Low-cost High-Definition DVD Creation.”)

Once you begin exporting HD MPEG-2, you will quickly discover how time consuming this task is. In the next installment, we will look at “Smart GOP Splicing” technology to understand its potential to reduce export time.

Steve Mullen owns Digital Video Consulting that offers eBooks on HD production. Currently, DVC (www.mindspring.com/~d-v-c) offers two eBooks: the Sony HDV Handbook and the JVC ProHD Handbook. In January 2007, DVC will offer a new title: the Sony V1/FX7 Handbook.

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