Mixing Surround in Sony Vegas Software
Mike Jones/ 2005 Sundance Media Group

HDV:What You NEED to Know

 

The ideas behind multi-channel surround sound; that of directing and panning sounds between speakers placed in an array around the viewer, are not particularly new. In many ways surround sound is simply an extension of the ideas that gave us stereo recording through dual channels of left and right. The principal centers on the idea that our perception of sound comes not just from hearing the modulation of frequencies but also of perceiving the sound as it exists in space through sounds placement, direction and reverberation.

Various forms of surround sound have been used in cinema houses for many years but even up until very recent times the idea of having surround sound mixing capability built into a software only non-linear editing system that can run from a laptop computer was a fairly far fetched idea.

Surround sound comes under a variety names that describe the number channels accommodated by the format; generally 7.1, 6.1 or 5.1. The first number refers to the number of principal, discreet channels. The .1 refers not to an extra channel as such but rather to a Low Frequency Effects (LFE) channel or subwoofer. The LFE reproduces and reinforces only the low frequencies below a prescribed threshold (usually around 100hz) from all of the discreet channels.

Vegas allows for the mixing of surround sound using the Dolby 5.1 specification. Sound can be placed and panned between five speakers placed relevant to the viewer at Center, Left, Right, Rear Left and Rear Right.

Of course this is all pretty irrelevant to you if you don't have surround sound speakers attached to your PC workstation. In order to be able to monitor your sound you will need to have fitted to your computer a multi-channel sound card that allows for surround sound and an array of speakers placed at the appropriate points around the room.

It is also possible to use three separate stereo sound cards together to create surround sound rather than a single multi-channel sound card. To set up multiple sound cards, from the drop down menus select Options - Preferences - Audio device. These controls tell Vegas what hardware the various channels should be directed to.

A stereo card can then be specified for the front speakers, a second for the rear and the third for the center and subwoofer. Vegas will then treat the combined channels of these individual sound cards in the same way it would multi channel 5.1 card.

We'll start first by setting up the Vegas project for surround sound. Open up the Mixer window in Vegas usually placed as a Tab at the bottom of the screen or otherwise go to the View menu and select Mixer.

Vegas will default the mixer to stereo when the program is opened so from the top of the Mixer window click the Project Audio Properties button.

This will open up the Properties Window that contains all the parameters and settings for audio on your project.

Where it says Master Bus Mode click the drop down box that says Stereo and select Dolby 5.1. Lower down it gives the heading of Cut-off Frequency for Low Pass Filter, this sets the point in the frequency range at which sounds will be directed to the subwoofer. Clicking the drop down box gives a range of specifications; in most cases the Dolby consumer/DVD 80hz mark is best. This means that frequencies from all channels falling below 80hz will be re-enforced by the subwoofer or LFE.

Click OK and your mixer window will immediately update to show not the usual single master fader but four; a front stereo pair of left and right, a rear stereo pair of left and right, a center and an LFE. These faders work in pretty much the same way as regular master buses.

Insert three audio files as separate tracks onto the timeline. It is best to use mono audio tracks in surround sound as tracks in stereo will essentially have their own spatial array built in and can making effective mixing more difficult. It is easy enough to convert stereo tracks to mono on the timeline; right-click on the audio track and select Channels - Combine.

The track control area differs in Surround mode by having a small surround sound panner on each track. This shows up as a box with five blue points to represent each of the five speakers and an orange point that indicates where in the surround sound field the sound is currently placed

To start with we'll direct each of our three audio tracks to a specific speaker. Click on the orange marker for each track and drag so each rests on a different blue speaker position.

Play back the audio and hear that each of tracks is coming from a different position. While the audio is still playing click on the orange marker from one of the tracks and drag it slowly around the panner. You should hear the sound shift smoothly around the surround field from point to point. Not that you don't have to place a sound completely towards a speaker point. The position marker can rest anywhere in the field. Positioning it between two markers or speaker positions will create and appropriate blend of the two channels.

Doing this obviously raises the db level of the sound on it's respective master fader. You may notice that moving fully towards one particular speaker position causes the master VU level to go into clip.

If you right-click in the middle of the surround sound panner on the track area a menu will pop up. Go down to Select Pan Type and a set of five options will appear. These controls set the parameters for governing how the audio levels are combined as they are panned from channel to channel.

The default is Add Channels which simply combines the db levels between points and directs all volume to the select point. This setting is the one most likely to cause clipping when channels are panned. The next three (0db, -3db and -6db center) set a threshold for the volume at a given point. For example setting the Pan Type to -3db center will mean that sound panned from the center speaker toward the front left speaker will raise the level of the front left master fader to a maximum of -3db and thus help to prevent clipping. The last setting is Constant Power and this type maintains constant audio levels evenly across the sound filed through panning.

Next we will create an automated pan that allows our sound to move via a pre-programmed path across the surround sound filed between channels. If you are familiar with keyframing effects and animation in Vegas then this process will be much the same.

First we'll get a better view of the surround panner so double click in the middle of the panner on the track area. This will open up a larger, more detailed, surround panner from where we will begin to create surround keyframes.

The larger surround panner window allows for additional controls over the surround sound field. Any given channel can be muted by simply clicking on the blue speaker shaped icon.

At the bottom of the Surround Panner is a slider for controlling and boosting the center field gain of the track. Use this if you want to boost the output and dominance of the center channel.

To create automation of the sound we have to insert surround pan keyframes. Return the cursor to the beginning of the timeline, select an audio track, open its' surround sound panner and then go to Insert - Audio Envelopes - Surround Pan Keyframes.

This will place a keyframe timeline running along the bottom of the audio track. The timeline will have one keyframe already on it at the beginning of the project. Drag the orange maker in the surround panner window to the channel you wish the sound to begin its pan from. This will become the first keyframe.

Select a point further along the keyframe timeline and click to place the cursor. Now move the orange maker to a different speaker position. A trail of small dots will show up on the surround panner indicating the movement path of the sound within the field.

As many keyframes as you like can be created on the keyframe line and by using the panner in conjunction with the video preview window it is possible to make very accurate audio spatial representation of the visual elements.

The velocity of audio keyframes can also be controlled from the keyframe timeline. Select any keyframe (showing as a small diamond shape) and right-click to bring up a small menu.

These settings control the acceleration of the pan movement between keyframes by assigning the movement a particular path shape; Fast shifts the sound rapidly and then slows down as it draws near the next keyframe, Slow does the opposite. Hold stops the sound from panning at all but rather holds it at the position it is set until the next keyframe.

When several keyframes are created the movement between them can also be adjusted with the Smoothness slider on the bottom of the Surround Panner window. This crates a panning movement that moves quickest in the middle but smooth and slow at the beginning and end of the pan. This type of keyframe movement is often called Ease-in Ease-out.

Through these techniques all audio tracks can be keyframe panned individually to create an audio environment that closely resembles spatially the images on the screen; whether it's the voices of actors or the sounds of space ships hurtling past the viewer. Final output levels for each channel need to be closely monitored on the master faders to ensure that individual channels aren't overdriven.

When it comes to rendering, surround sound needs to be encoded in an appropriate format that will be accepted by the DVD format and doesn't take up so much room there is nothing left for the video. It is possible to render out six individual mono PCM *.wav files which combine to form the surround sound mix but it is more common to use the Dolby AC-3 encoder which uses lossless compression to create a single surround audio file for DVD. It should also be remembered that for the MPEG-2 file format required by DVD video material audio and video are rendered as separate files. It's good practice to call both audio and video files the same name as this will mean DVD authoring applications, such as the Vegas partner application DVD Architect, will automatically associate the two files. If not it is a simple matter of manually directing the authoring program to the associate two files.

Lastly it is crucial in creating an effective and balanced surround sound mix that you carefully arrange and position your monitoring environment. Ensure that you are sitting equidistant from front and rear speakers and in line with the center speaker. If you're sitting off-center then you'll be listening off-center and subsequently your mix will be a bit skewed. It is also a common trait of inexperienced users making surround sound projects to have their subwoofer volume on the subwoofer itself up too loud. This may sound great in your room but your room is unlikely to have the same characteristics as the viewers'. Best to mix subtly and let the viewer adjust to their tastes.

 

Happy editing,
MIKE

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