Sound Advice: Audio on an Even Keel

Did you realize there is a non-stop video production master-class available to you at all times? It’s a little thing called television. Once you understand the video production process, it’s easy to watch a TV show and figure out how they accomplished the various elements in a program. This is great for shot composition, lighting and camera work, but it’s also a very valuable education for audio production.

Let’s use a news-magazine show as an example. Its stories are typically filled with interviews. If you’ve shot many interviews, you know the three minutes you see on TV were ruthlessly edited from an hour or more of footage, possibly recorded over several days. Somehow, the editors manage to keep the audio levels consistent throughout the piece, even though they may come from different tapes and even different locations. How do they do that?

Shoot It Right

It all starts with the original shoot. Attention to detail is the key. Using the same microphone throughout a shoot is a simple thing, but also easy to miss. Pay attention to the microphone position on your talent’s mouth and clothing. These notes help you recreate the shot after a break or even on another day, if necessary. If using an external mixer, write down all the settings for gain, volume and tone. Make a note of the audio cabling and how the signal enters the camcorder. It sounds like a lot of extra trouble, but one item out of place can completely change the sound on your video.

Those with consumer or prosumer camcorders have to deal with the automatic gain control (AGC). This circuit monitors the incoming audio signal and decides whether to turn the gain up or down. In theory this is fine. In practice, however, the continuous change results in audio that is difficult to deal with. There isn’t a good way around AGC, but you can minimize its effects. Make sure your video camera has a strong, clean audio signal. AGC can take even the slightest microphone buzz and turn it into a swarm of bees. When shooting another take of a scene, have the talent start speaking (e.g. countdown "3, 2, 1…") before the segment you want to record. This will put the sound level in a similar range to the previous take, which is handy if you’re cutting scenes together later. If you’re one of the few camcorder owners with manual audio control, switch the AGC off and learn to read the level meters. Of course, you should always monitor the recording with headphones.

We’ll Fix It in Post

Now it’s time to edit the raw footage. When you’ve chosen the best takes, put together a rough cut of the video. On playback, you’ll probably notice some variance in the audio volume and sound quality. These changes are common, even if you did the shoot correctly. Not to worry – we can fix this.

Believe it or not, those with analog capture cards have an advantage over IEEE 1394 users at the point of acquisition. You can pre-condition your audio as you capture from the source tapes. This process involves using an external mixer to compensate for volume differences. The biggest advantage is the ability to monitor audio as it enters the computer, allowing you to make changes to volume and tone as needed to match the other audio in your production.

Once in the software, the first line of defense is to normalize the audio clips. This process analyzes the audio in your clip and adjusts the volume to make the loudest portion full-scale or 100-percent volume. Normalizing audio clips ensures maximum volume from even the quietest take. On the Adobe Premiere timeline, right-click on the audio clip and choose Audio/Gain. This provides a dialog box to normalize your audio clip. After normalizing, you may find that a few of the clips sound out of place. If so, manually adjust the normalize percentage to a value that sounds identical to the surrounding clips. This requires some trial and error; don’t you just love the "undo" feature?

Normalize works great for entire clips, but what if you want to bring out a single word or phrase in that clip? Rubber bands to the rescue virtual "rubber bands" (a.k.a. envelopes) allow you to adjust audio levels as you please. The red rubber band line appears below the main portion of the audio track. If it is not visible, click the triangle to the left of the track name. When the arrow cursor is over the rubber band, the pointer will turn into a finger icon. Clicking on the rubber band creates a handle or node, which you can click and drag to change the volume. Add several handles to perform intricate volume changes. This is great for spot adjustments, creating voiceover holes (a.k.a. ducking the audio) or for gradual fade-ins or fade-outs. If you need to adjust a long segment without changing other handles, shift-click on the segment you want to change. This will show the actual volume of that portion of the clip, measured in percentage and decibels. Just drag the mouse up or down to make the desired adjustment. Some editing applications even have a simple audio mixer that looks and acts much like a multi-track real-world mixer. This allows you to adjust the relative volume of each audio track in your project in the final mix.

The Master’s Touch

In the audio recording world, when a producer finishes mixing an album, the project is sent out for mastering. This process takes the mixed songs and polishes them even further, producing a finished master that sounds great on everything from an MP3 player to a home theater. Mastering generally involves some degree of tone control and dynamics processing (adjusting the difference between loud and soft). This processing can be done with audio effects plug-ins that may be included in your video editor. Additional audio effects plug-ins are also available. For this article, we’ll use Sonic Foundry’s DirectX audio plug-ins, which are broadly compatible with many Windows video editing environments (and are included with Vegas Video).

Audio effects can be added to audio tracks by dragging them from the Audio Effects window (in Premiere, from the Windows menu, select Show Audio Effects) and dropping them on the target audio clip. Now, we’ll have some fun. First on the list is a little tone shaping, which is done with an equalization (EQ) effect. This is all a matter of taste, but if your audio lacks the gut-kicking sound you had in mind, some judicious equalizer use can help. Equalizer plug-ins work by modifying the volume of your audio at specific frequency ranges and may be familiar to you as sliders on your home stereo system.

Next on the list are two rounds of dynamics control. The first step is compression – minimizing the difference between the loud and soft passages of your video. Go easy on this: you have the power to completely destroy all dynamics. Great dynamic range (pianissimo and forte) is an important aspect of classical music and can be why it is so exciting. Conversely, a lack of dynamic range (loud, loud, loud) is one reason why rock music can be so monotonous. When using the Sound Forge Dynamics effect, for example, I like a threshold of -20dB, a ratio of 1.3:1, an attack time of 15mS and a release of 150mS. This is a good starting point, but listen to the results before moving to the next step. You may want more or less compression. The second step is limiting. It’s identical, but uses different settings. This time use a threshold of -3dB, a ratio of 10:1 and attack and release times of 0.0mS. The resulting image in the waveform view should be big and fat – just what you want. For comparison sake, turn the effect on and off as you preview the audio. The new version should be even and more consistently loud with a minimum of variance between vocal clips. This results in a significantly more clean and powerful sound track and, by extension, better video.

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