Time CompressionExpansion

[ LiB ]

Time Compression/Expansion

Time compression and expansion is the process of lengthening or shortening a piece of audio without altering its pitch or timber in any significant way. Time compression and expansion are very useful tools for sound design. You will often find sounds that are perfect for a certain situation except that they are not the correct length. Expanding or compressing them to fit the necessary space can save the day. Many times musical cues need some time adjustment to fit into a 30- or 60-second TV spot. Time compression or expansion can be the answer to these and other problems.

Audiosuite Time Compression/ Expansion

The Digidesign time compression/expansion Audiosuite plug-in, shown in Figure 7.11, has many parameters and functions that will help you correct the length of just about any sound. You must define how much compression or expansion needs to occur for your particular use. In most cases, your goal will be to correct the length of a sound to fit the visual timing.

Figure 7.11. The time compression/ expansion Audiosuite plug-in.

Although the only parameters that affect the processing are the ratio, crossfade , minimum pitch, and accuracy, Pro Tools allows you to use other methods to define what the ratio will be. The ratio parameter is a function of the desired length of the audio file divided by its original length (x:1). Thus, a ratio of 0.993:1 will create a shorter file than the original. Conversely, a ration of 1.237:1 will yield a longer file once processed .

The upper parameters are all used to generate a proper ratio for processing. With the measurements of the original region appearing in the left column in the plug-in window as Source parameters and the Destination parameters appearing in the right column, it is possible to enter numbers in the Destination column and alter the ratio. Take the ratio slider and move it around and see how the Destination numbers change. This is not really intuitive, but you'll understand it as you gain experience.

Directly inputting numbers in the Destination values will change the ratio as well. Using this method, very precise time compression and expansion is possible with accuracy down to one sample. Here's how to generate a time compression ratio to match a video accurate to one millisecond:

Measure the visual timing to get an accurate idea of how much compression or expansion will occur.
Use the Selector tool and set the Main counter to either time code, minutes and seconds, or samples.
Using Grid mode, select the area that you would like the resultant audio file to completely fill.
Look at the Length (duration) number in the Edit window, as shown in Figure 7.12. Write this number down somewhere handy.

Figure 7.12. Selecting the desired length for an audio file about to be processed. Write down the duration length for handy reference. In this xample, it is 10.567 seconds.
Select the region to be processed, making sure it is edited to your liking.
Choose Audiosuite > Time Compression Expansion. The plug-in window will open .
Check to see that the numbers on the source side of the parameters reflect the region you have selected.
Manually input your destination length you jotted down into the appropriate value, as shown in Figure 7.13. (With time code, only the end time should be changed to reflect the new audio length. The Destination value is not a length value but an end time.)

Figure 7.13. Manually entering the desired length into the min:sec Destination value of the time compression expansion plug-in.
Press the preview button to listen to the processed audio and tweak the settings below to achieve the desired sound quality. For dialog, the accuracy slider usually sounds best set to sound, not rhythm.
Decide if you want the processed file to replace the original in the playlist or just placed in the Audio Regions list. Set the options accordingly .
Click the process button. Viol ! Your audio file should now be the desired length.

NOTE

COMPRESSION AND EXPANSION LIMITS

Time compression and expansion will degrade the sound quality as the ratio increases or decreases further from 1:1. The degradation will vary depending on the type of program material. A good rule of thumb is that if you are expanding, ratios above 1.2:1 will start to sound strange , and compression will get a little funny below 0.8:1. But there are circumstances in which such degradation is the desired effect. Try processing with ridiculous ratios and see what happens. You could come up with a really cool effect.

Trimmer Tool Compression/ Expansion (TDM only)

In TDM systems, the Trimmer tool can be used to compress or expand audio files just like the Audiosuite plug-in but using visual methods. You can select the Time Compression Expansion (TCE) Trimmer tool from the Trimmer tool's pull-down menu or by pressing F6 repeatedly until the cursor changes to the TCE Trimmer tool's icon. Once selected, the TCE Trimmer tool looks like the Trimmer tool but with a small clock icon in the center, as shown in Figure 7.14.

Figure 7.14. The Time Compression Expansion Trimmer tool.

Instead of manually entering values, the TCE Trimmer will simple change the length of the audio file by the amount you trim or open up the region. This can be a really convenient way of quickly resizing audio files to match picture. When resizing a region, you will see a processing progress bar after letting go of the region, indicating that the time compression or expansion is occurring.

When you choose Setups > Preferences > Processing and select a preset for the Time Compression Expansion plug-in from the Pro Tools Preferences window, the parameters of that preset will be used each time you perform a time compression or expansion with the Trimmer tool, as shown in Figure 7.15. You may even choose a third-party time compression plug-in, such as Serato Pitch 'n Time or WaveMechanics Speed, to do the processing.

Figure 7.15. The Pro Tools Preferences window. Here you can determine what plug-in will process the Trimmer tool's Time Compression or Expansion mode and what preset will be used.

[ LiB ]