This article discusses common causes for audio distortion within movies created or edited in Final Cut Pro and how to avoid them.

When audible audio distortion (crackling, popping) occurs while using Final Cut Pro, it is usually due to:

• Capturing audio at a sample rate other than the one it was initially recorded with (when importing clips from DV)
• Mixing audio clips with different sample rates together on playback

Distortion when Recording

The most frequent cause of audio distortion heard while recording clips in Final Cut Pro stems from the captured audio sample rate not matching the sample rate initially recorded onto a DV Tape. For example, if you shot footage with your camera set to record at 48 kHz, and you have the sound recording sample rate set to 32 kHz, you'll hear distortion. This distortion will be recorded into the clip.

It's easy to remember which Bit Rates correspond to which Sample Rates, and the two must always correspond:

12 bit = 32 kHz

16 bit = 48 kHz

Note: This can also be a cause of lost audio sync. For more information on this subject, see article 58640: " Final Cut Pro: Dropped Frames and Loss of Audio Sync ".

Figure 1 The Sample settings of the Sound panel, showing the available sample rates

This setting can easily be changed. To do so:

1. Open the Capture Preferences tab.

2. Click Audio.

3. With the Sound panel open, set the upper left pop-up menu to Sample.

4. Select the appropriate sample rate from the Rate pop-up menu.

The most frequent reason for audio distortion when playing back a mix in Final Cut Pro is the combination of too many audio clips with different sample rates. This distortion will take the form of crackling, popping, and sometimes even skipping in the audio.

Final Cut Pro, when faced with clips of varying sample rates, will resample the audio in real time. One result of this is to reduce the number of tracks that can be mixed in real time as specified in the General Preferences tab.

The other result of varying sample rates is distortion. For example, a sequence with 32 kHz audio from a consumer camcorder mixed with 48 kHz audio from a Pro camcorder and 44.1 kHz audio from a CD transfer is virtually guaranteed to create audible distortion. One solution to this would be to redub your 32 kHz source material, rerecording the audio at 48 kHz. Another solution would be to resample all 32 kHz audio using dedicated audio software, such as Bias Peak.

You should note that 44.1 kHz and 48 kHz audio generally work fine together without distortion. This is because the sample rates are so close together that the resampling is generally inaudible. With this in mind, it is important to know that Final Cut Pro is better at upsampling then it is at downsampling. For this reason, you should have your Sequence Settings default set to 48 kHz when mixing the two.

In general, you should record DV source material at 48 kHz whenever possible. Most DV cameras provide a menu setting to toggle between 32 kHz and 48 kHz. See your camera manual for more information.

Distortion Outside of Final Cut Pro

Another potential problem is when you output a sequence to tape, and when playing back the tape you notice audible distortion not present when the audio was playing out of the computer. This is generally because of too much dynamic range in the mix.

Dynamic range refers to the range from the softest sound to the loudest sound in a given sequence of clips. If this range is too great, and you set the recording input level of your VTR to record the soft parts at an acceptable volume, the loud parts will distort. Conversely, setting the recording input levels of your VTR to reflect the loud part will cause the soft parts to be too silent to hear. This is especially problematic if you're planning on sending your output tape to a dubbing facility. When making hundreds of dubs from a tape, engineers generally set the audio level of all the VTR's in the facility to match the level of the audio tone at the head of your master tape (Assuming you output to tape with bars and tone. If your audio varies widely from this benchmark, you can be sure that there will be audio problems.

The solution to this is to mix the audio levels of the different tracks in your sequence to bring the audio levels of the softer clips up and the louder clips down, in an effort to make the levels of all the clips in your sequence acceptable relative to your 0 dB reference tone. This is done by using the Clip Overlays. Clip Overlays are enabled by clicking Clip Overlays at the bottom of the Timeline window.

Figure 2 The Clip Overlays button

Once enabled, a thin red line indicates the volume of an audio clip relative to the others in a sequence.

Figure 3 Clip Overlays are the red lines in the center of the audio clips

The simplest form of mixing is to set the level up or down for the duration of the entire clip by clicking and dragging this red line. A small yellow window will indicate the new level of the clip in + or - decibels (dB). (For a hint on how to do this more precisely, refer to article 58632: " Final Cut Pro: Using the Command Key Modifier to Make Subtle Keyframe Changes ")

Figure 4 Bringing down the audio level of the "Mahaiy" clip to -15 decibels

In this way, the relative levels of all audio clips in your sequence can be brought into an acceptable range for output to tape. More precise mixing can be achieved by setting keyframes with the Pen tool.

Use a Mixer to Set Your Audio Levels

Since it is difficult at best to accurately gauge the loudness of your mix by ear, it is a good idea to use the VU meters of an external mixing board to help you set your levels. In order for this to be effective, first play back the tone you placed at the head of your edit and adjust the level of your mixer such that the VU meter reads 0 dB Now, whenever your mix is too hot, you will have an accurate visual indicator warning you of the problem. Do not rely solely on your meter readings while mixing your tracks. Proper mixing requires careful attention to both the meter levels and to the sound coming out of your computer.

Note: It is a good idea to mix using high quality headphones. Inexpensive speakers, the physical space of a room, and the ambient noise in any home (such as refrigerators, traffic, purring cats) can all inadvertently alter the quality of what you are hearing, resulting in your producing a mix that may be louder than necessary. Wearing headphones while mixing minimizes this effect, and allows you to make better decisions about setting EQ and levels.

Using the Compressor to Reduce Dynamic Range

Other common sources of unwanted dynamic range that are not so easily fixed are music and voice-over. In both cases, the volume from one moment to the next can vary widely, but unfortunately any attempt to correct this by mixing will result in an audible change in volume, either in the background of the music or in the room tone of the voice-over. Either way, it is unacceptable to a high quality mix.

Fortunately, Final Cut Pro gives users a solution to this dilemma in the form of the Compressor/Limiter audio filter. This filter adjusts the dynamic range of the entire clip so that the loudest part is compressed to within a user selectable range of the quietest part of the clip.

Figure 5 The Compressor/Limiter filter, located in the Audio Filters folder of the Effects Pallette

To use it, simply:

1. Apply the filter to the audio clip in question via either the Effects menu, or drag and drop from the Effects bin in the Browser.

2. Double-click the audio sequence clip .

3. Select the Filters tab in the Viewer.

Figure 6 The Compressor/Limiter filter's parameters

By setting levels, mixing, and compression, you can create a professional sounding mix that will output to tape clearly and cleanly.

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