Recreating spaces for foley effects and dialog replacement

Frequently, dialogue recorded in the field is rendered unusable by ambient noise. If you are shooting in the field and realize that overdubbing will be necessary, you should create an impulse in each filming location. This allows you to overdub dialog during post­ production that is indistinguishable from dialog recorded on location.

If you intend to use the Acoustic Mirror effect as a film/video post-production tool, there are some factors to keep in mind:

•Distance information is determined by the distance between the source and the microphone when creating the impulse. Record multiple impulses at various distances for each location to create realistic dialog effects when matching audio processing to approximate camera positioning.

•The frequency response of the human ear changes as the volume of a sound increases. As a result, impulses created from a significant distance may sound unusual at high volumes.

•Placing a microphone off center allows you to create directional information in the recovered impulse. For example, placing a microphone to the left of the speaker produces an impulse that approximates a source located on the left side of the screen.

Panning with head-related transfer functions

A head-related transfer function (HRTF) contains the frequency and phase response information required to make a sound seem to originate from a specific direction in a three-dimensional space. The Acoustic Mirror Impulse Files\HRTF Impulses folder on the Sound Forge application disc contains a collection of impulse files that contain directional cues.

To achieve optimal results using these impulse files, the original file should be mono and playback should be monitored using headphones. To begin, convert the mono file to stereo by replicating the mono signal in each channel. After the audio is converted to stereo, choose an impulse file from the HRTF Impulses folder. You will notice that the HRTF Impulses folder is further divided into Left and Right directories. Opening the desired folder displays the available impulse files, all of which are named based on their elevation (up or down) and azimuth (left or right) angles in degrees. The following table provides some examples:

File Name

Impulse positioning

0E000L

Straight ahead

 

 

0E090L

Far left

 

 

0E090R

Far right

 

 

90E000L

Directly above your head

 

 

0E180L

Directly behind you

 

 

-20E120L

Below, behind, and to your left

 

 

Note: Refer to Readme.doc in the HRTF Impulses folder for more information.

Troubleshooting the Acoustic Mirror effect

The following sections describe problems that may be encountered when working with the Acoustic Mirror tool.

Stuttering during real-time previewing

It is not uncommon to experience problems when previewing processing in real-time. The following sections contain several suggestions to remedy the situation.

Lower the Quality/speed setting

Lower the value of the Quality/speed control on the General page. When previewing lengthy impulse responses, a setting of 1 or 2 may be necessary; however, the quality suffers. This setting should always be returned to 5 prior to processing to maintain effect quality.

224 CHAPTER 13