Audio Technology / Recording techniques/ Signal processing and special effects...

Signal processing and special effects

Long-term preservation preservation is one of the primary goals of oral history recordings. It is, therefore crucial to try to obtain the cleanest, highest-fidelity signal right at the very first stage of recording. One should try to capture a wide dynamic range (e.g., 96 db in 16-bit digital recordings) and a fairly wide frequency response (e.g., 0-20,000 Hz). It is also recommended to avoid using too many special effects at this stage, as well. The only exception may be a soft limiter to reduce the possibility of accidental signal overloading. Such "clean" recordings should be stored for preservation purposes. However, various audio delivery situations, such as radio broadcasts, web streaming, CD, etc. may require extra signal processing to make the audio sound better. Digital signal processing (DSP) effects are, therefore, best applied in the post-production process.

There are 4 basic types of effects (filters) that can be applied to speech recordings EXAMPLES :

Equalization (EQ) EXAMPLES

Equalization is selective amplification, or reduction, of a signal based on frequency. Audio signals consist of combinations of fundamental signals and their harmonics. Changes to the spectral balance of a signal involves altering the relationship of the fundamental to its harmonics. Each harmonic makes up one aspect of the audible character of a signal. Knowing these relationships allows you to quickly zero-in on the correct frequency range of the signal and apply boost or cut to enhance or correct what you are hearing.

Compressor EXAMPLES

The effect of a compressor is to make loud parts of a signal softer and to make very soft parts louder. Compression works particularly well in radio broadcasting and streaming audio where very soft passages may be lost due to the background noise in the listening environment and the data compression algorithm used in the streaming audio format.

De-esser EXAMPLES

The de-esser is a dynamic range controller specially designed to regulate high frequency content. The de-essing technique was developed for motion picture dialogue recording. Speech sounded more natural and pleasing with the reduction of sibilants (in words such as: church, test, zest, etc.). By sensing and limiting certain selected frequencies, the de-esser provides specific control over some of the higher frequency vocal sounds which may become overemphasized when the speaker or vocalist is close to the microphone.

Reverb EXAMPLES

Reverb (reverberation) is an effect that simulates natural room reverberation. It can be defined as the remainder of sound that exists after the sound source is stopped. The time of reverberation as defined as the time it takes for sound pressure level to decay to one-millionth of its former value. For good speech intelligibility, too much reverberation is a hindrance, and can be considered noise, although some reverberation may add a bit of character and presence to one's voice.