Extraneous Acoustic Sounds
Despite the hope for perfectly quiet recording spaces, there are often numerous sources of noise both inside and out-side of a recording space that must be dealt with.
Some of these are relatively constant, steady-state sounds, such as air-handling noise, whereas other sounds are unpredictable and somewhat random, such as car horns, people talking, footsteps, or noise from storms.
With most of the population concentrated in cities, sound isolation can be particularly challenging as noise levels rise and our physical proximity to others increases. Besides airborne noise there is also structure-borne noise, where vibrations are transmitted through building structures and end up producing sound in a recording space.
Distortion
Although engineers typically want to avoid or remove noises such as previously listed, distortion, on the other hand, can be used creatively as an effect, or it can appear as an unwanted artifact of an audio signal.
Sometimes distortion is applied intentionally—such as to an electric guitar signal—to enhance the timbre of a sound, adding to the palette of available options for musical expression. At other times, an audio signal may be distorted through improper parameter settings, malfunctioning equipment, or low-quality equipment.
Whether or not distortion is intentional, an engineer should be able to identify when it is present and either shape it for artistic effect or remove it, according to what is appropriate for a given recording. Fortunately engineers do have an aid to help identify when a signal gets clipped in an objectionable way.
Digital meters, peak meters, clip lights, or other indicators of signal strength are present on most input stages of analog-to-digital converters, microphone preamplifiers, as well as many other gain stages. When a gain stage is overloaded or a signal clipped, a bright red light provides a visual indication as soon as a signal goes above a clip level, and it remains lit until the signal has dropped below the clip level.
A visual indication in the form of a peak light, which is synchronous with the onset and duration of a distorted sound, reinforces an engineer’s awareness of signal degradation and to help identify if and when a signal has clipped. Unfortunately, when working with large numbers of microphone signals, it can be difficult to catch every flash of a clip light, especially in the analog domain.
Digital meters, on the other hand, allow peak hold so that if a clip indicator light is not seen at the moment of clipping, it will continue to indicate that a clip did occur until it is reset manually by an engineer.
For momentary clip indicators, it becomes that much more important to rely on what is heard to identify overloaded sounds because it can be easy to miss the flash of a red light. In the process of recording any musical performance, engineers set microphone preamplifiers to give as high a recording level as possible, as close to the clip point as possible, but without going over.
The goal is to maximize signal-to-noise or signal-to-quantization error by recording a signal whose peaks reach the maximum recordable level, which in digital audio is 0 dB full scale. The problem is that the exact peak level of a musical performance is not known until after it has happened.
Engineers set preamplifier gain based on a representative sound check, giving themselves some headroom in case the peaks are higher than what is expected. When the actual musical performance occurs following a sound check, often the peak level will be higher than it was during sound check because the musicians may be performing at a more enthusiastic and higher dynamic level than they were during the sound check.
Although it is ideal to have a sound check, there are many instances in which engineers do not have the opportunity to do so, and must jump directly into recording, hoping that their levels are set correctly. They have to be especially concerned about monitoring signal levels and detecting any signal clipping in these types of situations.
There is a range of sounds or qualities of sound that we can describe as distortion in a sound recording. Among these unwanted sounds are the broad categories of distortion and noise. We can expand on these categories and out-line various types of each:
• Hard clipping or overload. This is harsh sounding and results from a signal’s peaks being squared off when the level goes above a device’s maximum input or output level.
• Soft clipping or overdrive. Less harsh sounding and often more desirable for creative expression than hard clip-ping, it usually results from driving a specific type of circuit designed to introduce soft clipping such as a guitar amplifier.
• Quantization error distortion. Resulting from low bit quantization in PCM digital audio (e.g., converting from 16 bits per sample to 8 bits per sample). Note that we are not talking about low bit-rate perceptual encoding but simply reducing the number of bits per sample for quantization of signal amplitude.
• Perceptual encoder distortion. There are many different artifacts, some more audible than others, that can occur when encoding a PCM audio signal to a data-reduced version (e.g., MP3 or AAC). Lower bit rates exhibit more distortion.
There are many forms and levels of distortion that can be present in reproduced sound. All sound reproduced by loudspeakers is distorted to some extent, however insignificant. Equipment with exceptionally low distortion can be particularly expensive to produce, and therefore the majority of average consumer audio systems exhibits slightly higher levels of distortion than those used by professional audio engineers.
Audio engineers and audiophile enthusiasts go to great lengths (and costs) to reduce the amount of distortion in their signal chain and loudspeakers. Most other commonly available sound reproduction devices such as intercoms, telephones, and inexpensive headphones connected to digital music players have audible distortion. For most situations such as voice communication, as long as the distortion is low enough to maintain intelligibility, distortion is not really an issue.
For inexpensive audio reproduction systems, the level of distortion is usually not detectable by an untrained ear. This is part of the reason for the massive success of the MP3 and other perceptually encoded audio formats found on Internet audio—most casual listeners do not perceive the distortion and loss of quality, yet the size of files is much more manageable and audio files are much more easily transfer-able over a computer network connection than their PCM equivalents.
Distortion is usually caused by amplifying an audio signal beyond an amplifier’s maximum output level. Distortion can also be produced by increasing a signal’s level beyond the maximum input level of an analog-to-digital converter (ADC).
When an ADC attempts to represent a signal whose level is above 0 dB full scale (dB FS), called an over, the result is a harsh-sounding distortion of the signal.