MT1: Unit 1 DIGITAL AUDIO-Elements of multimedia

Digital audio is the result of sound reproduction, using pulse-code modulation and digital signals. This includes analogue-to-digital conversion (ADC), digital-to-analogue conversion (DAC), storage, and transmission. In effect, the system commonly referred to as digital is in fact a discrete-time, discrete-level while analogue is a continuously varying electrical analogue.

Digital audio has emerged because of its usefulness in the recording, manipulation, mass-production, and distribution of sound. Modern distribution of music across the Internet via on-line stores depends on digital recording and digital compression algorithms. Distribution of audio as data files rather than as physical objects has significantly reduced the cost of distribution. While modern systems can be quite subtle in their methods, the primary usefulness of a digital system is the ability to store, retrieve and transmit signals without any loss of quality.

The digital audio signal may be stored or transmitted. Digital audio storage can be on a CD, a digital audio player, a hard drive, USB flash drive, Compact Flash, or any other digital data storage device. Audio data compression techniques — such as MP3, Advanced Audio Coding— are commonly employed to reduce the file si ze. Digital audio can be streamed to other devices.

The digital audio chain begins when an analogue audio signal is first sampled, and then (for pulse-code modulation, the usual form of digital audio) it is converted into binary signals— ‘on/off’ pulses—which are stored as binary electron ic, magnetic, or optical signals, rather than as continuous time, continuous level electronic or electromechanical signals. This signal may then be further encoded to allow correction of any errors that might occur in the storage or transmission of the signal; however this encoding is for error correction, and is not strictly part of the digital audio process.

The last step is for digital audio to be converted back to an analogue signal with a DAC. Like ADCs, DACs run at a specific sampling rate and bit resolution but through the processes of oversampling, up sampling, and down sampling, this sampling rate may not be the same as the initial sampling rate.

PSYCHOACOUSTICS

Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound (including speech and music).

Hearing is not a purely mechanical phenomenon of wave propagation, but is also a sensory and perceptual event; in other words, when a person hears something, it arrives the ear as a mechanical sound wave traveling through the air, but within the ear it is transformed into neural action potentials. These nerve pulses then travel to the brain where they are perceived. Hence, in many problems in acoustics, such as for audio processing, it is advantageous to take into account not just the mechanics of the environment, but also the fact that both the ear and the brain are involved in a person’s listening experience.

Audio compression techniques, such as MP3, make use of this fact. In addition, the ear has a nonlinear response to sounds of different loudness levels. Telephone networks and audio noise reduction systems make use of this fact by non-linearly compressing data samples before transmission, and then expanding them for playback. Another effect of the ear’s nonlinear response is that sounds that are close in frequency produce phantom beat notes, or inter modulation distortion products.

3.1.1.1 PSYCHO-ACOUSTIC MODEL

The psychoacoustic model provides for high quality lossy signal compression by describing which parts of a given digital audio signal can be removed (or aggressively compressed) safely — that is, without significant losses in the (consciously) perceived quality of the sound. Psychoacoustics is based heavily on human anatomy, especially the ear’s limitations in perceiving sound as outlined previously. To summarize, these limitations are:

High frequency limit

Absolute threshold of hearing

Temporal masking

Simultaneous masking
Given that the ear will not be at peak perceptive capacity when dealing with these limitations, a compression algorithm can assign a lower priority to sounds outside the range of human hearing. By carefully shifting bits away from the unimportant components and toward the important ones, the algorithm ensures that the sounds a listener is most likely to perceive are of the highest quality

SELF ASSESSMENT EXERCISE

What do you understand by Psychoacoustics?

7.0 CONCLUSION

In this unit you have learnt about the digital audio and its features. You have also learnt that Psychoacoustics is the branch of science studying the psychological and physiological responses associated with sound (including speech and music). Psychoacoustics is based heavily on human anatomy, especially the ear’s limitations in perceiving sound . Some of its limitations are High frequency limit, absolute threshold of hearing, temporal masking and simultaneous masking

5.0 SUMMARY

Digital audio is the result of sound reproduction, using pulse-code modulation and digital signals. What you have learned in this unit concerns the features of the digital audio and sheds light on psychoacoustics. The units that follow shall give more information on other elements of multimedia.

6.0 TUTOR MARKED ASSIGNMENT

1).Distinguish between the digital audio and analogue audio

2).Explain what you understand by “Psychoacoustics ”

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