Special Tutorial – The Audio Characteristics in Networking

by Gina Tina on April 18, 2013

Audio Signal is Important Terminology to Networking

All audio signals are generated in an analog form; that is, the signal that is generated by a microphone varies continuously with time as the amplitude of the speech or audio varies. Hence before an audio signal can be stored in a computer and integrated with the other media types, it must first be converted into a digital form. This is done using an electronic circuit called an analog-to-digital converter(ADC). This involves sampling the amplitude of the speech or audio signal at regular time intervals; the higher the sampling rate, the better quality is the reproduced analog signal. The reverse conversion is done using a circuit called a digital-to-analog(DAC) and the reproduced signal is played out via speakers. In the case of a speech signal- as used in telephony – a typical sampling rate is 8 kHz with 8 bits per sample. This yields a digital signal of 64kbps, which is the bit rate used in telephone networks. For CD-quality audio, however, a common sampling rate is 44.1kHz with 16 bits per sample, which produces a bit rate of 705.6 kbps or, for stereo, 1.411 Mbps.

The raw transmission bit rate required for a communications channel far exceeds the available capacity of most network types. Hence, in almost all Internet applications the source signals re first compressed using a suitable compression algorithm.

The flow of data associated with the different applications can be either continuous or block-mode. In the case of continuous data, this means that the data stream is generated by the source continuously in a time-dependent way. In general, therefore, continuous data is passed directly to the destination as it is generated and, at the destination, the data stream is played out directly as it is received. This mode of operation is called streaming and, since continuous data is generated in a time dependent way, it is also known as real time data. With continuous data, therefore, the bit rate of the communications channel that is used must be compatible with the rate at which the (compressed) source data is being generated. Two examples of media types that generate continuous streams of data in real time are audio and video.

Audio – Block Mode Data Mode

In term of the bit rate at which the source data stream is generated, this may be at either a constant bit rate (CBR) or a variable bit rate (VBR). With audio, for example, the digitized audio stream is generated at a constant bit rate. In the case of video, however, although the individual pictures or frames that make up the video are generated at a constant rate, after compression the amount of data associated with compressed video is generated at fixed time intervals but the resulting bit rate is variable.

audio characteristicsIn case of block-mode data, the source data comprises one or more blocks of data that is or are created in a time-independent way, for example, a block of text representing an e-mail, a two-dimensional matrix of pixel values that represents an image, and so on. Normally, therefore, block-mode data is created in a time-independent way and is often stored at the source in, say, a file. Then, when it is requested, the blocks of data are transferred across the network to the destination where they are again stored and subsequently output or displayed at a time determined by the requesting application program. This mode of operation is known as downloading and,as we can deduce from this, with block-mode data the bit rate f the communications channel need not be constant but must be such that, when a block is requested, the delay between the request being made and the contents of the block being output at the destination is within an acceptable time interval. This is known as the round-trip delay (RTD) and, for human-computer interaction, ideally should be no more than a few seconds.

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