After reading this article you will learn about the sampling of audio waves.
The ear is a surprisingly sensitive organ and particularly sensitive to sound variations lasting for only a few milliseconds. Thus in a multimedia transmission, a jitter of only a few milliseconds affects the perceived sound quality much more than it affects the perceived image quality.
Audio waves can be converted to digital form by using an Analog to Digital Converter (ADC). An ADC takes an electrical voltage as input and generates a binary number as output. Figures 11.2 and 11.3 explain this concept clearly.
Figure 11.2 is an example of a sine wave. This represents a sound wave (similar to ‘C’ in Fig. 11.1). To represent this signal digitally, it is sampled every ΔT seconds, as shown in Fig. 11.3. After they are sampled, the samples are quantised and these quantised values are expressed as digital figures.
If a sound wave is not a pure sine wave but a linear superposition of sine waves where the highest frequency component is f, then it is sufficient to take samples at a frequency of 2f. Sampling at more frequent intervals is useless, since the higher frequencies are not present and hence one can be detected by increasing the sampling frequency and thereby decreasing the sampling intervals.
Digital sampling cannot be exact. The error introduced by the finite number of bits per sample is referred to as quantisation noise. If this error is too large, it will be detectable by the human ear. Two clear examples of sampled sound are the telephone and audio compact discs. Pulse code modulation in telephones uses 8-bit samples 8000 times per second.
This gives a data rate of 64 kbps (64,000 bits per second). With only 8000 samples/sec, frequencies above 4 kHz will be lost. Audio CDs are digital with a sampling rate of 44,100 samples per second. This sampling rate is adequate to capture frequencies up to 22 kHz (22,050 Hz to be exact). This limit is adequate for the human ear. The samples—16 bits each—are linear over the range of amplitudes utilised.
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Since 16 bits are used, the total number of distinct values that can be allowed will be 216 or 65536, although the dynamic range of the ear is about 10 lakhs. Therefore, in an audio CD 44,100 samples/second of 16 bits each, a bandwidth of 705.6 kbps for mono and 1411 Mbps for stereo, are required.
This means that it requires nearly a full El channel to transmit—unless digital compression is utilised. Use of digitized channels is advantageous in several other ways. Digitized sound can easily be processed using software. Large number of programs exists to permit users to record, mix, edit, display and store sound waves from multiple sources.
Because of these advantages from digitizing sound, most modern musical instruments have digital interfaces. Originally, each digital instrument had its own interface, but since then a certain standard has been developed. This standard—the MIDI or Music Instrument Digital Interface—has been adopted by the music industry.
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This standard specifies the following:
i. The connector,
ii. The cable,
iii. The message format.
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Each MIDI message conveys one musically significant event such as a key being pressed, a foot pedal being released or a slider being moved. In the message, the status byte indicates the event and the data bytes give the parameters. These parameters could include, for example, which particular key is being pressed and the rate at which the key is being moved. Every instrument has a MIDI code that is assigned to it.
A total of 127 instruments (including special sounds such as chirping of birds, the sound of a helicopter, canned applause from a studio audience, etc.) are assigned these MIDI codes. This is necessary to differentiate between instruments such as the grand piano, violin, tuba, etc. Otherwise, we may have the occasion that a piano concerto is played back as a violin concerto.
At the centre of every MIDI system is a synthesizer that accepts messages and generates music from them. The synthesizer (usually a computer) understands all the 127 codes for instruments and special sounds. The disadvantage of the use of MIDI is the fact that the MIDI receiver needs a MIDI synthesizer to reconstruct the music again and different MIDI synthesizer packages may give slightly different outputs.
This is just a brief introduction to MIDI. Music, of course, is a special case of the use of general audio. Another important case is that of human speech. Speech is made up of vowels and consonants, each having different propertied.
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Vowels are produced when the vocal tract is unobstructed, producing resonances whose fundamental frequency depends upon the size and shape of the vocal system and the position of the speaker’s tongue and jaw. These sounds are almost periodic for intervals up to 30 milliseconds. Consonants are produced when the vocal tract is partially blocked. These sounds are less frequent than vowels.