Sound travels as a series of waves - that is, as a continuous rise and fall in the pressure of the air at a given point in space. When we speak, our vocal chords vibrate at various frequencies, creating corresponding vibrations in the air around them. These vibrations - waves - are then transmitted by the particles that make up air, and the vibrations are passed along till they reach someone else's ears…or a microphone that is designed to pick up these vibrations.
For more on microphones, see Section B: Microphones on Page 198Analog Audio
The microphone, in turn, converts these vibrations into an electrical signal, that rises and falls in exact correspondence with the rises and falls in the sound wave that is reaching the microphone. If the signal is then recorded on magnetic tape, what we have thus far is a process where a continuous wave or signal - even if it is changing from sound energy to electrical energy and stored as an arrangement of magnetic particles - is being preserved throughout the entire process. Since in each case, we have a signal that is analogous to the original sound wave that was produced, such signals are called analog signals.
Digital audio
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As technology progressed, however, new ways to store information became available. One such technology was digital storage, where information of any kind - visual, audio, or a combination of both - could be stored as a series of numbers, which together represented the original information. The information was stored usually in combinations of ones and zeros, a system of mathematics called binary numbers. (In fact, the fact that the storage was in the form of numeral digits was why it began to be called digital storage in the first place!)
Digital signals and storage offer us vast advantages over the older analog system:
1. Digital signals can be stored more economically than analog signals can. An LP or long playing record could store about a half hour's worth of music per side. Today, a small digital music player the size of a matchbox can store ten times as much.
2. Digital signals can also be manipulated more easily than analog signals, both in terms of clearing out unwanted components, and in terms of making changes to the signals.
3. Digital signals can also be copied and duplicated more easily: There is no deterioration of the signal across copies, unlike what used to happen with analog storage methods. This also applies to transmission of the signal through cables or broadcast, where analog signals invariably pick up some noise, but digital signals do not by virtue of the way the information is carried in both cases.
Where audio was concerned, this presents a plethora of options, with high quality audio becoming easily accessible to everyone: The first item of digital audio to make it to the market was the compact disc or CD, which stores digital data on a spinning disc of plastic encased metal.
For more on compact discs, see Section B: Compact Discs on Page 138 and Section B: CD Writers on Page 145Increasingly, audio recording - professional or for personal use, large radio station or community radio station - relies on digital audio equipment. Computers store information digitally, as do CDs, VCDs and DVDs. However, there is a downside to using digital technology; and that has to do with the way analog sound is converted to a digital signal by the digital sound equipment we use.
A/D conversion
We have already seen that a sound, when it originates, is a continuously rising and falling wave, and hence analog in nature. To convert analog sound into a digital signal, there has to be a process where we convert the characteristics of the sound wave into a set of equivalent numbers that describes the wave. This process is called Analog to Digital (or A/D) conversion, and the first step in this is called sampling.
Sampling is essentially the process of dividing the original wave up into a series of smaller slices. Obviously, it is easier to describe each slice more accurately than one can describe the wave as a whole. And if we then have
a description of the value of each slice (as an equivalent number), and a description of that number's position in the overall list of numbers describing the wave, it is fairly straightforward to reconstruct the original wave from these descriptions.
Analog: Inform Socks (3-Pack)-multi/l
The figure above shows the original wave, and slices we have made. We can reconstruct the wave by joining the samples (dots) that we have created. The reconstructed wave is not totally an accurate copy - it is more jagged than the original. This is the negative side of digital technology. A/D conversion is essentially a matter of approximation: The slices only give you an idea of what the original analog signal was like. However, this is becoming rapidly less of a drawback, as modern technology is using higher and higher sampling rates to make finer and finer slices of the original signal. If we can make the samples/slices much finer, the wave we can reconstruct from this information becomes closer and closer in shape to the original analog wave. It is generally accepted that if the sampling rate is twice the highest frequency wave in the audio signal, the results of sampling will be indistinguishable to the human ear from the original analog sound wave. This is why digital audio on CDs are sampled at 44100 Hertz (or 44100 samples per second, 44.1 KiloHertz or KHz) - because the highest frequency detectable by the human ear is 20000 Hz, and this is more than twice that. Digital video recordings record sound at 48000 Hz, so their audio is a little higher in quality. FM stations often sample the audio at 32 KHz, as the higher frequencies - 16000 Hz and above - often get lost in the broadcast process, and we need to be concerned with recording and reproducing audio only upto that limit.
Data/Bit Rate
The other factor that controls the quality and accuracy of the digital signal is the amount of information we can store about each of the samples/slices:
The more information we store about each sample, the greater the accuracy in reconstructing the original wave. In digital storage, each 1 or 0 that we use is called a bit of information. CDs usually use 16 bit sampling - that is, 16 bits to describe each sample. More recent pro audio equipment uses 24, 32, 48 or 96 bit sampling, leading to ever more accurate storage and retrieval.
D/A Conversion
Once the audio is stored in a digital format, we need equipment and techniques to convert it back into the original analog sound as well. The CD players, DVD players and MP3 music players that we see all around us today - including the music players built into many mobile phones - perform just this function: They convert the digital signal back into the analog signal, a process known as D/A conversion. This is the exact reverse of the sampling process.
As noted previously, both the sampling and the D/A conversion process involve some loss of audio information. Some people can be sensitive to this loss, and can 'feel' the difference between the digital version and the analog version of the same audio recording. But increasingly, as digital audio equipment improves, even low cost consumer grade equipment can give you a high enough grade of storage and reproduction to satisfy the vast majority of listeners.
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