What’s Hot in DAP?
In this section, find out about Digital Audio Players. How does a CD-ROM work? What is MP3?
Sound is an analogue phenomena - cyclical compression’s and decompressions in the air that transmit movement to the eardrum - detected as sound by the
brain. Until recently the whole chain from sound source to ear was analogue, which means that there is a continuous range of air pressures (loudness) and rate of reversals (frequency).
For example, a sitar - an Indian string instrument - plucking a string causes it to vibrate which then moves the thin wood of the sitar’s sound box which transmits movements to the air which then propagate to peoples’ ears (at about 600 mph). So why go digital and what is digital anyway?
What is Digitising?
Quite simply, digitising of sound is the conversion of the continuous range of air pressures that make up sound into numbers or digits!. We can represent the sound as a graph where the horizontal
axis is time and the vertical axis is variation of air pressure around the mean. See how there is a continuous range. Well we could sample the waveform - that is take spot readings at defined
time intervals and get a series of numbers. For example if we sampled the waveform 5 times at the yellow spots, we would get values of 0, +1, 0, -1, 0. If we put those numbers into a DAC (digital to
analogue convertor) we would get out the same waveform! In practice there is a bit more to it, but the principle is the same. There needs to be
standards so a digital stream of numbers can be successfully converted back to the correct analogue signal no matter who produced the digits. For example, what is the sampling frequency? and what is the encoding
law? (if the values are not linear, which they aren’t, as more accuracy is needed with small amplitudes than with large amplitudes). For telephone
calls the sampling frequency is 8kHz and each sample is held in 8 bits to one of two international standard encoding laws. This means the maximum frequency that can be processed is 4 kHz (Nyquist’s law)
So Why go Digital?
Several reasons really:
- Many sound sources are now digital - synthesised keyboards for example
- Noise! Electronic analogue amplifiers always add a a little noise and distortion. This tends to be additive, so the sound gets less true
and more noisy the more amplifiers it passes through - which could be many. Also, analogue recording mediums also add noise and lose a little of the original. A digitised signal on the other hand is a
binary stream - just 0s and 1s. It is very easy to distinguish between a zero and a one even if the signal is very noisy and distorted and
then create a brand new ultra clean binary digit in its place - digital signals can be self-cleaning! Usually, there are also some redundant bits that enable you to determine if there is an error (a bit
was read as one rather than zero or vice versa) and even correct the error!
- Signal processing - it is now so easy with powerful DSPs (Digital Signal Processors) to modify the signal in so many creative ways
without introducing any extra noise. Signals can be mixed and compressed to take up less space (numbers of bits). You can modify the frequency response (Bass, Treble or graphic equalisers
for example), the amplitude (volume), echo cancellation or introduction (reverberation) and even the speed of speech without changing the frequency (so you can speed up a slow speaker or
slow down a fast speaker whilst it still sounds like them!)
- Recording Media and Transmission Media - are now almost completely digital, so no loss of signal quality by needing several
conversions. For example, the internet is a digital network, you hard disk is a digital device, and CDROMs are digital.
So if you can keep the sound signal in digital format whilst it is processed, stored and transmitted and only convert back to analogue for the final leg
of the journey to your ear you can get untold quality and introduce many effects.
Find out more about:
- Digital Recording Media - the CDROM
- Digital Audio Encoding - MP3
- Learn more about:
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