Digital to Analogue - Analogue to Digital
Digital - Has two states ON or OFF - Like a light. Why is everything Digital? Digital signals are much easier to transport around and are highly resistant to loss due to poor transmission links. Analogue signals vary from nothing to maximum and very quiet signals (close to the "nothing" end) tend to get swamped by interference and noise as they travel through the transmission medium. Hence the trend is to convert very delicate analogue signals to the almost "bomb proof" digital version as soon as possible. Convertors To get from one method of transmission to the other requires a convertor - either Analogue to Digital (A-D) or the reverse, Digital to Analogue (D-A). No process is perfect and the quality of conversion has a direct bearing on the accuracy (and hence musical quality) of the resultant signal.
An Analogy - Suppose you were given the task of building a curved wall for your garden - around a tree trunk for example and you were given large blocks (building breeze blocks) to do it with. What would the completed structure look like? Similarly with convertors - the more bits we use then the smaller is the size of brick. So an 8 bit convertor is like using large breeze blocks whilst a 24 bit convertor can be likened to house bricks. No doubt in time the bit rates will rise higher to give an even better approximation. Filters It is possible to apply filters to the convertor output (D-A) to get a better approximation. Using our analogy above this is like plastering over the bricks with mortar to smooth out the irregularities. Unfortunately, no matter how good the builder is, there will always be rough bits on the wall - it will never be a perfect circle. Similarly our convertor may have a number of filter circuits built in to smooth out the rough bits but it cannot achieve perfection. Of course the higher the bit rate, the less there is to fill in and smooth over so obviously we need to focus on bit rate as an initial guide to sound quality. Choosing a Convertor Most equipment these days have convertors built in so the issue of choosing one never arises. However, some systems will allow the insertion of a stand alone unit and so what should we look for? Like any other piece of equipment, the basic specifications tell us a lot about technical performance but our ears tell us most and again it is impractical to personally test every convertor in your own listening space. So the best we can do is to compare specifications.
With convertors, it is the high frequencies that present most problems. The convertor is like a person taking a series of photographs - one after the other. Each "picture" happens in an instant of time (very fast) and records exactly what is seen. Likewise the convertor (A-D) takes a "photograph" of the audio, then goes away and works on it. First, the convertor looks at the sample and compares it with a table it has of values. It finds the nearest value and then looks up on another table what the numeric equivalent is in digital speak. So the original sample is converted into a digital "word" and sent out as a data stream. The convertor then takes another "photograph" and repeats the procedure to send another digital word out and so on. The analogue input signal (our music) is thus converted to a continuous stream of digital information which consists of a very large number of ON or OFF pulses which represent our music. Specifications
Distortion - Is the key parameter and needs to extremely low as distortion due to digital abberations is awful.
|