Analog and Digital Conversion/Sampling and Reconstruction
Relationship Between A & D
Analog and digital signals both transmit information, and on occasion both signals can be used to transmit the same information. Sometimes digital information can be put through a machine called a "modulator" so that they can be transmitted over an analog network. Similarly, sometimes analog signals need to be digitized, to travel over a digital network. The modern telephone system, for instance, received analog signals from the handset, that are then turned into digital signals for transmission over a digital, fiber-optic network. Digital information stored on DVDs and CDs need to be converted to analog music and video signals, for use on analog TVs and Stereos. Both types of signals are highly useful, and they have moments where they each excel. However, there are plenty of times when we need to convert from one to the other, and that conversion process will be the primary focus of this wikibook.
A device that converts an analog signal to a digital signal is called a sampler. There are other names for samplers, including "Analog to Digital Converters". This is usually abbreviated as ADC, A2D, A/D, or other similar abbreviations. The important point is that analog signals are converted into a digital equivalent, according to some particular rule.
The process of sampling, by necessity, causes a loss of information. If we are only sampling at particular times, for instance, all the information between those two times is lost. Also, because digital signals have less accuracy than analog signals, the sampled values may not even be expressed correctly. The effects of sampling on a signal have a number of names including "Sampling noise", "sampling error", or "converted signal degradation". While this may sound like a terrible situation, there are methods to decreasing this error, and we will discuss them in this book.
Similar to samplers, but performing the exact opposite task, are Reconstructors. Reconstructors are frequently also known as "Reconstruction Circuits", or "Digital to Analog Converters". The later is frequently abbreviated as DAC, D2A, D/A, or other similar abbreviations.
Because digital signals don't have the same accuracy as analog signals, the reconstructed analog signal will have a certain amount of error associated with it. This error is frequently called the "Reconstruction noise", or "reconstruction error", and can become a large problem in sensitive applications.
Effects of Conversion
Sampling noise and Reconstruction noise not only combine in the final signal, but they can also be compounded over multiple stages of conversion. As a rule, every single conversion can lose data, or the amount of data can stay the same. However, a conversion can never "gain data". This can be demonstrated in computer audio or video, where every successive conversion between one file format to another can cause a loss of data, and the file never gets any better.