The switch to digital form for audio and video has multiple reasons. Some of them are:
- Storage - analogue storage (particularly) in tapes has a limited life. Magnetic surface degrade and lose the data. Though a vinyl can store data literally permanently, they also degrade with use. Digtal data, once stored, never changes.
- Space constraints - Analogue information need huge space either as tapes for music or as film reels for movies. With the dramatic improvement in computer storage, hard disks are getting physically smaller with an ever increasing capacity to hold data.
- Convergence - All forms of gadgets that you use have now have become compatible with one another. So why not use digital form of music and videos also to take advantage of this convergence?
But digital transmission, at least in real time, is not that easy as you will understand as your read.
Analogue signals are simple and represent the information they convey through a continuous waveform analogous to the information itself. A 500 Hertz tone, for example, is represented by a voltage varying from positive to negative and back again, at 500 times per second. This is called a sine-wave.
A digital signal, unfortunately, does not bear any resemblance to the information it is carrying. The logical '0's and '1's of digital data are represented as a rapid series of fluctuation or transitions in the voltage. These fluctuations are creating an instantaneous square wave. Thus the same 500 Hertz tone is represented differently, and is subject to interpretation at the receiving end.
There are two types of digital circuits. One is called Transistor-To-Transistor Logic or TTL. TTL circuits use bipolar transistors which are current-controlled devices. TTL circuits are restricted to voltages between 4.75 to 5.25 volts. TTL circuits are very fast, but need a constant regulated power supply of 5 volts. Any significant variations in that power supply voltage will result in the transistor bias currents being incorrect, which then results in unreliable or unpredictable operation.
The other digital circuit is CMOS and uses field effect transistors more commonly known as MOSFETs. CMOS circuits operate in either the saturated or cutoff modes and never in the active mode. Their inputs are, however, sensitive to high voltages generated by static electricity, and may even be activated into "high" (1) or "low" (0) states by spurious voltage sources. CMOS circuits can operate in any voltage from 3 to 15. CMOS circuits are easier to operate as they do not need precise voltages. The only effect of a voltage variation in a CMOS circuit is the logical conclusion arrived at. For example, if you use a 5 volt system with CMOS circuit, an voltage near to 5 will be considered as high or a logical 1. Similarly any voltage near to zero will be considered low or a logical 0. One disadvantage of CMSO circuits is their speed. They are slow, but this compensated by using buffers at the output signal to increase the overall voltage gain.
Generally systems followed is shown by the following example where VCC is the supply voltage.
CMOS
Low= 0V to VCC/2
High= VCC/2 to VCC
TTL
Low= 0V to 0.8V
High= 2V to VCC
But the voltage is not important for what we are discussing.
Binary representation are actually very easy to represent in physical terms. A binary bit can have only one of two different values - a '0' or a '1'. Any physical entity capable of switching between two states can be used to represent a bit.
One such entity are transistors in electronic circuits. When operated at their bias limits, transistors may be in one of two different states - either cutoff (no controlled current) or saturation (maximum controlled current). If a transistor circuit is designed to maximize the probability of falling into either one of these states, it can serve as a physical representation of a binary bit. A voltage signal measured at the output of such a circuit can also serve as a representation of a single bit - a low voltage representing a binary "0" and a relatively high voltage representing a binary "1."
But that is where the conundrum comes in. Let us say I design a digital circuit that has zero volt to represent logical 0, and 1 volt to represent logical 1. What happens when we get 0.5 volts? This kind of difference could come in because of various factors including attenuation in the cable or an external surge. A incorrect voltage would be invalid and would occur only in a fault condition or during a logic level transition, as most circuits are not purely resistive, and therefore cannot instantly change voltage levels. However, few logic circuits can detect such a fault, and most will just choose to interpret the signal randomly as either a 0 or a 1.
In a computer system, this does have too much of an effect. Digital transmission in computer circuits have what is called error correction in the form of a checksum carried at the end of a fixed quantum of data. The receiving circuit uses a program to calculate the checksum by itself from the data received and compares it with the checksum received. If there is a mismatch, the receiving circuit simply instructs the transmitting circuit to resend the data. This is done till the checksums match.
In audio and video circuitry, as they are in a real time, such checksum calculations or what is called error correction are difficult to perform. So when digital data are lost, the loss is final. Depending on what is lost, this may result in one of three options:
1. The receiving circuit will interpolate or guess the missing data.
2. The receiving circuit will completely misread the incoming data
3. The receiving circuit will completely miss the data, where the signals disappears for a time.
This will result in what we all know as jitter. As the digital signal has sharp transitions it is more sensitive to degradations as compared to a analogue signal.
In an analogue signal, the change of an analogue waveform is progressive and continuous--the more noise is introduced, the more noise will come out at the other end. Disturbance en route will not have such a serious effect on the waveform, and will be minor in nature.
A digital signal, on the other hand works differently. Because of its sharp transitions, it is highly sensitive and subject to sharp degradation in its waveform. A originating square wave will never arrive in it's original form. The corners of the square wave are rounded off to a greater or lesser degree, making the wave uneven. This makes it very difficult for the receiving circuit to correctly interpret the incoming signal and clock the signal. This is what we all know as jitter.
Jitter is introduced as the digital signal travels through the cable and depends upon the characteristic impedance of the cable, the capacitance, and the impedance match between the source and load devices.
Thus it is important to use a cable of high quality for digital circuits.
Cheers