Topic 14: Digital Technology

Electronic devices deal with signals- the transfer of information from one plac to another. Electrical signals are sent through a changing electic field, which our electronic devices can interpret.

Signals can be in analogue (continuously varying) or digital (on/off) form. Examples are shown below.

Considering sound signals, we can see from the oscilloscope trace produced from a microphone that it is an analogue signal.

Use the graphs below to explain how is this signal produced.

What devices do you know that deal with analogue signals?

Analogue and Digital Devices

Most modern devices, use digital technology rather than analogue. Digital devices give us the ability to be able to use microprocessors, with which we can manipulate signals, and add vast quantities of information to simple sounds or pictures.

A microprocessor contains millions of tiny switches, which are either on or off. When on they give out a signal, and when off there is no signal. In this way a microprocessor is a device that can process digital signals.

Logic gates give us the ability to manipulate digital signals. By employing the correct combination of logic gates, different functions can be performed, and hence allow us to do more with digital data. At a basic level when dealing with a single or double input these devices seem rudimentary, however, when used with millions of signals, they enable great verstility.

As you see in the diagram, digital data (on/off) is represented by 1s and 0s. Using binary code, we can use a string of 1s and 0s to make up all numbers, and hence send all information.

We use a base 10 number system. This means that each 'place' in our number system is made of one of 10 symbols (0-9). In this way we can represent any number. For example 255...

We can equally well do this with a number system of base 2, only we need more 'places'.

Convert numbers to/from binary form: Questions 1 and 2 from Topic 14 teaching questions

Each digit in a digital signal is called a bit. In computing we work with 8 bit binary code. This means that each computing symbol is represented by an 8 digit binary code, following the ASCII (American Standard Code for Information Interchange) protocol (http://www.asciitable.com/).

Because eash character in ASCII is 8 bits long, we store information in 8 bit sequences. 8 bits is called a byte.

Write and save any single word you choose in a notepad document. Look at the properties of the saved document. What do you notice about its file size compared to the number of characters in your word? Explain this. Why is the file size much larger in when saved in a word documant?

What do you think is meant by the terms 'most significant bit' (MSB), and 'least significant bit' (LSB)?

Questions 3 and 4 from Topic 14 teaching questions

Sampling to Convert Analogue to Digital

With modern technology digital signals are more versitile then analogue ones, so we need to be able convert analogue to digital. We use Analogue to Digital Conversion (ADC) to do this.

ADC involves measuring or sampling an analogue signal at regular intervals called the sampling rate. The greater the sampling rate (more samples per second), the closer to the analogue signal the digital can be.

Nyquist theory states that to reproduce an analogue signal digitally the sampling rate must be twice the frequency of the analogue signal. This is so that at least one point on each peak and trough is recorded per cycle.

Why are sounds usually sampled at a rate of 40,000Hz?

Questions 5-7 from Topic 14 teaching questions

Kirk and Hodgson pp274 q1, 2, 3.

Finally, a number must be associated with each sample made, to represent the value of the analogue signal at that time. Pd values of analogue signals are assigned a 'quantum' value in order to do this. With a 4 bit-digital signal it is possible to assign 12 quantum levels, and so analogue signals can be given one of 12 values at each sample.

Kirk and Hodgson pp270-275, Kirk pp112-113

Applications of Digital Signals

Consolidate the ideas of sampling rate and quantum levels with these introductory questions

A complact disc is probably the most obvious and commonly used example of digital data storage. The standard CD-R can store up to 700MB of digital information.

If a CD with a 700MB storage capacity reads 16bit music at rate of 44.1kHz, how long can the music last for?

A CD is made up of a set of irregularities. The irregularities are called pits and lands, and occur in a sequence appropriate to the data that is stored on the CD.

The pits and lands occur in a very thin track, which spirals outwards from the centre of the disc. It is so thin, that the complete track for one CD is over 4km long!

The laser in a CD player scans over the CD track, and reads the lands and pits, by the phase difference between different reflected laser beams.

A phase difference between successive laser beams tells the CD player that a pit has changed to a land or visa versa.

It is these changes that return binary data. A change is recorded as a 1, and no change is recorded as a zero.

There is a problem here. If the CD rotates at a constant rate, then the speed at which the laser moves over the surface of the disc increases as it moves radially away from the centre of the disc. This could potentially lead to problems because the laser would move over the surface of the disc so quickly at the edges that it could miss depth changes. To account for this the speed of rotation of the CD changes. It slows down as the laser moves radially outwards so as to ensure a constant speed over the surface of the disc.

Find out and explain the main differences between CDs and DVDs, which allow DVDs to hold significantly greater quantities of data. You should consider the laser, the track and layering. Please write your answers in as simple terms as possible, with sources referenced.

A good resource- a direct comparison of the CDs and DVDs

Questions 9 and 10 from Topic 14 teaching questions

Advantages of Digital Storage

Manipulation of digital data is really easy. Changing binary coding is a simple process using digital microprocessors such as those used in computers. Software such as Adobe Photoshop, Audacity, and Windows Movie Maker allow us to simply and easily manipulate images, sounds and videos in a way that with analogue data was only possible with extremely expensive, complex machinary.

Corruption of data is much less likely also, and more easily fixed. A small change to an analogue signal changes its meaning completely, however that does not happen so easily with digital data.

Social Implications of Digital Data Storage

Write a paper on the implications for society of the advent of large capacity, widely available data storage.

Consider in your research both positive and negative aspects, and quote your sources.

Kirk and Hodgson pp275-278, Kirk pp114-115

Capacitance and the CCD

Much of the digital data we use today is used in image storage and manipulation. The CCD (Charge Coupled Device) is the component that allows us to store images as digital data. You should have a basic appreciation of how a conventional film camera works, and the concept of capacitance in order to understand the CCD.

Capacitance is essentially a device's ability to store charge. Capacitors are electrical components designed specifically to do this. They usually consist of two parallel conducting plates, separated by an insulator of some sort (called a dielectric). If a PD is applied to the capacitor, then charge builds up on the plates. The charge is trapped, it can not move because of the dielectric. We define capacitance as the charge stored per unit PD applied to a component. The units of capacitance are the Farad. A Farad of capacitance is a very large amount. we usually measure capacitance in mF.

The CCD is a small slice of silicon that has been divided into many tiny squares (pixels). Each pixel is essentially a capacitor that stores charge which is released from the silicon when light shines on it. In the case of a CCD pixel, it is a capacitor that uses silicon as its dielectric.

When silicon absorbs a photon of light, an electron is excited, and becomes free to move within the sturcture of the silicon, and thus is collected on the capacitor plates. The amount of charge, and hence capacitance of a CCD pixel is therefore directly related to the amount of light falling on that particular CCD.

The optics in a digital camera are exactly the same as those in a conventional film camera. The difference is the recording media. Instead of film, digital cameras use CCDs of varying quality. Light from the optical system is focussed onto an array of CCDs. The greater the number of CCDs in the array, the greater resolution the recored image can have. An 8 million pixel (8 mega pixel) CCD array is considered very good (however when you read this in a couple of years it won't!).

The overall structure of the CCD array must be such that each pixel is isolated, and charge can not 'leak' from one pixel to another.

Now all we need to do is read the information from the 6 million pixels!

Data is stored in the form on charge stored in each of the pixels in a 2D array on the CCD. This needs to be turned into a binary code for digital storage. The following process allows this to happen.

1. A Pd is applied across the chip, moving all the electrons down one row.

2. The bottom row is the serial register, in which each pixel's pd is measured between it's two plates. The Pd is proportional to the amount of charge stored in each pixel.

3. As each row is measured, the Pds are converted into a binary code by an ADC.

4. After measuring the charge is removed from the row, and the process starts over untill all the rows have been measured.

Now write an explanation about how a digital camera works.

Kirk and Hodgson pp279, q2, 4, 5, pp280 q2, 3, 4, 5, 6 do q1 for homework.

Kirk and Hodgson pp278-80, Kirk pp116

Applications and Limitations of the CCD to Optical Devices

Resolution is a measure of the smallest detail that can be discerned in an image. The higher the resolution the finer and more detailed an image becomes. The resolution that can be achieved by an optical device depends on the number of pixels that the CCD has, and the magnification the device is operating at.

In order to be resolved (seen as distinctly separate) in a digital picture, objects must form images on the CCD array that are separated by at least one pixel.

Magnification is simply the ratio of image height to object height. M= h_i / h_o.

The magnification that digital devices can achieve depends on the size of the CCD, as this will limit the possible image size. Common CCD sizes range between 6mm and 16mm (diagonally). The larger the CCD, the larger the image can be, and the more pixels it can be made up from.

Quantum efficiency is a measure of how good a photoelectric effect a device has. It is defined as the ratio of photons absorbed to electrons emitted. A low quantum efficiency can result in loss of image data.

eg: If 3 electrons are emitted for every 5 photons incident on a pixel, then its quantum efficiency is 3/5 = 60%.

So far we not dealt with how colours are defined. Pixels are not discerning regarding the wavelength of photon they absorb, so additional measures must be taken. Each pixel in an array has a colour filter covering it, allowing through to the pixel, only specific wavelengths of photons. Red, Green and Blue colour filters are used, allowing a picture to be build up based on quantities of these colours present in different areas of the image.

Digital video is difficult to achieve, because of the high processing rates required. For good quality video, a frame rate of 30 fps is needed. Thus 30 digital images, each of sufficient quality to be acceptable must be processed every second. This is further complicated by the need for colour video, in which case 3 CCDs are now commonly used, one for each colour, so light is dispersed in the camera's optical system such that each of the 3 promary coloured lighs are directed to the dedicated CCD. Now the three sets of data must be comined to give a colour film.

1. Add to your explanation of how a digital camera works, a list of advantages and disadvantages of digital photography.

2. Write about the uses of CCDs in one other application other than conventional photography (eg: Radiography, Astronomy, Scanning, etc)