FURTHER INFORMATION ON
THE NOVEMBER SUBJECTS
Computer and TV
screens
If you viewed the computer or TV screens with a lens you will
have noticed that there are many tiny groups of red, blue and
green stripes or dots depending on your type of monitor or TV.
Inside the TV or computer the signals for the red, blue and green
image which build up the picture are each sent to a different
electron gun. The electrons fired from each gun hit the back of
the screen and are targeted so that the electrons from each gun
only activate a part of the phosphor which emits light of the
appropriate colour. This allows a red, blue and green image to be
built up on the screen by the sets of phosphor dots or stripes.
If the red, blue and green stripe were equally illuminated in one of the groups, that tiny spot would be seen by the eye as white light. If you look at the red, blue and green text and compare it with the white area surrounding it, you will see that only the red stripes are bright in the red text and similarly for the relevant colour in the blue and the green text. In black areas of the screen none of the stripes are illuminated.
A computer is viewed from a much closer distance than a TV screen, and modern high resolution video modes demand that much more detail is presented on the computer screen than contained in a 'still' from a TV broadcast. The stripes on the computer screen are therefore much closer together than those on a TV. On the author's 15 inch monitor the repeating triplet of red, blue and green stripes is approx. 0.26mm wide whereas on his 14inch TV screen it is approx. 0.7mm.
If you take a snapshot of your PC and TV screen with a macro attachment on a 35mm camera (the author used a 50mm lens with 3 extension tubes), you can compare the phosphor dot/stripe density on the snapshots. To take a photo' of your PC and TV use a tripod and a shutter speed slower than 1/25th of second.
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The digital information on a CD is stored as a spiral track of pits of varying length in the flat polished disc.
A laser scans the spinning disc and the light hitting the pits is scattered whereas light is reflected off the disc surface in between the pits. The variation in the light scattering is detected and converted to electrical impulses to reproduce the digital information stored on the disc. The pits are typically 1-4 microns in length and the distance between the centre of adjacent 'tracks' of digital information is 1.6 microns. Unlike an LP the information is tracked from the centre outwards.
The disc rotates at a speed of 200-500 revolutions per minute, and the speed changes to maintain a constant linear velocity and hence steady transfer of digital information. This is unlike the LP record which must maintain a steady spinning speed of 33.3 rpm to accurately reproduce the music stored in analogue form.
If you own the Microsoft Encarta Encyclopaedia on CD, there is an excellent animated sequence showing how the CD works in the 'Sound and Reproduction' section.
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How the grooves store the information on an LP record
When the stylus is on the record it rests on the angled walls of a groove but does not touch the groove bottom. One wall contains the left channel of stereo music information and the other wall contains the right hand channel. As the stylus tracks the spinning record groove the stylus tip traces out the modulation of the groove ie the complex up and down and side to side movements of the groove.
The stylus is on the end of a thin bar called the cantilever which is attached to the record cartridge on the tone arm. At the opposite end of the cantilever to the stylus is usually a tiny magnet or coil which sits inside four electrical coils. The mechanical movement of the cantilever in the coils as the stylus tracks the groove is converted into electrical signals which are amplified and played through speakers to reproduce the stereo music. The rate of oscillation of the stylus (and hence the cantilever) determines the frequency of the sound whereas the amplitude of oscillation determines its loudness.
Did you know:
- the groove is typically 0.025mm deep and the groove is 0.013mm
wide at the top.
- the tip of the stylus must be less than 25 microns (0.025 mm)
to track the groove.
- the area of the stylus tip which touches the groove is so small
that it exerts a pressure of 50 000 pounds per square inch (300
000 000 Pascals).
- while the record is played the stylus is subjected to
accelerations of 16 000 metres/sec/sec which is more than 1600
times the force of gravity.
Source and further reading Practical Hi-Fi
Sound by Roger Driscoll, Hamlyn, London 1980. ISBN 0 600
43627 7.
Encyclopaedia Britannica, Sound Recording section, volume 27,
p571. 15th Edition, 1993.
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Cassette record / playback head
The cassette head image shown on the left was the
cheaper and poorer specification of the two. High quality
cassette heads are typically manufactured to higher engineering
tolerances and the left hand one appears to be the poorer
engineered.
There are two electromagnets in each head (for the left and right music channel) which are seen in the images as polished bands. Each electromagnet's core is made of sheets or laminates of a magnetisable material and has a wire coil wrapped around it. The laminates which form the magnet core can just be seen on the cheaper cassette head as dark lines but not on the higher quality head. Each electromagnet is 'C'-shaped and although it is not usually visible, they each have a tiny vertical gap in the centre.
A cassette tape consists of a plastic backing (typically 20-35 microns thick) coated with very fine iron oxide and/or chromium dioxide powder which can be magnetised to store the sound information. On a double-sided stereo music cassette there are four separate tracks of information ie a left and right channel for each side of the tape. When a cassette is played the tape passes over the gaps in the two 'C' shaped electromagnets. As the magnetised particles on the tape move across this gap, an electrical signal is induced in the coils in the cassette head which can be amplified to reproduce the stereo music.
When recording sound, an audio signal is applied to the coils surrounding the electromagnets. This sets up a varying magnetic field across the gap which magnetises the particles on the tape as it passes over the head thus storing the musical information on the tape.
The cassette head is only magnetised when a current passes through the coils of the electromagnets, if it was a permanent magnet it could potentially erase the recorded material on the tape! That's why tape head demagnetisers are sold to remove any residual magnetism.
Project for younger readers: If your family possesses both a hi-fi cassette recorder and a portable 'Sony Walkman' type cassette player, have a look at the cassette heads by eye perhaps using a torch to get a clearer look. (Ask your parents permission first). The Walkman probably has four bands of electromagnets whereas the hi-fi recorder probably has only two. Can you think why?
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Disclaimer: all the information in this series is given in good faith. However, no responsibility is accepted for damage to property or injury to persons as a result of readers investigating the subjects described. It is up to the reader to judge whether the subjects can be safely viewed in their own home. Younger readers should consult their parents or teacher as appropriate before examining either their or somebody else's property.