External Hardware Devices Flashcards
Digital camera:
Shutter opens and light enters the lens
Light is focused onto a sensor (Charge Coupled Device or Complementary Metal Oxide Semiconductor), which is made of millions of transistors, each of which stores data for one pixel by converting it into electrons and recording the amount of charge
There will be multiple sensors or filters to get red, green and blue
The data for each pixel is stored and combined into one file, typically stored on flash memory
How many pixels is a megapixel:
1000000 (1 million)
Barcode reader:
An LED or other light is passed over an image
The reflected light is measured by a light sensor, like CCD or a photodiode
White areas reflect more light, so the waveform can be analysed to find the pattern
The waveform is analogue, so is converted to digital with an ADC
The UPC (universal product code) is the most common type of barcode - the numbers are a manual override and contain a check digit
QR codes are another type
RFID:
Radio frequency ID
Small tags the size of a grain of rice are attached to things and data is stored in them
They also have a modem for converting between radio and digital, and an antenna to send/receive signals
When they get near a reader, data is transferred over radio waves - either just location or data about what it’s attached to
They are either active or passive - active has it’s own power source, whereas passive takes power from the reader
Laser printer:
A laser charges a drum at certain places
Toner is dispersed onto the drum, and sticks to the drum where the charge is opposite to itself, creating a negative of the image
Charged paper is passed over the drum, opposite to the toner, so it sticks together
The paper moves on with the image and the charge is removed
Fuser melts the toner on to the paper
This is repeated for each colour (black, cyan, magenta, yellow)
Laser printers are more expensive than ink upfront, but the toner is cheaper and it’s faster
Magnetic hard disk:
A number of disks stacked on top of each other coated in a thin magnetic film - changes in the direction of magnetism correspond to 1s and 0s
They spin at 3600-12500 rpm, and the heads never touch the surface of the disk
There is often redundant space because space is allocated in clusters that may not all be full
1TB capacity
Optical disk:
1 spiral track on a disk with pits and lands
The edges of the pits are curved, so light isn’t reflected straight back up to the scanner, which is how 0s and 1s are generated
For rewritable disks, photosensitive dye is used over reflective aluminium and burned in certain places by the laser to create 0s and 1s because the density of the dye changes, so it only reflects in the places that are still translucent
Solid state drive/disk:
Made of memory cards of semiconductors No moving parts Uses NAND memory A controller allocates space The semiconductors use floating gate transistors - data is stored by a voltage being applied to the control gate which sends a current from the source to the drain. The electrons gain enough energy to pass through the insulating oxide layers, where they become trapped in the floating gate, giving it a negative charge, even when power is removed. To remove data, high voltage is applied to the source, giving it a positive charge, so the electrons move back to the drain.
Comparisons of storage, lowest to highest:
Capacity - optical, ssd, magnetic Cost - optical, magnetic, ssd Portability - magnetic, optical, ssd Power consumption - ssd, optical/magnetic Speed - optical, magnetic, ssd Latency - ssd, magnetic, optical Reliability - optical, magnetic, ssd
