Sound becomes a signal: the microphone
Sound is a pattern of pressure changes in the air. A microphone has a thin sheet called a diaphragm. When sound hits it, it shakes. The microphone changes this shaking into a tiny electric voltage that rises and falls in the same pattern. This voltage is the audio signal.
- Dynamic microphone: the diaphragm moves a small coil near a magnet, which makes a voltage. It is strong and good for stages.
- Condenser microphone: the diaphragm is one plate of a capacitor; as it moves, the capacitance changes. It is very sensitive and needs a small power supply. Phones use tiny ones of this kind.
Making the signal bigger: amplifiers and mixers
A microphone signal is only a few millivolts. An amplifier makes it bigger so it can drive a speaker. A mixer joins several signals (voice, guitar, music) and lets you set the loudness of each. An equaliser lets you boost or cut the bass (low) or treble (high) parts.
Loudness is measured in decibels (dB). Sound that is 10 dB louder has ten times the sound power.
The signal becomes sound: the loudspeaker
A loudspeaker does the microphone's job backwards. The signal flows through a coil attached to a paper or plastic cone. The coil sits in the field of a magnet, so it is pushed back and forth in the same pattern as the signal. The cone shakes the air and we hear sound. A larger signal moves the cone more, so the sound is louder. A woofer is a big speaker for low sounds; a tweeter is a small one for high sounds.
Recording sound: analogue and digital
Old tape and vinyl records stored the wave itself (analogue). Modern equipment turns the wave into numbers. It measures the voltage many times each second; each measurement is a sample. The number of samples per second is the sampling rate. A music CD uses 44,100 samples per second (44.1 kHz). The more samples and the finer each measurement (bit depth), the closer the copy is to the real sound. Files like MP3 then squeeze the numbers to take less space.
Light becomes a signal: camera and image sensor
A camera has a lens that bends light so a small, sharp picture falls on an image sensor. The sensor is a grid of tiny light detectors called pixels (picture elements). Each pixel measures how much red, green and blue light falls on it and gives an electric signal. The chip then turns the signals into numbers.
The resolution is the number of pixels, for example 1920 × 1080. More pixels show finer detail.
Video and screens
A video is a quick series of still pictures called frames. The frame rate is frames per second (fps): films use 24 fps, many phones use 30 or 60. Your eye joins them and you see smooth motion.
A screen does the camera's job backwards. It is also a grid of pixels. In an LCD each pixel is a tiny light gate over a backlight; in an LED or OLED screen each pixel makes its own light. By mixing red, green and blue in each pixel, the screen makes every colour. The picture signal is sent frame after frame, pixel by pixel.
Try it
In the 3D scene, press Video and choose 4 × 4, then 16 × 16. Which picture shows the house better? Now at home: look at a phone screen with a drop of water on it (it works as a magnifier) and see the red, green and blue dots. Also, speak into your phone's voice recorder and play it back softly, then loudly, and think about which part of the chain you just changed.
Key formulas and definitions
- Resolution = pixels across × pixels down (1920 × 1080 = 2,073,600 pixels)
- Frame rate = frames per second (fps)
- Sampling rate = samples per second (Hz); CD audio = 44,100 Hz
- Data per second (uncompressed audio) = sampling rate × bit depth × channels
- Sound level change: +10 dB = 10 times sound power; +20 dB = 100 times
Worked examples
1. A camera sensor has 4000 pixels across and 3000 pixels down. How many pixels (in megapixels)?
4000 × 3000 = 12,000,000 pixels = 12 megapixels.
2. A Full HD screen is 1920 × 1080. How many pixels is that?
1920 × 1080 = 2,073,600 pixels, about 2.1 million.
3. A video lasts 10 seconds at 30 frames per second. How many frames?
10 × 30 = 300 frames.
4. CD audio takes 44,100 samples every second. How many samples in a 3 minute song, for one channel?
3 minutes = 180 s. 180 × 44,100 = 7,938,000 samples.
5. Uncompressed CD audio uses 16 bits, 44,100 samples/s and 2 channels. How many bits per second?
16 × 44,100 × 2 = 1,411,200 bits per second (about 1.41 Mbit/s).
6. A speaker is turned up by 20 dB. How many times has the sound power increased?
Every 10 dB is 10 times power, so 20 dB is 10 × 10 = 100 times.
7. A film at 24 fps is shown at 48 fps by showing each frame twice. How long does a 2 hour film last now?
Frames are shown twice as fast, but there are twice as many shown, so the time is unchanged: 2 hours.
Common mistakes
- Thinking the microphone and speaker are different kinds of machine. They are almost the same part used in opposite directions.
- Mixing up resolution and screen size. A small phone can have more pixels than a big old TV.
- Confusing pixels (a picture's dots) with frames (a video's pictures per second).
- Believing a higher sampling rate means a louder sound. It means a more exact copy, not more volume.