Analog and Digital Flashcards
1
Q
Signal and Sound Process
A
- varying wave, typically carrying information we care about
- sound is vibrations in the air over time – essentially small changes in air pressure over time
- when a violin plays, the wood top and back of the instrument flex in and out, producing little ripples of air pressure which flow out from the violin, like ripples in a pond
- when the ripples hit an ear drum, it vibrates back and forth with the pressure changes, translating them into something your brain can sense
2
Q
Oscilloscope
A
- embodies the idea of analog
- a device that connects to wires, looks at the electrical signal varying over time, and draws that signal on its screen in real time
- voltage is on the vertical axis, and time is the horizontal axis
3
Q
Frequency, Amplitude, and Timbre
A
- how often the waves repeats per second
- higher frequency = higher note
- up an octave = doubling the frequency
- the height and depth that the signal makes on each cycle, corresponding to loudness
- bigger changes in air pressure = bigger waves = louder
- the fine pattern of shapes and wiggles that can seen on the signal
- different instruments playing the same note
4
Q
Chord, Discord, and Noise-cancelling
A
- peaks and valleys match up nicely, sounds naturally good (e.g. double the frequency)
- peaks and valleys don’t match up, sounds bad
- detect the ambient sound and create an exactly opposite sound, so when added together, it all cancels out to near silence
5
Q
Analog Technology
A
- in analog technology, a wave is recorded or used in its original form
- so, for example, in an analog tape recorder, a signal is taken straight from themicrophone and laid onto tape
- the wave from the microphone is an analog wave, and therefore the wave on the tape is analog as well
- that wave on the tape can be read, amplified and sent to a speaker to produce the sound
6
Q
Analog Phone System Process
A
- a person talks into the phone receiver
- their voice is vibrations in the air – a signal
- the phone receiver contains a microphone where the vibrations in the air move a tiny coil of wire
- the movement of the wires sets up tiny electrical flow that is in one-to-one correspondence with the air vibrations
- essentially we translate a signal in the air, to an analogous signal of electricity in wires
- the electrical signal travels out of the house to the phone company, gets amplified etc. and is eventually delivered to the phone at the other end
- there the electricity goes into a speaker – a speaker is just an arrangement of wires and magnets to translate electrical variations back to sound (the reverse of the microphone)
- the key feature of analog signaling is 1-1 correspondence .. variations (a signal) in one medium such as sound in the air, are translated to variations in some other domain like electricity in wires
- the signals in the different domains are in 1-1 correspondence – one goes up, the other goes up
7
Q
Analog Noise
A
- the pure sound signal you wanted but it’s been distorted by little up/down errors – like fuzzy variations around the true signal
- this is the “hiss” you hear on the phone line or an AM radio or a cassette tape
8
Q
Digitital Technology
A
- the computer “samples” the analog wave very rapidly over time, noting the value (the height basically) of the curve each time, and recording that value as a number
- on a CD, the sampling rate is 44,000 samples per second
- so on a CD, there are 44,000 numbers stored per second of music
- to hear the music, the numbers are turned into a voltage wave that approximates the original wave
9
Q
Digital Technology Advantages
A
- the recording does not degrade over time
- as long as the numbers can be read, you will always get exactly the same wave
- groups of numbers can often be compressed by finding patterns in them
- it is also easy to use special computers called digital signal processors (DSPs) to process and modify streams of numbers
10
Q
Digital Recording and Playback Process
A
- CD quality audio samples the sound signal 44000 times per second, noting the “height” of the sound signal once for sample
- each sample is a whole number in the range -32768 .. 32767 (this is the range of number that can be stored in 2 bytes)
- each sample number is coded as a series of 0’s and 1’s on the disk
- say, a dark spot on the disk is a “1”, a light spot is a “0”…now to play the CD, the player reads back this signal with the 0/1 information on it
- noise resistant
- as a second layer of error-correction, the CD stores in a sense, redundant copies of the sample data
- if one section has errors, detected with a checksum scheme, the CD player can switch to use an alternate copy
11
Q
Lossless Digital Compression
A
- as a practical matter, each sample number tends to be very close to the sample numbers that come just before and just after it in time
- so one way to “compress” the audio, so it takes up less space, is to record just the changes – for each sample, record how much change it is from the previous sample
- these change numbers tend to be small, so it turns out they can be recorded more compactly (requiring fewer 0’s and 1’s)
12
Q
Lossy Digital Compression
A
- drops every other sampled number and on playback, guesses the missing samples as the average of the surrounding numbers
- MP3 is complicated, reducing the space required by 10x, and also it is lossy
- discards little bits of the original signal in a way which the human auditory system tends not to notice
- JPEG also
13
Q
Suppose we have two musical notes, X and Y, and note X is higher than note Y. What must be true?
A
note X has more cycles per second than note Y
14
Q
Clearly it is possible to be in a room and hear two distinct sounds at the same time. Describe how this works.
A
- the sound signals mix in the air, forming a sum signal
- the ear takes in this sum and recovers the original signals
15
Q
Describes the digitization of sound.
A
converting the sound signal into a series of numbers representing the signal’s difference from zero at each moment
