Data representation - sound Flashcards
What is analogue data?
Data which varies in a continuous way
What is digital data?
Data which takes the form of discrete values
What is an analogue signal?
An electrical signal which varies in a continuous way
Describe how sound is dealt with in an analogue sound system
Sound is an air pressure wave. The wave is often captured by a TRANSDUCER to produce an electrical current or voltage which varies proportionally to the wave
The electrical signal can be transmitted and used to recreate sound by vibrating a mechanical surface such as a loudspeaker to reproduce the original pressure wave
What is a transducer?
A device which converts variations in a physical property (e.g. pressure in a sound wave) to an electrical signal or vice versa
If vibration in a sound wave increases, what happens to the pitch?
It gets higher
What is a pure tone?
A regular sine wave
A more complicated sound is superposition of sine waves and is wobblier looking
How does a vinyl LP work?
The sound wave is recorded and a similar shape to the real sound wave is created in the grooves of the record
The needle follows the groove and the changes in this create changes in electrical signal
The signal is amplified and fed to a loudspeaker
Give two examples of a transducer
microphone
loudspeaker
Describe how sound is dealt with in a digital sound system
The electrical signal from a transducer is converted to numerical values with a size proportional to the strength of the signal. These are stored then converted back to an analogue electrical signal and then to a sound wave
In a CD how many sampled voltage values are there per second?
44,100 samples in TWO stereo channels
How many bits per voltage sample in a CD?
16 bits
How many possible voltage values does 16 bits give?
65,536
How many bytes per minute and per hour does a CD use?
minute=10MB
hour=650MB
What is a digital signal?
An electrical signal with voltage changes that are in discrete steps
Ina two level binary digit signal, which voltages are used?
5 volts and 0 volts
What is a binary digital signal?
A digital signal which uses only two voltage levels
This stops there being more than one binary pattern for 0
What is ADC?
Analogue to digital converter (hardware)
What is DAC?
Digital to analogue converter (harware)
What properties of a sound wave does a microphone record?
Frequency(pitch) and amplitude(volume)
What does PCM stand for?
Pulse code modulation
What is PCM?
A process for coding sampled analogue signals by recording the height of the signal at each sample point in binary electrical equivalent
What is the first step of using ADC to produce PCM?
samples of the analogue signal are taken at regular, fixed intervals of time
The sample is represented as narrow pulses of height
What does PAM stand for?
pulse amplitude modulation
What is PAM?
A process for sampling analogue signals at regular time intervals to produce electrical pulses with height proportional to the strength of the original signal’s amplitude
What is the sampling frequency/sampling rate?
The number of samples taken per second when digitising a continuous sound
What is the second step of using ADC to produce PCM?
The pulse amplitude modulation data is quantised
The height of each sample is approximated by an n bit integer
(e.g. 8 bit = 0..7 levels, values are made to fit within this range)
What is the third step of using ADC to produce PCM?
The quantised height of each pulse is encoded with n bits in binary form
How is the output from a PCM encoder shown?
A series of fixed-height pulses with the least significant bit first. The pulses can be stored in memory groups, in which each group has 3 bits
What is the staircase effect?
It happens when a DAC is used to change the output from a PCM encoder back to an analogue signal. The appproximation in the PCM conversion means the wave has fewer values in it, leading to a ‘blocky’ effect
What is quantisation noise?
The difference between the original amplitude and its sampled value
What is the sampling rate measured in?
Hz
What is the sampling resolution?
The number of bits assigned to each sample
How could you get a more accurate representation of the original signal?
Using more bits per sample
Increasing the sampling rate
What is Nyquist’s theorem?
We must sample at a frequency at least twice the rate of the highest frequency in the sample
What are the disadvantages of more frequent sampling or a higher sampling resolution?
More space in memory is needed to store the data
What is bandwidth?
Information carrying capacity
the amount of information which can be transferred per second
How can we reduce the bandwidth needed to transmit sound?
reducing the range of frequencies which are transmitted (e.g. compression techniques can be used on the voice without a problem in phone calls as a very high sound quality is not needed)
Describe WAV format
A file format for storing digitised sound
one minute of sound = 2.5 MB disk space
Used for CDs
Describe MPEG
MPEG audio is a file format with extensions such as mp2, mpa, mp3, mp4 etc
MPEG is a compression algorithm which can be applied to WAV files to cut out the frequencies which the human ear will not hear anyway
This reduces the file size to about 0.25 MB per minute
Used for most music available on the internet
What does MIDI stand for?
musical information digital interface
What is MIDI?
A way of representing the sound made by musical instruments by storing the notes to be played, their instrument and their duration
No sound wave is stored
Why is MIDI popular?
its not but the textbook wants me to talk about this
It is compact and flexible and it is easy to transpose music into a different key, play on different instruments or synthesis musical notation
What is synthesising sound?
Using digital means to generate audio signals resembling instrument or the voice
How does streaming audio work?
The streaming client recieves audio data and puts it in the buffer to be stored until a few seconds later when the player will read and play data from the buffer
If the buffer runs out of audio data then the player will pause until more data is recieved
What are the advantages of streaming?
There is no need to download an store large audio files
(no file space is occupied on the hard drive)
In theory it prevents copying
Give examples of ways in which stored, digitised sound can be edited
Notes removed, notes added, frequency of notes changed, background noise eliminated, merging or mixing seperate recordings
Give an example of a sound editing package?
e.g. audacity
Give three audio file formats from least storage space used to most storage space used
MIDI
MP3
WAV
