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analogue/ digital audio/ gain staging > Digital Audio > Flashcards

Digital Audio Flashcards

1
Q

What is digital audio

A

analogue voltages that have been turned into simple mathematical values

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2
Q

difference between analogue and digital audio

A

analogue describes a waveform that is continuous in time and has a continuous range of amplitude values

digital is a signal whose independent time variable is quantised, the value of the signal is only known at discrete instances in time. A signals amplitude is also quantised. its time signal not responded by a continuous waveform but instead a series of values

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3
Q

what is the advantage of digital audio

A

numbers representing a waveform can be effectively communicated and recorded with greater precision than continually changing analogue voltages

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4
Q

what are the different ways of transmitting digital digits:

A

pulse number modulation (PNM)

pulse position modulation (PPM)

pulse width modulation (PWM)

pulse amplitude modulation (PAM)

pulse code modulation (PCM)

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5
Q

which method of transmitting digital digits do we use

A

pulse code modulation (PCM)

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6
Q

what type of code does pulse code modulation use

A

binary code

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7
Q

what is binary code

A

it is digital information stored using the base 2 system

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8
Q

what is the base2 system

A

only uses 2 symbols, zeros or ones (ons and offs)

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9
Q

what does a 0 digit in base2 binary signify

A

an OFF message

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10
Q

what does a 1 digit in base2 binary signify

A

an ON message

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11
Q

what is a 1 or 0 digit called in binary

A

a BIT

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12
Q

what do we use bits and bytes to show

A

binary word lengths

size of hardrives

digital datae transfer rates

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13
Q

what is a bit

A

either a 0 or a 1

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14
Q

what is a byte

A

8 bits

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15
Q

what is a nibble

A

4 bits

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16
Q

how are digital and analogue audio differen

A

analogue = signals are carried as alternating current, with a continuously varying voltage

digital audio signals are sampled versions of the analogue waveform, samples are taken a certain number of times per second. each sample will then have a number of bits assigned to it to store the instantaneous amplitude level

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17
Q

what is the quality/ resolution of digital signal based on?

A

the sample rate and the bit depth

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18
Q

what is the sample rate of a signal

A

the number of discrete samples of an analog waveform taken per second. it is measured as a frequency in Hertz

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19
Q

the same rate determines the…..

A

frequency response of the digital signal

20
Q

what are common sample rates

A

44.1kHz - CD quality
48kHz - originally from DAT machines
96 kHz - the most common high quality format
192kHz - uber high quality format

21
Q

what is sample and hold

A

the process of taking digital samples defined by the sample rate. how long the initial sample is held for before the next sample is taken. this is what gives digital waveforms their square like shape

22
Q

what is bit depth

A

bit depth is the number of binary bits assigned to represent the instantaneous amplitude value of the digital waveform

23
Q

what does bit depth determine

A

because it represents amplitude, it roughly determines dynamic range also

the higher the bit depth, the more accurately we can measure amplitude. more bis means more discreet values can be represented.

24
Q

what does the digital dynamic range mean

A

larger bit depths (24 bit) will more accurately represent a much larger and varying dynamic range than lower ones (16bit)

25
Q

1 bit approx represents what dynamic range?

A

6 dB

26
Q

16 bit approx represents what dynamic range?

A

96dB

27
Q

24 bit approx represents what dynamic range?

A

144dB

28
Q

what do we refer to as the dynamic range for digital signals

A

signal to error ratio

29
Q

what is quantisation in digital audio

A

the act of passing a binary value to the voltage (amplitude) that is read for each sample in the sample and hold circuit. voltage values need to be rounded up or down to fit a value

30
Q

what is quantisation noise?

A

the raising and lowering of the voltage/ amplitude is call Q distortion aka quantisation noise, which could be referred to as the noise floor for digital signals as its more prominent for low signal levels

31
Q

what is the nyquist theorem?

A

states that the sample rate must be at least twice the highest frequency you wish to represent in the digitised audio.

this is because at least 2 samples must be taken per frequency in order to reproduce that frequency.

32
Q

what is aliasing

A

if the sample rate is less than twice the highest frequency you wish to sample, you will get aliasing. this is the introduction of false or alias frequencies that were not present in the original song and are a form of harmonic distortion

33
Q

what are aliasing frequencies

A

the difference between the sample rate and the frequency you are trying to sample

34
Q

what do anti aliasing filters do

A

they are used to remove frequencies above the nyquist frequency to prevent aliasing. sample rates are set slightly higher than twice the required frequency to accomodate the roll of off these filters.

low pass filters used to remove frequencies above the nyquist frequency to prevent aliasing.

35
Q

What is oversampling

A

sampling at a much higher rate than the eventual sample rate which gives the analogue anti aliasing filter a much wider range to operate in

36
Q

what is big endian/ little endian

A

the different ways in which the bytes are arranged.

big endian (apple) = byte is on the right

little endian (microsoft) = byte is on the left

37
Q

what is unsigned integer fixed point notation?

A

you can only represent positive values

38
Q

what is signed integer fixed point notation?

A

you get an extra digit so you can show if its a positive or negative value (0 is positive, 1 is negative)

39
Q

what is fixed point binary

A

this is when the decimal point remains in the same place and divides the binary into two sections. left of the decimel point the values double, and the right side it halfs.

Flawed system as the numbers get small very quickly

40
Q

what is floating point binary

A

when the decimal point can move, meaning you can get more accurate values.

the uses an exponent which tells us where the decimal point is, and the mantissa to represent the value

41
Q

what are sample clocks

A

they are digital clocks that provide the timing intervals between sample playback. the timing intervals determine the rate. clocks primary function is to set a steady pace

42
Q

what does a typical clock run at?

A

about 256 times the sample rate

43
Q

what is jitter

A

the loss of a sample or block of samples in digital audio bitstream.

44
Q

what are the types of jitter?

A

random jitter - similar to noise as the timebase is altered in a random fashion causing a random voltage error, unpredictable effect on audio

periodic jitter - occurs at regular intervals, its effect on audio is to cause sidebands to occur at specific frequencies that are mathematically related to the fundamental, this type of distortion is called frequency modulation because the freq jitter interacts with the source audio resulting in noise

45
Q

what is dither

A

low volume noise intorduced into digital audio when converting from higher bit resolution to a lower bit one.

46
Q

what are the data loss compression types

A

lose-less - all original data is kept, allowing original data to be perfectly reconstructed

lossy - uses inexact approximations and partial data discarding to represent the content

analogue/ digital audio/ gain staging (3 decks)

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