audio and midi protocols

Question 1

analogue is ….
digital is ….

Answer 1

continuous time signals
discrete signals, snapshots

Question 2

midi is a … process

Answer 2

serial

Question 3

what was the first analogue recorder?

Answer 3

Thomas Edison, tin foil cylinder 1877

made of a horn, a diaphragm a needle and a tinfoil cylinder into which the needle impresses the analogue signal from vibrations

Question 4

what is tape made of and what are its characteristics in terms of sound?

Answer 4

made of polyester or iron oxide or ferric oxide, something that can store a pattern of magnetisation

the strength of magnetisation varies in strength and direction to represent the original waveform.

tape sound has dynamic and harmonic distortion that gives it colour compared to digital.

Question 5

how does digital recording work

Answer 5

converts analogue signal into numbers and stores them

analogue wave played back from digital is the exact same unless it has been corrupted

Question 6

what is ADC and DAC

Answer 6

analogue to digital converter
digital to analogue converter

Question 7

define sample rate and quantisation resolution

as these two factors increase…

Answer 7

sample rate - samples taken per second or snapshots

quantisation resolution - the number of possible gradients available when taking those snapshots, discrete amplitude resolution

…fidelity increases

Question 8

what do 1 and 0 represent in binary?

Answer 8

1 is on
0 is off

Question 9

how many midi numbers are there

Answer 9

128

Question 10

what are the minimum bytes needed to be sent over for midi to work, what do these 3 bytes contain

Answer 10

3 bytes, one status byte and two data bytes

SB has
- action - whether note is on or off
- midi channel - ch 1 is 0000, ch2 is 0001… ch 16 is 1111

first data byte has
pitch

second data byte has
velocity - from 1 to 127 and 0 is off

Question 11

in the status byte under action, what denotes note on and note off?

Answer 11

status byte box - 1

on - 001

off - 000

Question 12

what are 2 ways you can represent a note that is turned off in the midi table?

Answer 12

in action have 000 and max velocity
or
in action have 001 and 0 velocity

Question 13

how many bits in a byte

Answer 13

8

Question 14

give limitations of an analogue system

Answer 14

copies are never as good as original

analogue recording always adds noise

Question 15

how many samples would be taken with sample rate 50kHz of 1 minute of stereo audio?

Answer 15

2 x 50000 x 60 = 6,000,000 samples

times by2 because its stereo

Question 16

give the Nyquist criterion

Answer 16

the sampling frequency must be at least twice as fast as the highest frequency in the signal

this is to capture the peaks and troughs

Question 17

define critical sampling

Answer 17

when samples are taken at the peaks and troughs to produce the most accurate representation possible of the wave form

Question 18

what is aliasing?

what would happen to a pitch frequency of 30kHz running thru a 48k sample rate?

Answer 18

when the sample rate is not high enough for the pitch frequency of whatever it is sampling.

results in a wave to be produced that is lower than the pitch frequency, an aliased frequency, you cannot reconstruct the original higher freq if aliasing has occurred

the 30kHz frequency would become an 18kHz frequency because 48-30=18

Question 19

what does an antialiasing or low pass filter do and where is it found?

Answer 19

found between the preamp and the ADC
it makes sure no frequencies are occurring that are more than half that of the sample rate

Question 20

what does a smoothing filter do and where is it found?

Answer 20

found between the DAC and the amplifier
it changes the samples taken into a smooth representation of the original

Question 21

what was the problem with early anti aliasing filters?

Answer 21

they were tight brick wall filters that created resonant peaks at those cut offs
now they are smooth

Question 22

how do you calculate dynamic range

Answer 22

amount of bits x 6

Question 23

what is dithering?

Answer 23

adding white noise to a signal on purpose to remove quantisation distortion

quantisation distortion refers to the signal noise and error that is created when lowering bit depth of a signal, quantisation values are reduced when lowering bit depth so forces some values into values that do not represent them well.

dithering turns the quantisation distortions into low level hiss sounds, getting rid of unwanted harmonics

retains the audio quality of the original sound wave