Digital Audio Theory

Question 1

What are the three steps of digital audio?

Answer 1

• conversion of analog signal into binary numbers (A/D)
• processing/handling/storage of digital audio data
• reconstruction of analog signal from binary numbers (D/A)

Question 2

What are the 7 basic steps of digital audio from microphone to speakers?

Answer 2

microphone –> analog signal –> A/D conversion –> storage (binary numbers) –> reconstruction (D/A) –> analog signal –> speakers

Question 3

What is the best analogy for analog vs binary audio?

Answer 3

analog: dimming switch
binary: on/off switch

Question 4

What are the 7 basic steps of analog to digital conversion?

Answer 4

analog signal –> anti-aliasing –> PAM –> sample and hold –> quantizing –> PCM –> storage

Question 5

What is pulse amplitude modulation (PAM)?

Answer 5

a modulator signal causes a carrier signal’s amplitude to move in direct relation to the modulator’s amplitude pattern
-when the modulator’s amplitude increases, the carrier’s amplitude increases

Question 6

What is the signal being modulated by the input signal called in PAM?

Answer 6

pulse wave

Question 7

What is the frequency called in PAM?

Answer 7

sample frequency

Question 8

What is the speed of the pulse wave in PAM called?

Answer 8

sample rate

Question 9

Which process takes snapshots of the instantaneous level of the input signal at regular intervals?

Answer 9

pulse amplitude modulation (PAM)

Question 10

What happens if too few samples of a waveform are taken per cycle (

Answer 10

aliasing (uneven representation of the original waveform)

Question 11

What is produced due to aliasing?

Answer 11

sum and difference tones

Question 12

If there is a 13kHz sampled frequency and the sample rate is 22kHz, what sum and difference tones will be produced?

Answer 12

sum: 35kHz
difference: 9kHz

Question 13

What does an anti aliasing filter look like?

Answer 13

a low-pass filter

Question 14

What is sample and hold?

Answer 14

takes input voltage of the moment and assigns it a binary value

Question 15

What is quantization?

Answer 15

maps values to set of discrete options, rounding up or down to nearest available number

Question 16

What is pulse code modulation (PCM)?

Answer 16

uses modulated pulse wave to capture amplitude info by numerical code (binary); results in digital signal

Question 17

What is the Nyquist theory?

Answer 17

highest frequency a digital audio system can record/reproduce is ½ the sampling rate of the system

Question 18

What is the Nyquist limit for CD?

Answer 18

22 kHz (sample rate is 44.1 kHz)

Question 19

What is bit resolution?

Answer 19

word size; number of 0’s and 1’s in each sample

Question 20

How does one find the signal to noise (S/N) ratio based on bit resolution?

Answer 20

6dB per bit

Question 21

What is the S/N ratio of a 44kHz 16-bit CD?

Answer 21

96

-16 bits x 6 = 96

Question 22

What is the S/N ratio of a 24 bit DAW?

Answer 22

144

-24 x 6 = 144

Question 23

How many MB of storage are taken up per track minute in 16 bit, 44.1kHz audio?

Answer 23

5MB

Question 24

Knowing what we know about how many MB of storage are taken up per track minute in 16 bit 44.1kHz audio, how many MB of storage are taken up per track minute in a CD?

Answer 24

10MB (because CDs are stereo)

Question 25

How many MB of storage are taken up per track minute in 24 bit, 44.1kHz audio?

Answer 25

7.5MB

Question 26

What would be the storage requirement, in GB, for a 5min, 24 track song at 24 bit 88.2kHz audio?

Answer 26

``` 24bit/44.1k=7.5MB/track min 88.2k/44.1k=2 7.5X2=15MB/min 24bitX5minX15MB=1800MB 1800MB=1.8GB ```

Question 27

What is dither?

Answer 27

low level random signal (noise) added to analog signal before A/D conversion

Question 28

What is re-dither?

Answer 28

noise/random bits are added at the very end of gain staging

Question 29

What is the difference between dither and re-dither?

Answer 29

- dither is an analog process and happens in the analog domain - re-dither is a digital process and happens in the digital domain

Question 30

Why does re-dither occur at the very end of the digital audio conversion process?

Answer 30

- gain stages make us lose data, so the output must be reduced to the word length of the converter - if you round off at every stage, your output will not be as accurate as when you round at the end

Question 31

What is word clock?

Answer 31

time reference for digital audio interconnections | -square wave that runs at the sample rate

Question 32

When does one need to use word clock?

Answer 32

when multiple audio devices are being used (one has to be master clock)