Digital Audio Theory Flashcards
What are the three steps of digital audio?
- 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)
What are the 7 basic steps of digital audio from microphone to speakers?
microphone –> analog signal –> A/D conversion –> storage (binary numbers) –> reconstruction (D/A) –> analog signal –> speakers
What is the best analogy for analog vs binary audio?
analog: dimming switch
binary: on/off switch
What are the 7 basic steps of analog to digital conversion?
analog signal –> anti-aliasing –> PAM –> sample and hold –> quantizing –> PCM –> storage
What is pulse amplitude modulation (PAM)?
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
What is the signal being modulated by the input signal called in PAM?
pulse wave
What is the frequency called in PAM?
sample frequency
What is the speed of the pulse wave in PAM called?
sample rate
Which process takes snapshots of the instantaneous level of the input signal at regular intervals?
pulse amplitude modulation (PAM)
What happens if too few samples of a waveform are taken per cycle (
aliasing (uneven representation of the original waveform)
What is produced due to aliasing?
sum and difference tones
If there is a 13kHz sampled frequency and the sample rate is 22kHz, what sum and difference tones will be produced?
sum: 35kHz
difference: 9kHz
What does an anti aliasing filter look like?
a low-pass filter
What is sample and hold?
takes input voltage of the moment and assigns it a binary value
What is quantization?
maps values to set of discrete options, rounding up or down to nearest available number
What is pulse code modulation (PCM)?
uses modulated pulse wave to capture amplitude info by numerical code (binary); results in digital signal
What is the Nyquist theory?
highest frequency a digital audio system can record/reproduce is ½ the sampling rate of the system
What is the Nyquist limit for CD?
22 kHz (sample rate is 44.1 kHz)
What is bit resolution?
word size; number of 0’s and 1’s in each sample
How does one find the signal to noise (S/N) ratio based on bit resolution?
6dB per bit
What is the S/N ratio of a 44kHz 16-bit CD?
96
-16 bits x 6 = 96
What is the S/N ratio of a 24 bit DAW?
144
-24 x 6 = 144
How many MB of storage are taken up per track minute in 16 bit, 44.1kHz audio?
5MB
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?
10MB (because CDs are stereo)
How many MB of storage are taken up per track minute in 24 bit, 44.1kHz audio?
7.5MB
What would be the storage requirement, in GB, for a 5min, 24 track song at 24 bit 88.2kHz audio?
24bit/44.1k=7.5MB/track min 88.2k/44.1k=2 7.5X2=15MB/min 24bitX5minX15MB=1800MB 1800MB=1.8GB
What is dither?
low level random signal (noise) added to analog signal before A/D conversion
What is re-dither?
noise/random bits are added at the very end of gain staging
What is the difference between dither and re-dither?
- dither is an analog process and happens in the analog domain
- re-dither is a digital process and happens in the digital domain
Why does re-dither occur at the very end of the digital audio conversion process?
- 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
What is word clock?
time reference for digital audio interconnections
-square wave that runs at the sample rate
When does one need to use word clock?
when multiple audio devices are being used (one has to be master clock)
