Digital Audio

Question 1

Analog Sound

Answer 1

Changes continuously and has an infinite number of possible states.

Example: A dimmer light switch is similar to an analog representation of sound. It has an infinite amount of values between “on” and “off”

Question 2

Digital Sound

Answer 2

Has a fixed number of states.

Example: A light switch with on/off has 2 possible states.

Question 3

Sampling

Answer 3

The ADC (Amplitude Digital Converter) measures the amplitude of the incoming waveform some number of times every second and assigns a numerical value to the amplitude.

Question 4

Sample Rate (Hz)

Answer 4

The number of samples taken per second by the ADC.

Example: CD quality digital audio is 44,100 Hz. This means that the ADC is measuring the incoming sound 44,100 times per second.

Question 5

Nyquist Frequency

Answer 5

Digital audio can contain no frequency higher than half of the sample rate.
- CD Quality audio is sampled at a rate of 44,100 Hz.
- 1/2 of 44,100 Hz = 21, 0505 Hz
- The highest frequency that CD quality audio can contain is 21,505 Hz.
21,050 is above the human audio range (20-20,000 Hz)

Question 6

Higher Sample Rate

Answer 6

Often times, audio is recorded at higher than CD-quality same rate.

• 48 kHz - slightly better than CD-quality. Standard for audio that accompanies video (DVD).
• 96 kHz/192 kHz contain frequencies far above human hearing used to capture ultrasonic frequencies.

Question 7

Quantizing

Answer 7

The ADC must assigns a numeric value for the incoming continuous analog signal. Data that falls in between numeric a values is rounded to the closest value.

Question 8

bit depth

Answer 8

The number of values that are available for the ADC to measure the amplitude of the incoming waveform at each sample.
1 bit = 2 values
2 bits = 4 values
3 bits = 8 values
8 bits = 256 values
16 bits = 65, 536 values (CD-Quality)
24 bits = 16,777,21 values (standard for modern recording)

Question 9

Signal-to-Err Ratio

Answer 9

Ration of the overall signal to the sampling error. Sampling errors introduce a small amount of noise to the system. Higher bit depths and quality ADC’s mean more signal and less error.

Question 10

File formats

Answer 10

Digital audio is stored in a file format that determines what type of information is in the file and how it is organized on the disk.

Question 11

uncompressed file formats

Answer 11

Digital audio is stored as a series of 16 or 24 bit amplitude values. The most common uncompressed file formats are: AIFF, WAV

Question 12

Lossless compressed file formats

Answer 12

Reduce the size of an audio file (up to 50% of the original file) in such a way that the original data can be perfectly recovered. Lossessl file formats include:

• zip(non-audio file)
• mp4 (.mp4 or m4a)
• FLAC (.flac)

Question 13

Lossy compressed file formats

Answer 13

reduce the size of an audio file (up to 10% of the original file) in such a way that the original data cannot be perfectly recovered. Lossy compressed file formats achieve smaller size by decreasing the bit depth of the audio file. File formats include:
- mp3(.mp3)
- mp4(.mp4)
- FLAC(.flac)
Common lossy bit rates include:
-128 Kbps & 256 kbps
- 256 kpbs is considered by many to be indistinguishable from CD-quality.

Question 14

Loop File Formats

Answer 14

are designed for composing with audio. There are several file formats for loops.

Question 15

WAV and AIFF Loop Files

Answer 15

are audio files that are designed to be repeated a loop

Question 16

Acid Loop(Sony)/Apple Loop(Apple)

Answer 16

are regular uncompressed audio files that contain information called metadata. The metadata allows the audio files to be stretched or compressed in order to change the tempo of the loop.

Question 17

Rex Files

Answer 17

Start as a regular uncompressed audio, then get processed by Propellorhead’s Recycle software in order to change tempo.