Analog to Digital Analog to Digital ConversionConversion

For Introduction to Music Production for CourseraBy DJShirlee

Sound in the AirSound in the Air

As we recall from our last lesson, sound is a pressure variation in the air.

• Sound waves travel through the air by the change of air pressure at different moments in time.

•The stronger the air pressure, the louder the sound.•The stronger the air pressure, the louder the sound.

•As the speed of the air pressure changes become faster the perceived pitch is higher.

•These changes of amplitude (loudness) and frequency (Pitch) are the two factors of sound.

Sound Recording Analog MediumSound Recording Analog Medium

• Capturing recordings of sound on tape via a microphone (electrical voltage) the change in voltage represents changes in amplitude and frequency and is recorded on to a medium such as a tape.

•The recording on to tape is continuous.

Sound Recording to DigitalSound Recording to Digital

•Capturing recordings of sound microphone (electrical voltage) the change in voltage represents changes in amplitude and frequency.

•This is not a continuous process but instead digital records in steps, at regular time frames. digital records in steps, at regular time frames.

•At each step amplitude and frequency are recorded – this is called, Sampling.

Binary Information and FormBinary Information and Form

Computers translate sound into binary information, represented in 1’s and 0’s.

Binary form is the decimal counting system that is used for storing digital data.

Binary numbers are expressed in bits (Binary digit).

Bits are a single kind of memory location.

A single bit can only represent ON or OFF or 0 or 1.

Large numbers of bits are collections of “WORDS”.

Bytes and Words

A byte is 8 bits.

With 8 bits you can store any number between 0 and 255, since there are 256 different combinations of 1 and 0 to choose from.

A word is the number of bits that are manipulated as a unit by the particular CPU of the computer.

CPUs have a word size of 32 or 64 bits.

Keep in mind, WORD LENGTH IS RELATED TO AMPLITUDE

Values Represented by BitsValues Represented by Bits

It’s important in music producing to understand the how many values represent a bit.

Bit representation of a word

1 Bit word 2 Bit word 3 Bit word Decimal #

0 00 000 0

1 01 001 11 01 001 1

10 010 2

11 011 3

100 4

101 5

110 6

111 7

Values Represented by BitsValues Represented by Bits

Bits are always to the power of 2

2 to the power of Word length:

Every time you add a bit you double the value

For Example:

21

= 2

25 =32

22

= 4

26 =64

23

=8

27 =128

24 =16

28 =256

Bit Depth Bit Depth

Word length is also known as Bit Depth

Bit depth (sample-width) determines the accuracy of the amplitude measurements per sample.

A higher bit depth allows for greater audible perception in the subtle changes in loudness (the dynamic range).

This is a measurement of the ratio of loudest available undistorted noise and the quietest available.

16 bit is the standard for consumer level audio. i.e. CDs.

24 bit is used for the standard of recording level which offers a greater dynamic range (amplitude).

Sample RateSample RateHow often a sound is measured is called, Sample Rate

We have to measure the sound in the air over 40,000 times per second in order to accurately represent the continuously variable signal in the air as digital representation.

The higher the sample rate the higher the frequency that can be represented accurately.represented accurately.

A good recording sample rate is 44,100 hertz (i.e. CD)

The human ear can only hear approximately at 20,000 hertz.

Thus recording at this higher sample rate will enable to capture the audio at a level good for human hearing.

Referenced by Nyquist – Shannon theorem

Recording in your DAWRecording in your DAW

It’s important to set up your DAW for recording at the right Bit Rate and Buffer size when recording.

The following slides will outline how to set up your audio in Ableton Live 9

• Connect your audio interface to the Mac (via USB or Firewire most likely) and to

a power source if needed (most do). Then switch it on.

• Check the manual if there are any other setup steps necessary.

• Start Live and go to the Preferences > Audio. The shortcut for it is Cmd + ,.

• Make sure Core Audio is chosen as the Driver Type.

• Choose either the built-in input or the one of your audio interface as Audio Input

Device. You can also choose the built-in microphone, but then do yourself the favour

and use headphones. Otherwise arming a track will greet you with some nasty

feedback.

• Choose the built-in output or the one of your audio interface as Audio Output

Device.

• You can activate the needed mono and/or stereo in- and outputs under Input and

Output Config respectively.

• Choose the Sample Rate as needed. It's not saved with the Live Set, but is set universally so beware

if you tend to switch, e.g. between live and studio. You can set the Bit Depth in Preferences >

Record/Warp/Launch.

• A Buffer Size of 512 seems to work for most. Or you can try to find a setting that gives you low

latency without audio drop outs. This should always be a power of 2 (64, 128, 256, 512, 1024 and

so on) to avoid issues with the driver of the interface.

Now you should be all set. If you're using an audio interface, don't forget to plug in headphones or connect it to speakers.