DIGITAL SOUND

Analogue vs digital signals

Sound (e.g. music, speech) is an air pressure (analogue signal) that issensed by our ears.

A transducer such as microphone converts continuously varyingsound pressure into a continuous varying electrical signal.

To change into Digital signal, the electrical signal is converted intosequence of numerical value proportional to the strength of thesignal and the numerical values are stored and processed.

The digital sound can be converted back into analogue form using aDAC.

Analogue Sound vs Digital Sound

Conversion from analogue signal todigital and vice-versa

Analog transmission is not particularly efficient. When analogsignals become weak because of transmission loss, it is hard toseparate the complex analog structure from the structure ofrandom transmission noise. If you amplify analog signals, it alsoamplifies noise, and eventually analog connections become toonoisy to use.

Digital signals, having only "one-bit" and "zero-bit" states, are moreeasily separated from noise. They can be amplified withoutcorruption. Digital coding is more immune to noise corruption onlong-distance connections.

WHY DIGITAL SOUND?

PCM is a process for coding sampled analogue signals by recording theheight of each sample in a binary electrical equivalent

PULSE CODE MODULATION

Time Level(rounded)BinaryRepresentation

0 0 0001 1 0012 4 1003 6 1104 7 1115 6 1106 3 0117 1 0018 0 000

Sampling and quantization of a signal (red) for 4-bit PCM

More PCM

PCM STEPS

1. Samples are taken of the analogue signal at fixed and regular intervals oftime. The sampling rate or sampling frequency is at least twice the highestfrequency in analogue signal (Nyquist criterion). This process is known as PAM(Pulse Amplitude Modulation)

PAM

2 . To produce PCM data, the PAM samples arequantized.

PCM Quantization

Finally, the height of eachPCM pulse is encoded in nbits to produce the digitaloutput. The output fromPCM encoder is a sequenceof fixed height pulses. Thetrain of pulses may then bestored in memory in groups ,where each group consistsof 4 bits.

PCM ENCODING

Sampling Rate - The number of samples taken per second. It isexpressed in hertz (Hz).

Hertz (Hz) - the SI unit of frequency defined as the number ofcycles per second of a periodic phenomenon

The discrete approximations (in red) can be used to recreate theoriginal sound (grey). However, due to limitations in the numberof samples we take we are often unable to truly represent asound wave, though we can get close enough for the human earnot to notice the difference.

Why Sampling?

To create digital music that sounds close to the real thing you need to look at theanalogue sound waves and try to represent them digitally. This requires you to tryto replicate the analogue (and continuous) waves as discrete values.

Lower sampling rates decrease the quality of the sound (very distant from the onebeing recorded).

Higher sampling rates increase the quality of the sound recording but requiremore storage space than lower sampling rates.

Sampling Rate (1)

Sampling rate (2)

Sampling resolution - the number of bits assigned to eachsample

Different sounds can have different volumes. The samplingresolution allows you to set the range of volumesstorable for each sample. If you have a low samplingresolution then the range of volumes will be very limited, ifyou have a high sampling resolution then the file size maybecome unfeasible.

The sampling resolution for a CD is 16 bits used per sample.

Sampling resolution

Sampling Rate and Resolution

Bit rate - the number of bits required to store 1 second ofsound

To work out the size of a sound sample requires the followingequation:

File Size = Sample Rate * Sample Resolution * Length ofsound

This is the same as saying: File Size = Bit Rate * Length of sound

File sizes

Sample Rate = 8,000Hz Sample Resolution = 16 bit Length of Sound = 30 seconds

Therefore the total file size would be:

8,000 * 16 * 30 = 3 840 000 Bits = 480 000 Bytes

Calculation of file size

DIGITAL AUDIO FILE FORMATS

Editing Tools : Format Converter, Mixer, Equalizer,Frequency Tuner, Track Splitter, Edits Multiple Tracks

Filters: Band Pass, High/Low Pass, Silence Reduction,Noise Reduction, Band Stop

Effects: Invert, Pitch, Reverb, Echo, Cross Fading,Silence, Delay, Stretch, Noise, Trim, Resample,ReduceVocals

Recording Ability: Line-In, Streaming Audio, MIDI Files

Features of Audio tools

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