Fall 2004 EE 3563 Digital Systems Design

Audio Basics

Analog to Digital Conversion Sampling Rate Quantization Aliasing Digital to Analog Conversion

Fall 2004 EE 3563 Digital Systems Design

Audio Basics - Analog to Digital Conversion

Process of digitizing a signal (such as music)

Human hearing range is roughly 20 Hz to 20 KHz CD’s are sampled at 44,100 Hz – that’s no coincidence Nyquist Theorem – Must sample at a minimum of twice the

highest frequency– If not, undesirable aliasing will occur

Music ADC QuantizerLPF

Clock – Sampling Rate

Samples

Fall 2004 EE 3563 Digital Systems Design

Audio Basics - Analog to Digital Conversion

LPF – Low Pass Filter – used to remove frequencies higher than the Nyquist rate– It’s like turning down the treble on your stereo

Clock is the sampling rate If clock is 44.1 KHz, then the LPF should remove all frequencies above

22.05 KHz– In practice, you need a little extra removed, so 20 KHz is the cutoff

Sampling rate determines the frequency response– Too low and it will sound like an AM radio– Tradeoff is in data storage space

Music ADC QuantizerLPF

Clock – Sampling Rate

Samples

Fall 2004 EE 3563 Digital Systems Design

Audio Basics - Analog to Digital Conversion

The Analog to Digital Converter gets a sample every time the clock ticks

The sample is passed on to a quantizer The quantizer outputs a number corresponding to the

amplitude of the music at that point The range of values depends upon how many bits per sample For CD quality, 16 bits are used (-32768 to +32767) For voice quality, 8 bits are used (-128 to +127)

Music ADC QuantizerLPF

Clock – Sampling Rate

Samples

Fall 2004 EE 3563 Digital Systems Design

Audio Basics - Analog to Digital Conversion

The fewer the bits, the larger the quantization error, resulting in lower quality

Suppose you gave the teller $93 and asked for change– Suppose she only had twenties (5 steps) – the “quantization” error would be

$13– Suppose she had tens (10 steps) – the error would be $3

The tradeoff is in the amount of data to store CD Quality: 2 channels * 16 bits/sample * 44100 samples/sec

– = 176400 bytes/sec Voice Quality: 1 channel * 8 bits/sample * 8000 samples/sec

– = 8000 bytes/sec

Music ADC QuantizerLPF

Clock – Sampling Rate

Samples

Fall 2004 EE 3563 Digital Systems Design

Audio Basics - Analog to Digital Conversion

The sampling rate and the number of bits/sample together determine the overall fidelity

Music ADC QuantizerLPF

Clock – Sampling Rate

Samples

Fall 2004 EE 3563 Digital Systems Design

Audio Basics - Aliasing

Aliasing occurs when the sampling frequency is below the Nyquist rate

It manifests itself as low frequency noise Sampling at Nyquist frequency Sampling below Nyquist frequency

Fall 2004 EE 3563 Digital Systems Design

Audio Basics – Example Waveform

Fall 2004 EE 3563 Digital Systems Design

Audio Basics – Example Waveform

Fall 2004 EE 3563 Digital Systems Design

Audio Basics – Example Waveform

1/5000th second

Note the “squareness”

Fall 2004 EE 3563 Digital Systems Design

Audio Basics – Analog to Digital Converter

• A basic Analog to Digital Converter(ADC) is shown• Note that these comparators are ANALOG comparators• The voltage at each point along theladder drops• The comparator output is high whenanalog input voltage exceeds the reference voltage• There is an 8-bit priority encoder internally to produce the digital output

Fall 2004 EE 3563 Digital Systems Design

Audio Basics - Digital to Analog Conversion

For converting back to music, the process is reversed The LPF is required on the output because it has a staircase

shape– In reality, a staircase shape is composed of an infinite number of sine

waves of increasing frequency

– These frequencies must be removed or the output will be noisy

MusicDAC LPF

Clock – Sampling Rate

Samples

Fall 2004 EE 3563 Digital Systems Design

Digital to Analog Converter

There is a resistive ladder that

must be very precise Each of the switches is essentially

a mux that switches between the

reference voltage and ground The final output is called a

summing amplifier

– it is simply an analog adder