Audio Basics

1. Audio Basics • Analog to Digital Conversion • Sampling Rate • Quantization • Aliasing • Digital to Analog Conversion EE 3563 Digital Systems Design

2. 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 LPF ADC Quantizer Samples Clock – Sampling Rate EE 3563 Digital Systems Design

3. 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 LPF ADC Quantizer Samples Clock – Sampling Rate EE 3563 Digital Systems Design

4. 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 LPF ADC Quantizer Samples Clock – Sampling Rate EE 3563 Digital Systems Design

5. 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 LPF ADC Quantizer Samples Clock – Sampling Rate EE 3563 Digital Systems Design

6. Audio Basics - Analog to Digital Conversion • The sampling rate and the number of bits/sample together determine the overall fidelity Music LPF ADC Quantizer Samples Clock – Sampling Rate EE 3563 Digital Systems Design

7. 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 EE 3563 Digital Systems Design

8. Audio Basics – Example Waveform EE 3563 Digital Systems Design

9. Audio Basics – Example Waveform EE 3563 Digital Systems Design

10. Audio Basics – Example Waveform Note the “squareness” 1/5000th second EE 3563 Digital Systems Design

11. 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 the • ladder drops • The comparator output is high when • analog input voltage exceeds the • reference voltage • There is an 8-bit priority encoder • internally to produce the digital • output EE 3563 Digital Systems Design

12. 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 Samples DAC LPF Music Clock – Sampling Rate EE 3563 Digital Systems Design

13. 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 EE 3563 Digital Systems Design