By: Logan Sealover

 Analog-to-Digital Conversion (ADC)  Digital-to-Analog Conversion (DAC)  History of Class D Amplifiers  Audio Amplifier Classes  What Makes it Class D?  Class D Distortion Disaster  Frequent Future

 Today’s world—smartphones & laptops  Class D amplifiers currently at the top of the line ◦ Rated at 90% efficiency—less heat  The “D” in Class D amplification  Analog audio signals

 Can be any continuous value  Unwanted background noise  12” LP Record faults  Signals can only understand binary  Can compress analog into smaller files  CD “skips”

 Analog signals ◦ Must be converted to binary for use in digital equipment  Phone line  Audio CD ◦ Figure 1—analog audio wave signal  Y-axis—Voltage (V)  X-axis—Time (t)  ADC Sampling Figure 1

Figure 2—sampled analog signal  Sampling Rate ◦ A sample rate of 1 Hz uses 1 sample point/sec ◦ 22,050 Hz samples 22,050 points/second ◦ 44,100 Hz samples 44,100 points/second ◦ Space Requirements 34 Hz/second

 Nyquist Theorem ◦ Sampling Rate > 2 * Highest recorded frequency (Hz) ◦ Determines the best sampling rate during ADC for the best storage and sound quality  Human Hearing Range (20 Hz – 20,000 Hz) ◦ Typical music sampling rate (44,100 Hz) ◦ Phone system sampling rate (8,000 Hz or 8 kHz)

 Sample point value and bit size ◦ n-bits 2 n = N available states ◦ 0 – N defines the frequency of the sample point ◦ Greater sample point size means better sound quality, but also means more storage space is needed n = 8 bitsn = 16 bits

 Signal-to-Noise Ratio (SNR) ◦ SNR = 6.02 * n (# of sample point bits) dB ◦ Calculates the desired noise level of your audio application’s tolerable noise level ◦ Higher SNR provides better quality  Sample Point Bit Resolution Sizes ◦ 8 bit – Phone Systems ◦ 16 bit – Audio CDs ◦ 20 and 24 bit – High-end DVD audio

 Ex:) Coheed & Cambria – Welcome Home (8 bit)  With the sampling rate and sample point bit size, we can determine the necessary storage space  Phone System Quality: ◦ Sampling Rate – 8,000 Hz (8 kHz) ◦ Sample Point Size – 8 bits (1 byte) ◦ Transmission Rate: 8,000 Hz * 8 bits = 64,000 bps = 8,000 bytes/second OR 480,000 bytes/minute

 Audio CD Quality: ◦ Sampling Rate – 44,100 Hz ◦ Sample Point Size – 16 bits ◦ Two Independent Channels – Left & Right ◦ 44,100 Hz * 16 bits * 2 channels = 176,400 bytes/second OR 10,584,000 bytes/minute (~10 MB/minute) ◦ 720 MB of available space = 72 minutes  CD-ROM Quality: ◦ Storage space is slightly less due to error-correction code ◦ 650 MB of available space

 DAC converts binary number patterns into voltages and currents for your speakers  The DAC only connects the points that were captured by the previous ADC ◦ Audio signal not always the exact same as it was recorded ◦ Skipped values without curves Figure 3

 Much like a reverse process of ADC  Suffers sound quality loss  Digital-to-Analog Converters are unavoidably expensive SHARP SM-SX1

Pulse-Width Modulation

 For decades now, Class D amplifiers have been used in devices where high efficiency is important ◦ Medical field—Hearing aids ◦ Large controllers for bulky motors and electromagnets  Recently released to public ◦ Tripath Technology, Texas Instruments, Cirrus Logic, etc.  MP3/CD players  Laptops  Cellphones/PDAs  Home audio (TVs and stereos)

 Class D amplifiers ALWAYS have their transistors operating either fully on, or fully off  Able to accept a stream of bits from a CD/MP3 player and convert it to an analog signal  Older models are entirely analog ◦ Amplify digital signals only after conversion to analog ◦ Figure 5

Figure 5

 Different topologies and classes depending on how much current is allowed to pass while passive  Common Designs: ◦ Class A ◦ Class B ◦ Class AB ◦ Class D  Other Designs: ◦ Class C ◦ Class E & F ◦ Class G & H

 Class A ◦ No crossover distortion ◦ Wastes 50% of power ◦ Excess heat  Class B ◦ Crossover region near 0 V ◦ Push-Pull one at a time ◦ Two transistors

 Class B cont. ◦ Crossover distortion ◦ Fairly efficient—78% of power used ◦ Remaining power dissipated as heat  Class AB ◦ Push-Pull simultaneously ◦ Two transistors ◦ Smoother transfer rate  Less distortion ◦ Lower efficiency than B

 Class D uses Push-Pull between two transistors ◦ Switches between two voltage values (e.g., ±40 V) ◦ Can connect the output to both transistors simultaneously ◦ Neither transistor wastes any power ◦ Binary wave signals must be converted (DAC)  Other Amplifier Designs: ◦ Class C amps are used only for radio frequencies ◦ Class E & F are used for higher radio frequencies ◦ Class G & H amps use more complex variations of other amp classes for specialized applications

 Class D is prone to distortion ◦ Imperfect power supply regulation ◦ Timing errors  Power Supply Modulations caused by variations in the amount of current drawn by the amplifier ◦ Extra noise, or hum, from power supply fluctuations  Timing Errors due to changes in how long the transistors take to switch from on to off

 Frequency Response is the accuracy and equality of the sounds being produced during DAC ◦ Helps keep different frequencies at equal volume levels  These problems can be fixed using an analog feedback system to compensate for output-stage distortion ◦ Some of these systems handle frequency-response problems too

 Research will increasingly focus on Digital Signal Processing to correct inevitable analog errors ◦ Controllers that can sense voltage and modify their signals accordingly  Circuits that perform digital modulation by measuring analog error data and modifying the switch control signal as a result

 Digital sampling rate and its quality/storage  ADC/DAC not perfect  Amplifier classes  Class D distortion  Future prospects

 “Amplifier.” Wikipedia. Wikimedia Foundation, 13 Feb Web. 17 Feb  Putzeys, B. "Digital Audio's Final Frontier." IEEE Spectrum 40.3 (2003): Print.  Torres, Gabriel. "How Analog-to-Digital Converter (ADC) Work." Hardware Secrets. 21 Apr Web. 18 Feb

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