Transcript Digital Audio and Class D Amplification

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

Signals can only
understand binary

Unwanted background
noise

Can compress analog
into smaller files

12” LP Record faults

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

Sampling Rate
Figure 2—sampled analog signal
◦ 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
2n = N available states
n = 8 bits
n = 16 bits
◦ 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

Signal-to-Noise Ratio (SNR)
◦ SNR = 6.02 * n (# of sample point bits) + 1.76 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)
http://www.youtube.com/watch?v=ggahA5kQjHI

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. 2013. Web. 17 Feb. 2013.
Putzeys, B. "Digital Audio's Final Frontier." IEEE
Spectrum 40.3 (2003): 34-41. Print.
Torres, Gabriel. "How Analog-to-Digital Converter
(ADC) Work." Hardware Secrets. 21 Apr. 2006.
Web. 18 Feb. 2013.
END
(Applause)