Media Advances: HighFidelity Audio from Integrated Audio Components

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Media AdvancesHigh-Fidelity Audio from
Integrated Audio Components
Hakon Strande Program ManagerWMDGhakons _at_
microsoft.com Microsoft Corporation

• David Roach
• Audio Evangelist
• droach_at_sigmatel.com

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Session Outline

• In Search of Better Audio
• Why
• Terminology
• Attributes of Good Audio
• Audio is Part of the System
• Common Complaints
• Considerations
• Testing
• Resources

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Why Focus on Fidelity From Integrated Audio?

• Much can be gained with minimal effort
• Design board with audio in mind
• Our motivation for providing these guidelines
• 95 of PC audio solutions are integrated
• Microsoft would like the Windows PC to achieve
  parity with Consumer Electronics fidelity
• Microsoft Universal Audio Architecture initiative
• Standardizing external and integrated audio
  around established technology specifications
• Integrated audio UAA compliant solution is Intel
  High Definition Audio
• Has great potential if device and board
  implementation is done with fidelity in mind

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Terminology

• In order to understand and communicate about
  audio, consistent terminology must be used
• Volt
• Decibel (dB)
• Hertz
• Octave
• Frequency Response
• Signal To Noise Ratio (SNR)
• Total Harmonic Distortion plus Noise (THDN)

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What Is A Volt?

• The expected line input and output level of a PC
  is 1 Volt RMS for PCs using 5 Volts DC for the
  analog audio power supply (AVdd)
• PCs using 3.3V power supplies for AVdd are
  expected to put out at least 0.707 Volts RMS
• RMS stands for Root Mean Square, and is a
  method for measuring voltages that corresponds
  more closely to the perception of the human ear
• 1 Volt RMS
• Is the same as 1.414 Volts Peak
• Which is the same as 2.828 Volts Peak-to-Peak

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What is a Decibel or dB?

• A Bel was named after Alexander Graham Bell, the
  inventor of the telephone It is a unit which
  defines a doubling of loudness
• A decibel is 1/10th of a Bel
• A decibel is commonly expressed as a ratio of two
  values
• 0 dBV is referenced to 1 Volt RMS Each doubling
  or halving of voltage is approximately 6 dB
• dB FS (Full Scale) is a measurement of digital
  signals relative to the maximum possible value
• 0 dB FS maximum value possible, all other
  levels are expressed as minus
• The average human ear has a dynamic range of over
  10 Bels, or 100 dB
• Voltage is linear scale, dB is log scale
• A range of 60 dB is a ratio of a thousand to one
• A range of 120 dB is a ratio of a million to one
• 3 dB is the smallest volume step that a typical
  untrained listener can easily discern Trained
  listeners can identify smaller steps

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What Are Hertz?

• Hertz is an measure of frequency expressed in
  cycles per second
• A pure sine wave has a tone at only one
  frequency
• A complex waveform will contain combinations of
  many different frequencies at any one time
• The typical range of human hearing is 20 Hertz
  (Bass) to 20 kiloHertz (Treble)

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What Is An Octave?

• An octave is a doubling of frequency
• The average human ear has a frequency response of
  over 10 octaves
• Hertz is linear scale, octave is log scale
• 10-octave ISO (International Standards
  Organization) center frequencies
• 31.25 Hz
• 62.5 Hz
• 125 Hz
• 250 Hz
• 500 Hz
• 1000 Hz
• 2000 Hz
• 4000 Hz
• 8000 Hz
• 16000 Hz
• Note that the ear has log response for both
  frequency (octaves) and volume (db), but our test
  equipment normally uses the linear scales such as
  Volts and Hertz

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Attributes of Good Audio

• High Dynamic Range (DR)
• Can accommodate both soft and loud signals
• Adequate Headroom
• Ability to reproduce peaks without distortion
• High Signal to Noise Ratio, or SNR
• Freedom from noise
• Low Distortion, or THDN
• Freedom from distortion
• Full-range frequency response
• Match the range of human hearing
• Low channel-to-channel crosstalk
• Minimize leakage between audio channels

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High Dynamic Range, or DR

• The dynamic range, or DR, is the ratio between
  the loudest signal that can be reproduced
  accurately and the softest signal that can be
  reproduced accurately.
• A dynamic range ratio of 100 dB means that the
  loudest signal is at 0 dB (typically 0 dBV or 1
  volt RMS) and that all measured noise signals
  are no greater than -100 dB (typically 100 dBV
  or 10 microvolts)
• A dynamic range of 100 dB is a minimal goal for
  media-centric designs This requires a
  high-quality codec with greater than 16-bit
  resolution along with a good layout
• For 100 dB dynamic range performance, all stray
  noises together must equal less than 10
  microvolts RMS, or 28 microvolts peak to peak

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Adequate Headroom

• Digital signal overload, or clipping, sounds
  very bad
• To avoid clipping, a normal signal should be
  roughly 20 dB below the maximum output level, so
  that clipping does not occur on peak material
• Running a signal at a level higher than this will
  likely result in clipping, and should be avoided

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High Signal to Noise Ratio or SNR

• The signal to noise ratio, or SNR, is the ratio
  between the typical signal level and the softest
  signal that can be reproduced accurately.
• DR SNR Headroom
• For example, 80 dB SNR 20 dB headroom 100 dB
  of dynamic range
• SNR and DR are often confused, and the term SNR
  is often used when the term DR should be used

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Low Total Harmonic Distortion or THDN

• A pure sine wave has a tone or energy content at
  only one frequency a complex waveform will
  contain combinations of many different
  frequencies at any one time
• Distortion is a change or deviation between input
  and output signal
• Harmonic Distortion is deviation from a pure sine
  wave, which includes the energy at frequencies
  which are multiples (harmonics) of the original
  signal. For instance, a sine wave of 100 Hz has
  harmonics at 200 Hz, 300 Hz, 400 Hz, and so on
• Harmonic distortion is generally undesirable, but
  is harder to hearthan noise
• THDN measures both harmonic distortion and
  noise. If a system is very noisy, then the SNR
  and the THDN will probably be the same. For a
  well designed system, SNR will usually be better
  than THDN
• THD is mainly influenced by the codec and
  amplifier designs
• Noise is mainly influenced by both the components
  and the circuit layout
• A THDN of -90 dB is a minimal goal for
  media-centric designs

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Frequency Response

• The frequency response is the bandwidth of the
  audio passband, and should match the range of
  human hearing, that is 20 Hertz to 20 Kilohertz
  The edges of the frequency response are the
  frequencies at the upper and lower end where the
  level drops below -3 dB compared to normal A
  flat response curve is desired (less than /- 0.5
  dB or less)

-3dB down points define bandwidth
Octaves
Hertz
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Audio is Part of the System

• Everything affects audio!
• Audio sub-section Layout
• Audio Power supply
• CD/DVD analog outputs and power supply
• CPU Power supply
• External power supply (laptop)
• Battery charge circuit (laptop)
• Case grounding
• Jack grounding
• EMI suppression techniques

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Common Complaints

• Noise sources
• Noise from mouse movement
• Noise from stepper motors on disk drives
• Pops and clicks due to uni-polar power supply
• Inadequate power supply traces
• Poor grounding.
• Inadequate power supply
• Microphonics
• Ground loops

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Different Types of Noises

• There are many different types of noises
• Hissing noise or white noise is typically steady
  state, and usually derives from the thermal noise
  of the components in the subsystem
• Occasional or intermittent noise usually comes
  from other components, such as mouse movement,
  hard disk noise, CD stepper motors, RF circuits,
  memory strobing, video activity, and switching
  power supplies
• Be sure to check noise while other components are
  in use
• Reduce this noise by using components designed
  for low noise operation
• The choice of audio codec is critical for low
  noise

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Passive Components

• The way that a resistor or capacitor is
  constructed or the materials that are used can
  add noise or distortion to a circuit
• Not all passive components are suitable for audio
• If a polarized capacitor is being used as a
  coupling capacitor, make sure that it is biased
  properly
• For microphones, be sure to use a low-leakage
  capacitor, as any leakage may cause a DC offset
  which will be amplified by the microphone preamp
• Avoid using resistor, capacitor, ferrite, or
  inductor packs which have more than one device in
  a single package. These are a possible source of
  crosstalk
• Avoid using capacitors with varying tolerances or
  that have microphonic characteristics

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Considerations for Laptop and All-in-One Models

• Models with built-in speaker systems
  haveadditional considerations
• Typically the larger the screen, the louder the
  expected listening volume
• Loud listening volumes require a well-filtered
  power supply capable of providing peak currents
  in excess of the maximum current consumption of
  the power amplifiers
• If the power supply is not well-filtered and
  regulated, or if the power supply traces are not
  wide enough, then the output amplifier can become
  unstable at high volumes

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Considerations for Microphones

• Microphones typically require extra preamp or
  gain stages, which add between 20 dB and 62.5 dB
  of additional gain
• For external microphones, make sure that the
  microphone bias supply is super-clean (i.e.,
  noise-free)
• Keep microphone connections to a minimum length,
  preferably within an inch or two of the codec
• For longer microphone runs, use a microphone with
  a built-in preamp of 20 dB or more Generally
  avoid preamps which are not inside the microphone
  or near the microphone
• When wiring for stereo microphones, maintain
  maximum isolation between bias supplies to ensure
  low crosstalk this is especially important for
  beam-forming or phased-array microphones, as
  excessive crosstalk will cancel out the
  beamforming effect

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Power Supply Considerations

• Use a codec with a good PSRR (Power Supply
  Rejection Ratio) to minimize noise being induced
  from the power supply. This is especially
  important for designs which dont have a
  dedicated voltage regulator.
• Be especially careful with power supplies which
  provide mic bias voltage. Any noises on this
  supply will be amplified by the microphone
  preamp. For instance, a small 10 microvolt noise
  on the supply, amplified by a typical 40 dB
  preamp, will result in a noise floor of -60 dB,
  which cannot pass WHQL requirements.
• Whenever possible, leave a stuffing option for a
  voltage regulator even if a voltage regulator is
  not planned as part of the final design. This can
  prevent an expensive board re-spin late in the
  design cycle.
• Try to always bring up digital power supply
  before analog power supply, and return analog
  power supply to zero before removing digital
  power supply. Fast turn-on or turn-off of analog
  power supply will cause pop noises.

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ISOLATE!

• HD Audio link allow a 5-wire serial connection
  between the codec and the controller
• Take advantage of this to locate the codec as
  close as possible to the jack, and as far away as
  possible from other circuits in the computer
• Use an isolated voltage regulator designated
  exclusively for analog audio power supply, as
  close to 5V as possible
• Try to eliminate any coupling (magnetic,
  capacitive, or electronic) between the audio
  circuitry and any other circuits in the computer
• If possible, build a guard ring around the
  audio circuitry, and dont allow any other
  circuits within this area
• Avoid routing audio traces near other circuitry,
  especially high-speed digital circuits
• For external analog connections use twisted-pair
  shielded wires with the shield connected only at
  the input end avoid using ribbon cable or
  unshielded cables for analog connections
• Locate audio circuitry away from wireless LANs
  such as 802.11 and Bluetooth

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Star Grounding

• Star ground is accomplished by running a separate
  trace for each analog ground (return) shown on
  the schematic back to a central grounding point
  underneath the codec. Ground planes should only
  be used for shielding, and should be attached at
  a single point. Ground planes should never carry
  any current.
• Connect digital ground and analog ground together
  directly underneath the codec
• Ensure that there are no other ground loops or
  additional return paths, as these can cause
  noises to be induced into the audio
• Take special care with shielding around the
  jacks, as well as EMI and ESD circuitry. Shields
  must be grounded in only one place, and connected
  directly to the main ground underneath the codec.
• Ground return traces must be capable of handling
  as much current as each associated supply trace.
  An overly small ground return trace can cause
  noise due to the voltage drop over the resistance
  of the trace.
• Star grounding is counter-intuitive for board
  designers who are unfamiliar with audio layout
  best practices
• Use Optical SPDIF output or use transformer for
  coaxial SPDIF Out

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Testing

• Be sure to test under dynamic conditions
• Listen to and measure SNR on both record and
  playback paths
• While moving the mouse
• While running Disk Defragmenter on each hard disk
• While copying large files to and from CD/R
• While copying large files over Wi-Fi and/or LAN
• While exercising memory
• While exercising video

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Call to Action

• Start at the beginning
• Start with the understanding that audio must be
  designed in from the beginning, not added in at
  the end
• Consider your performance target
• Consumer equipment for the living room has Signal
  to Noise Ratios of 115 dB or higher, but PCs are
  typically about 85 dB and can be as low as 65 or
  70 dB
• Utilize new Microsoft Designed for Windows
  required standards
• High Definition Audio, part of the Microsoft
  Universal Audio Architecture initiative removes
  the digital format barriers which previously
  limited computer audio to 48kHz and 20 bits
• Use good design and layout implementations
• Even when using codecs with high-quality analog
  circuitry, audio performance depends on good
  design and layout to realize good quality audio
• Take care at every stage
• To realize living room quality, care has to be
  taken at every step of development, including
  schematic design and especially layout

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Additional Resources

• Email
• SigmaTel hdaudio _at_ sigmatel.com
• Microsoft uaa _at_ microsoft.com
• For assistance in your audio design
• Contact SigmaTel or your codec vendor for
  guidance and assistance
• Use reference designs where possible
• Attempt Golden Layouts
• Once you have the perfect audio design, re-use it
• Do not re-use designs that you know have audio
  issues You will continue to have the same issues.

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Community Resources

• Community Sites
• http//www.microsoft.com/communities/default.mspx
  
• List of Newsgroups
• http//communities2.microsoft.com/communities/news
  groups/en-us/default.aspx
• Attend a free chat or webcast
• http//www.microsoft.com/communities/chats/default
  .mspx
• http//www.microsoft.com/seminar/events/webcasts/d
  efault.mspx
• Locate a local user group(s)
• http//www.microsoft.com/communities/usergroups/de
  fault.mspx
• Non-Microsoft Community Sites
• http//www.microsoft.com/communities/related/defau
  lt.mspx

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