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The Well-Tempered Computer

An introduction to computer audio

Connecting audio
- Analogue
- Digital
  - SPDIF
  - USB
  - Bluetooth
  - Kleernet
  - SKAA
  - HDMI
  - MADI
  - WiSA
  - Network
    
    Airplay
    
    Chromecast
    
    DLNA
    
    DTS Play-Fi
    
    HEOS
    
    Matter
    
    MusicCast
    
    Prosumer
    
    Sonos
  - Wireless transmitter
  - Protocol converters
- Connecting speakers
Audio PC
Raspberry & Co
All in Ones
- Advance Acoustics
- Auralic
- Cambridge
- Cyrus
- Cary Audio
- Gold Note
- iFi
- Marantz
- Matrix Audio
- Moon
- Music Fidelity
- Oppo
- T+A
- Technics
Bluetooth
Chromecast
Streaming audio player
- A
- B
- C
- D
- E-G
- H-J
- K
- L
- M
- N-O
- P
- Q
- R
- S
- T-V
- W-Z
Music server
- A
- B
- C
- D
- E-L
- M
- N
- O
- P
- Q
- R
- S
- T-V
- W-Z
USB Audio
- USB DAC/Amp
  - Asynchronous USB 2
    
      A
    
      B
    
      C
    
      D
    
      E
    
      F
    
      G
    
      H-J
    
      K
    
      L
    
      M
    
      N
    
      O
    
      P
    
      Q
    
      R
    
      S
    
      T
    
      U-Z
- USB to SPDIF converters
- SPDIF to USB converters
- USB Interface
- USB-C
- USB Cables
- USB Portable
  - ADL
  - Apogee
  - Astell&Kern
  - Audioengine
  - Audioquest
  - Aurender
  - Beyerdynamic
  - Cambridge
  - Celsus
  - Chord
  - Cyrus
  - Hegel
  - HiFimeDIY
  - iBasso
  - iFi
  - Meridian
  - Onkyo
- Extenders
- Isolators
  - Alldaq
  - Audioquest
  - Ideon
  - iFi
  - Intona
  - Mutec
  - Oehlbach
  - SOtM
  - Uptone
- Power
- USB Trouble shooting
Real-time audio over IP
- AES67
- AVB
- Cobranet
- Dante
- Ethersound
- Ravenna
FireWire DAC
Multi-channel DAC
NAS
Remote control
Vinyl rippers
Acoustic materials
Active crossover
Active speakers
- A
- B
- C
- D
- E
- F
- G
- H-I
- J
- K
- L
- M
- N
- O
- P
- Q
- S
- T-U
- V
- W-Z
Single driver speakers
Invisible speakers
Sound card
Digital room correction
Headphone
- Types
- Drivers
- Impedance, damping and sensitivity
- Connect
- Crossfeed
- EQ
- DSP
- Measurements
- Amplifiers
Portable media players

USB-DAC

A USB DAC is a very flexible solution to convert digital audio as almost every computer (including smartphones) has USB.

Basically a USB DAC is a external sound card..
If you use a laptop this is the way to go if you want to improve on the onboard audio.

Asynchronous USB Audio Class 2 DAC

USB AUdio Class 2 (UAC2) uses High Speed mode (USB 2, 480 MHz).

Today you see specs like 32bit / 768kHz PCM, DSD512 USB.

Indeed the USB receiver accepts PCM audio up to 32 bits / 768 kHz and DSD512 ( 1 bit / 22.5792 MHz).

USB2 in high speed mode is fast enough to easily handle this amount of data.

From mid-2010 on both OSX and Linux supports USB audio class 2 natively.

In April 2017, an update of Win10 finally included UAC2 drivers.

You need to install a third party Class 2 Audio driver at the PC side if you use older versions of Win.

For a while I maintained a list of DACS with UAC2 input. Today, almost all USB DACs are UAC2 compatible. This dated list can be found here.

24 bit Asynchronous USB Class 1 Audio DACs

Around 2008, the first models featuring asynchronous synchronization appeared on the market.

This is considered the best solution as the DAC is in control of the buffer management allowing a free running clock.

Basically this architecture means zero input jitter.

At that time USB Audio class 1 was the standard.
It can do up to 2 channel 24 bits PCM audio with a 96 kHz sample rate.

All major operating systems (Win, OSX and Linux) do have native mode USB Audio class 1 drivers.
This allows for a plug&play installation.

Ayre QB-9, made USB audio salonfähig in 2008 employing asynchronous USB.

A couple of models.

24 bit Adaptive mode USB Class One Audio DACs

The problem with adaptive mode is indeed the DAC has to adapt its speed to the data rate as send by the PC.
A bit like SPDIF.
Before 2010 adaptive mode was common. From 2010 on, asynchronous mode took over.

A star of those early days of USB audio is the Benchmark DAC1.

Benchmark USB DAC 1 $1275.00

Digital input:

USB, Optical, Coaxial, AES/EBU(XLR)
Input Sample Frequency Range: 28 to 195 kHz
Maximum Input Word Length: 24 bits
Advanced USB™ computer audio interface for native 24-bit/96-kHz USB audio
Compatible with Windows Vista/XP/2000 and Mac OS X without driver installation

Analog output: balanced (XLR) and unbalanced (RCA)

A couple of models

16 bit USB

They are typical for the period up to 2010 when USB audio was in its infancy.

Audio brands offering USB-DAC's at that time where rare.
Most of the time it was a computer manufacturer or a startup producing this type of product.

The Silverstone EB01 ($89.99) was produced by Silverstone, a case maker for PC's

Most of these products where limited to 32kHz, 44.1kHz, 48kHz
This because you could get a simple and cheap receiver supporting this rates.
As a consequence, a lot of people believed USB audio to be limited to this range.

The pioneers of USB audio can be found here.

Vendor specific

Today almost all USB audio is UAC2 (USB Audio Class 2).
In those early days of USB audio, some manufacturers didn't use UAC1 but developed their own solution.
Instead of the isochronous mode, they used bulk mode.
Bulk mode is asynchronous by design.
As it is bulk mode,
• No guarantee of bandwidth or minimum latency
• Error detection via CRC, with guarantee of delivery.
Isochronous mode is exactly the reverse.

It is not trivial to develop a driver.
You have to do so for each OS supported and check its working for each update of the OS.
In practice it often turned out to be a very buggy road.
Today almost all audio brands buy a USB receiver from a specialist.

DSD over USB

USB audio was PCM audio only.

A couple of guys formulated an open standard (DoP) allowing direct DSD playback on a PC to a USB DAC. The DSD is camouflaged as being PCM so can be transmitted over USB.

Not only must your hardware (DAC) support DSD but your media player must support it as well.

Native DSD
UAC2 (USB Audio Class 2) has a UAC_FORMAT_TYPEI_RAW_DATA This is used to transfer DSD straight to a DAC.
This make DoP superfluous.

References

More technical information about USB audio can be found here.

DSD - The welltempered computer

DoP - The welltempered computer

USB Audio Software Design Guide - XMOS 2015