USB-DAC

Digital audio is a stream of bits. To become audible, conversion to a analogue signal is needed. This is done by a Digital to Analogue Converter.
In the audio world this is called a DAC, in the computer world this is called a sound card.
Of course both have a DAC chip on board.

In the audio world the DAC receives its input from a CD player.
If it is build in, the I2S protocol is used, if it is a separated unit, the input is S/PDIF (coax, toslink) and/or AES/EBU (XLR).
To connect a conventional DAC to your PC, you need a sound card with a digital out. High sound quality can only be obtained if the digital out is not only bit perfect but also low on jitter.

Sound cards receive their input from a bus. PCI , PCI card, Firewire, USB, etc.

A very flexible solution is a outboard USB-DAC as almost any PC offers USB.
If you have a laptop this is probably the way to go if you want to improve on the sound card.

The simple models accept 44.1/ 48 kHz 16 bit input only.
Something to keep in mind if you want to play 24 bit audio or higher sampling rates.

USB is not designed with audio in mind. It might generated audible input jitter.
Some manufacturers claim their DAC is immune to input jitter by buffering the input and reclocking it. Other control the the USB by using asynchronous mode.

Basically there are two types, well three types of USB DACS.
1) Adaptive 16 bit units that use the Cmedia or TI based chip sets. These are not variable and usually support 32k, 44.1k and 48k.

2) Adaptive 24 bit dacs that use either the Centrance or their own code and an intelligent programable design (TAS1020 or other USB Audio Controller). These include the Benchmark, the newer BelCanto, PS Audio and Emperical.

Adaptive means the computer controls the USB receiver Audio clock and can change that at a rate of every 1ms which will inject jitter into the system. All of the above products have to incorporate some kind of jitter rejection beit upsampling or reclocking.

3) Asynchronous 24 bit dacs which I make and others that control the clock internally and tell the PC to send data in a kind of flow control method that reduces the injected jitter to a very low amount.

The difference between the 16 and 24 bit issue is where do you want to be in the future. Do you want to rip DVD Audio disks or download 24/96 capable music?

Others feel the 24 bit dacs just seem better sounding and if done correctly I would agree with that.
J. Gordon Rankin

Drop out.

Some users complains about drop outs when playing USB audio.

There are also claims that there are differences in sound quality using different ports.

This might be due to different devices sharing the same USB-Hub.

If your audio and your graphics card are on the same hub, the bandwidth required by the graphics might cause the audio to stutter.

It might also generate jitter.

Solutions

Using another port for your audio might cure the problem.
You might try the windows device manager and see if you can configure a usb-hub in such a way that it is used for audio only.
Upgrading to Vista might be a easy solution as Vista not only has better audio, the USB drivers are said to be improved also.
A simple solution is to buy a PCI to USB card. As it is PCI it gets 1 exclusive interrupt. Use this one for USB audio only and the audio device can get 100% bandwidth.

http://www.audioasylum.com/cgi/vt.mpl?f=pcaudio&m=33658

http://www.audioasylum.com/cgi/vt.mpl?f=pcaudio&m=33930

Silverstone EB01 $89.99

Sampling: 32kHz, 44.1kHz, 48kHz

Fubar II USB DAC $137.00

Input: USB

Burr Brown PCM2702 DAC, 16 Bit 44.1 KHz / 48KHz sampling rates

Output: RCA

E-MU 0404 USB 2.0 $ 199.99

24-bit/192kHz A/D and D/A converters

Two 1/4" Balanced Outputs (117dB SNR)

Stereo 1/8" Speaker Output (117dB SNR)

24-bit/96kHz S/PDIF Optical and coaxial In/Out (switchable to AES/EBU)

MIDI Input/Output

Stereo Headphone Output (114dB SNR)

Trends UD-10 USB Audio Converter €179,-

Burr Brown PCM2704.

Input: USB 16 bits/ 32, 44.1, 48 kHz PCM
Output:

It does have a analogue headphone out but its stronghold is the conversion from USB to Optical, Coax (RCA and BNC) and XLR.

Pro-Ject USB Box $159.00

A/D converter 16-Bit Delta Sigma

Sampling rate 44,1kHz

Devilsound $249

USB in/ RCA out

Non oversampling DAC

Cambridge Audio DacMagic $399

32 bit Texas Instruments Digital Signal Processor (DSP)

DAC: Twin Wolfson WM8740

Digital input:

2x S/PDIF (coax & Toslink)

USB input connects direct to PC without drivers

Digital input word widths supported:	16-24bit (16 bit for USB)
Digital input sampling frequencies supported:	32kHz, 44.1kHz, 48kHz, 88.2kHz, 96kHz
Audio output up-sampling:	Fixed 24bit 192kHz

Output

Analog: RCA/XLR

Digital: 1x S/PDIF (coax & /Toslink)

Cord Chordette Gem € 549,-

Digital input

Blue Tooth supporting A2DP Stereo Audio
USB input

Sampling rate: 44KHz – 96KHz

Analog output:

Unbalanced (RCA)

PS Audio Digital Link III $995.00

Digital input

Optical input
Coaxial input
USB input

DAC

TI PCM1798 balanced DAC
24 Bit / 192 kHz upsampling

Analog output:

Class A discrete FET based output stage

Balanced (XLR) and unbalanced (RCA)

Aqvox USB 2 D/A MKII - € 1000,-

Upsampling: 24 bit/192 kHz

Fully balanced design

Digital input:

AES/EBU:
16kHz, 32kHz, 44.1kHz, 48kHz, 88.2kHz, 96kHz, 176.4kHz, 192kHz at 16bit / 24bit
COAX input
16, 32, 44.1, 48, 88.2, 96, 176.4, 192 at 16bit / 24bit
TOSlink input
16, 32, 44.1, 48, 88.2, 96 ( sometimes up to 176.4 and 192kHz) at 16bit / 24bit
USB - INput and OUTput
16, 32, 44.1, 48, at 16bit

Dual Mono DAC (2 x Burr Brown PCM1796)
USB-Codec: Texas-Instruments PCM2906 Transceiver

Analog output:

Single ended Class A amplifiers

balanced (XLR) and unbalanced (RCA)

Digital output: COAX, TOS, AES/EBU-Input are routed to the USB-output (up to 48kHz/16bit).

Apogee Mini Dac $ 1000,-

Sample rates: 44.1/48-88.2/96-176/192 . All +/- 10%

Max word length: 24bit

Digital input:

USB
2 x AES-EBU on 9 pin D-Type (breakout cable to two female XLR-3 required) handling sample rates: 44.1k-192k single-wide and 88.2 k–192k double-wide.
S/PDIF optical on TOS-LINK 44.1/48k
S/PDIF coaxial on RCA 44/1-192k
ADAT 44/1-48k
ADAT/SMUX II for 88/2/96k
ADAT/SMUX IV for 176.4/192k
Optional FireWire card

Output:

2 x XLR (pin 2 – hot) for pro-audio stereo
1/8” jack for consumer-level stereo (standard 1/8” to RCA cable required) also able to drive headphone’s
1/4” jack headphone
Optional FireWire card

CEC DA53 € 1200,-

Digital input: USB, Optical, Coaxial, AES/EBU(XLR)

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

Benchmark USB DAC 1 $1275.-

Digital input:

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

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

April Music Stello DA220MK2 $1,650

Supports up to 24Bit/192kHz upsampling
Sampling Rate Selectable: Bypass, 48kHz, 96kHz, 192kHz
True 24Bit Delta-Sigma DAC
4 Digital input (1 AES/EBU, 1 Coaxial, 1 Coaxial or USB selectable, 1 Toslink)
3 Digital Outputs (1 AES/EBU, 1 Coaxial, 1 Toslink)
2 Analog Outputs: 1 Balanced Output (XLR) & 1 Unbalanced Output (RCA)
Full Balanced Class A Operation Analog Stage

Wavelength Brick $ 1750,-

Input: asynchronous USB

16/44.1 NOS DAC (TDA1543N2)

Amp: 12AU7A/ECC82 output tube

Analog output: RCA

Bryston BDA 1 $ 2000,-

32, 44.1, 48, 88.2, 96, 176, 192K sampling

Digital input:

USB (1), COAX (2), OPTICAL (2),AES-EBU (1) BNC (2)

Sample rates: 16-24Bit PCM, 16Bit 32K-48K USB

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

Bel Canto e.One DAC3 $2495

Digital input (5): AES/EBU (XLR), S/PDIF (BNC, RCA, TosLink), USB.

Sample rates: 32–96kHz (S/PDIF, AES/EBU), 32kHz–48kHz (USB).

Analog output: RCA (1 pair), balanced XLR (1 pair).

BD-Design € 2.769,-

Digital input: Asynchronous USB

No oversampling and no analog or digital filter

16bit @ 44.1kHz WAV/ FLAC only

Analog output: RCA (1 pair)

Chord QBD76 £ 3000,-

Digital input:
2 x SP/DIF BNC Coax
2 x AES Balanced XLR Input
2 x Plastic fibre optic ( TOSlink )
1 x USB ( B type )
1 x Bluetooth supporting A2DP Stereo Audio

Sample frequencies 32KHz – 96KHz Single Cable,
176KHz & 192KHz Dual Cable ( Dual data mode )

Random master clock jitter of less than 3pS cycle to cycle

Analog output: 2 X RCA / 2 X balanced (XLR)

Empirical Audio Spoiler USB Tube DAC $ 6000

Digital Input:

USB

S/PDIF

Sample rates supported:

16-bit/44.1kHz
24-bit/48kHz
24-bit/88.2kHz
24-bit/96kHz

Analog output: RCA

Spoiler Pricing, options and Warranty (shipping not included)

Spoiler with Siemens cca grey-plate tubes - MSRP $5999.00

Spoiler with USB Pace-Car inside - adds $1650 - one Superclock4, $2300 two Superclock4's

Optional I2S input, rear-panel switchable from USB - adds $300.00
(allows use of the Northstar I2S Transport)

Optional AES/EBU input - replaces S/PDIF input - adds $100

Optional BPS power supply and charger - $425.00

dCS Scarlatti upsampler with USB € 10.900,-

Digital input:

USB2.0 interface on a B-type connector. Operates in asynchronous mode.

AES3 on a 3-pin female XLR connector.

4x SPDIF on 2x RCA Phono, 1x BNC connectors and 1x TosLink optical connector.

All digital inputs will accept PCM data at up to 24 bit PCM at 32, 44.1, 48, 88.2 or 96kS/s.

Data from any input may be converted to 24 bit PCM at 32, 44.1, 48, 88.2, 96, 176.4 or 192kS/s or DSD (1 bit data at 2.822MS/s). The output sample rate must be equal to or greater than the input sample rate.

Digital output:

IEEE 1394 interface on 2x 6-way connectors. In DSD mode, the interface outputs dCS-encrypted DSD (1 bit data at 2.822MS/s).

2x AES3 on 3-pin female XLR connectors. Each outputs 24 bit PCM at 32, 44.1, 48, 88.2 or 96kS/s,
OR as a Dual AES pair at 88.2, 96, 176.4 or 192kS/s.

2x SPDIF on RCA Phono and BNC connectors. Each outputs 24 bit PCM at 32, 44.1, 48, 88.2 or 96kS/s.

1x SDIF-2 interface on 2x BNC connectors, outputs 24 bit PCM at 32, 44.1, 48, 88.2 or 96kS/s.

It is a upsampler so you still need a DAC for the conversion to analogue. Well, they have one:

Scarlatti D/A-Converter € 18.900,-

At € 29.800,- you have a USB DAC, except of course if you decide to synchronize the two with a Scarlatti Master Clock: add € 8.500.