- Introduction |
- Hardware |
- Software |
- Sound Quality |
- Reference |
- Enjoy
An introduction to computer audio
- Amplifier
- ASIO driver
- Audio cables
- Bit perfect jitter
- Bits: 16 or 24
- Clock
- Crossover
- CDtext
- CDP vs DAP
- Cue sheet
- DAC
- Digital cable
- Direct Sound
- DLNA
- Driver
- DSP
- FireWire
- Headphone listening
- Hearing
- I2S
- ID3
- Kernel Streaming
- Memory player
- Music Server
- Perception
- Performance
- RAID
- Ripping
- RFI
- Router
- Sampling, up and over
- Sample Rate Conversion
- S/PDIF
- Tagging
- Toslink
- Transcoding
- Units
- UPnP
- USB
- VST
- WASAPI
- WAV
- Wave Out
USB
Universal Serial Bus (USB) is a serial bus standard to interface devices. USB was designed to allow many peripherals to be connected using a single standardized interface socket and to improve the plug-and-play capabilities by allowing devices to be connected and disconnected without rebooting the computer (hot swapping). Other convenient features include providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.
Just another bus but a very popular one. It inspires a lot of people to make all kind of products.
USB Audio
USB audio devices can use USB 1 or USB 2 but all confirm to the USB Audio 1 specs.
The final specs for USB Audio 2.0 has been released early (2009).
Transfer modes
Data is exchanced over USB using one of the four possible modes:
- Control Transfers: command and status operations,
- Interrupt Transfers: device requires the attention of the host
- Bulk Transfers: large volumes of data like print jobs
- Isochronous Transfers: time sensitive information, such as an audio or video stream
- Guaranteed access to USB bandwidth.
- Bounded latency.
- Stream Pipe - Unidirectional
- Error detection via CRC, but no retry or guarantee of delivery.
- Full & high speed modes only
Transfer modes explained in detail.
Synchronous, adaptive and asynchronous
When the computer sends the audio stream to an USB port, if first reads the data from the hard disk and caches blocks of the data in memory.
It is then spooled from memory to the output port in a continuous stream (Isochronous mode - 12 Mb/s USB 2).
This can be done with three possible types of synchronization.
- Synchronous is very prone to jitter.
- Adaptive mode is less jitter prone and used by most of today's USB audio devices.
- Asynchronous mode, the audio device does the clocking. It tells the PC to regulate its speed. As the clock is running on the audio device this solution can be implemented in such a way that the the DAC receives a signal with a very low jitter.
More details on these modes can be found here.
Asynchronous mode
At this moment a few brands offer asynchronous mode.
The warm reception in the audiophile world of asynchronous USB as developed and promoted by Wavelength inspired other brands to offer asynchronous USB DAC's .
Both adaptive and asynchronous USB uses the same Isochronous protocol.
This makes one wonder why asynchronous USB should have an advantage over adaptive mode.
In case of asynchronous USB the DAC is doing the clocking.
Asynchronous mode is not better by design but by implementation because you can implement a top quality (low jitter) clock in the DAC.
There is actually a good example of this case of its the implementation of the clock thats important, not the asyncness itself that is important. The recent inexpensive Musiland devices use an asynchronous protocol but then use a frequency synthesizer to generate the local clock rather than use a fixed frequency oscillator. The result is jitter that is actually worse than some of the better adaptive implementations!
John Swenson
More details can be found here.
A good general introduction to USB concepts.
Resolution
Most USB audio devices 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.
This is by and large a matter of the chip set used.
Adaptive 16 bit units often use the Cmedia or TI (PCM270x) based chip sets. These are not variable and usually only support 32k, 44.1k and 48k.
Adaptive 24 bit DAC's use either the Centrance or their own code and an intelligent programmable design (TAS1020 or other USB Audio Controller).
These include the Benchmark, the newer BelCanto, Wavelength
This chip enables 24 bit/ 96 kHz over USB.
96kHz is the upper limit when using native mode drivers on the PC.
E-MU, Edirol (UA 101), Musiland are one of the very few offering bit rates up to 192 kHz.
This requires installation of a custom USB device driver.
A nice story about the development of the USB interface: The D/A diaries: A personal memoir of engineering heartache and triumph by Hitoshi Kondoh.
Configuring
A clear and well written step by step guide to setup and USB DAC using XP, Vista or OSX can be found at the Ayre website.
Drop out.
Some users complain about drop outs when playing USB audio.
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.
Anti-virus programs polling the internet with high priority might interrupt the audio.
Trouble shooting
- Using another port for your audio might cure the problem if the new port it is attached to another hub
- You might try the windows device manager and see if you can find an usb-controller using an IRQ not used by other devices.
- Open the task manager and check for processes peaking or doing a lot of I/O
- Run a program like DPC Latency Checker
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.
More:
http://www.audioasylum.com/cgi/vt.mpl?f=pcaudio&m=33658
http://www.audioasylum.com/cgi/vt.mpl?f=pcaudio&m=33930
http://www.m-audio.com/index.php?do=support.faq&ID=de0c17eee57afeeba3978a213b6d86ac
