USB versus SPDIF

Vincent Kars

WBF Technical Expert: Computer Audio
Jul 1, 2010
860
1
0
Sample rate and bit depth
SPDIF allows 20 bits, optional 24 bits audio and the sample rate could be as high as 192 kHz.

USB audio devices can use USB 1 or USB 2 but all confirm to the USB Audio Class 1 specs.
All operating systems (Win, OSX, and Linux) support USB Audio Class 1 natively.
Class 1 allows for up to 24 bit/ 96 kHz. This is the maximum resolution.
The sample rates 176 and 192 kHz are not supported.

A lot of USB DACs are limited to 16 bit 32/ 44.1/ 48 kHz.
This is obvious not a limitation of the USB Audio Class 1 driver but a matter of a simple and cheap hardware solution.
Today the price difference between 16 and 24 bit DACs has become small.
I recommend going for a full Audio Class 1 implementation as a minimum.

The final specs for USB Audio 2.0 have been released early 2009.
USB Audio Class 2 supports 176 and 192 kHz.
At this moment (august 2010) USB audio class 2 drivers are available in OSX 10.6.4 and Linux.
It is unclear if Microsoft is going to support USB Audio 2.

Jitter
The send rate of the SPDIF bus is used to generate the sample rate.
SPDIF is real time, any fluctuation in the clock of the sender translates into input jitter.

USB in isochronous mode (used for audio) runs at a fixed rate.
Frames are send to the receiver.
These frames contain a variable amount of data.
USB is quasi real time as the data is send in discrete packages with the frame rate of the bus (12 MHz or 480 MHz ) but the total amount of data send is proportional to the sample rate.

In adaptive mode the sample rate is guessed from the rate the data is pouring in.
The DAC has to adapt its speed to the sender to avoid buffer over/under run.

In asynchronous mode the DAC tells the PC how many data to send.
This allows for using a fixed clock at the DAC.
By design asynchronous USB allows for a implementation with a very low intrinsic jitter.

Error correction
Both protocols are unidirectional, the sender starts sending and when an error occurs, no retry is possible.
Isochronous USB does have an error detection mechanism (CRC), but no retry or guarantee of delivery.

Sound quality
There are high quality implementations of SPDIF.
Small wonder from the 80's on audio industry had the time to optimize the product.
However a PC might have a SPDIF header but probably not one of the highest possible quality.

The advantage of asynchronous USB is that due to this protocol the DAC is independent of the source as far as the timing is concerned. This allows for using a high quality fixed clock inside the DAC. Often asynchronous USB is combined with other measures like galvanic isolation to shield the DAC as much as possible from the electrical noise of the PC.

Technically both SPDIF and USB are capable of bit perfect transmission with low jitter.
As usual the result will be dependent on the implementation.

Bit more detail: http://thewelltemperedcomputer.com/KB/USB.html
 
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