In the new DAC generation the USB port is fundamental for the reproduction of the hi-res digital formats. Standard digital audio transmission protocol like SPDIF and AES/EBU are limited in the signal transmission. The USB port allows the connection of the DAC to a computer or a media transport for the reproduction of hi-res music in PCM and DSD formats.
The USB port is made for computers and so is not perfect for audiophile use. In our implementation we have solved the problem with some technical solutions achieving the best from this connection. The advantage of the USB port is its speed. A USB 2.0 port has a maximum transfer rate of 480Mbit/s. The speed is enough to transfer very high resolution two channel audio without channel saturation. What are the problems in an audiophile implementation? It is easy: we want to send to the DAC chip a time perfect digital signal, a signal without jitter. The PC is a perfect jitter machine, so a technical solution is necessary to avoid this problem. Some years ago the first USB digital audio transmissions were based on the USB1.1; the maximum channel speed was 12Mbit/s good to transfer 96kHz-24bit. The quality is very poor since the data speed is near the limit of the channel transmission speed. The protocol for the signal transmission is isynchronous using the time reference of the PC so highly jittered.
The use of a USB2.0 interface changed the way to transfer the signal; the higher USB channel speed allows the asynchronous transmission. The data is sent in packets at 480Mbit/s and the channel is unused for most of the time since the data rate is a fraction of the maximum channel speed. If there is an error in the signal reception, the USB receiver requires a new re-transmission of the corrupted packet. This is possible since the USB bus is free between two transmissions of packets. Another detail is that the data transmission is required from the USB receiver only when the previous data packet was processed.
But what is the strategy to control the jitter?
Asyncronous transmission at high speed allows transmission and reception of a packet of data in a very small fraction of time. At the USB receiver on board the DAC unit the data is sent at a F.I.F.O. memory (First In First Out) and recovered at the output with the precision of a reference clock.
In this way the jitter of the received signal is equal to the jitter of the reference clock on board. We are no longer restricted to the PC precision in transmission. The only thing necessary for perfect transmission is not to lose data packets; this task is accomplished by the driver.