In the new DAC generation the USB port is foundamental 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. USB port allows the connection of the DAC at a computer or a media transport for the reproduction of hi-res music in PCM and DSD formats.
The USB port is born for a computer use so is not perfect for an audiophile use. In our implementation we solve the problems with some technical solutions acheiving the best from this connection. The advantage of the USB port is the speed. A USB port 2.0 have a maximum transfer rate of 480Mbit/s. The speed is enough to transfer the very high resolution two channel audio without a channel saturation. What are the problems in an audiophile implementation? It is easy: we want to send at the DAC chip a time perfect digital signal, a signal without jitter. The PC is a perfect jitter machine, so it is necessary a technical solution to avoid this problem. Some years ago the first USB digital audio transmissions are based on the USB1.1; the max channel speed is 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 isochronous using the time reference of the PC so higly jittered.
The use of a USB2.0 interface change the way to transfer the signal; the higher USB channel speed allows the asynchronous transmission. The data are 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 require a new retransmission 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 to transmit and receive a packet of data in a very small fraction of time. At the USB receiver on board of the DAC unit the data are 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 at the jitter of the reference clock on board. We are not more related at the PC precision in transmission. What is only necessary for a perfect transmission is to don’ t lost data packets; this task is accomplished by the driver.