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There is a lot of talk about Asynchronous USB. Is the LA100 with Asynchronous USB?
USB is an easy, fast and safe way to transfer data. USB can be very easy for the user, but not the designer! And what is the difference between Asynchrounous and Synchronous USB?
In USB there are several transfer modes:
Bulk: Here you can transfer data, when time isn’t critical, and a lot of data can be send without clogging the bus. Bulk transfer will wait until time is available on the bus. Not very useful for streaming data, unless you have a large receiving buffer, and can deal with long delays.¨'
Useful for transferring data without delay. With interrupt transfers there is a guaranteed response time.
This is the one for streaming Audio and for real time data. Here the bandwidth is guaranteed, and the transfer rate is fixed.
The USB is built up with frames sent at a rate of 1ms (full speed USB), and this 1ms (or 1khz ) is the basic clock for the data.
All modes are bit true.
Receiving USB data
Now we enter into the Audio world, and here we want everything perfect.
Audio data are okay, but they arrive with an unknown clock jitter due to the PC's reference clock. As long as jitter is below 1 clock cycle , the data will be reconstructed at the receiving end correctly. At the receiving end a clock synchronous with the data should be present ,and this is derived with a PLL. This PLL (normally running at 48Mhz) will track the data, and will because of that make jitter on a Digital to Analogue converter, if used directly as master clock. But timing jitter doesn’t normally matter.
Everybody has to do this on the USB, the differences comes in how you make a jitter free Master clock for the Audio.
In Asynchronous Usb , a feed back is made to the PC to request more data, or hold a little while, so the streaming is constant. This is an efficient way to do it , only it gives more load to the USB bus.
In the receiver a FIFO Buffer is required to deliver constant rate of data to the DAC.
Lead Audio USB
In Synchronous USB, data is delivered at a constant rate determined by the PC's clock and jitter can (will) happen, if used directly for the D/A converters master clock.
If you have a slow PLL(Phased Locked Loop) then there will be little jitter(attenuation of jitter will be high), but it will not track the data correctly,and errors will occur.
If you have a fast PLL, then there will be jitter(attenuation of jitter will be low), but data will be correct. So there will be jitter. So data will be correct but, timing will not.
This is how low cost DAC's do it, and one of the main reasons for their ”not-so-good” sound.
But serious Audio companies does not use this clock for Master clock.
There are several ways to clean up the timing:
Use of several PLL's , converting USB to Toslink and then to I2S , FIFO RAM buffer and Asynchronous Sample Rate converter (ASRC).
In Lead Audio after the USB PLL (that gets the data),we use an adaptive Clock generator , that maintains the number of master clocks between USB's SOF events(Start Of Frames). This clock is better to use for a DAC(it reduces jitter), but we want to separate the PC clock completely from the DAC's master clock.
Therefore next step is to use an Asynchronous Sample rate converter . This ASRC will output 192k samples/S, at all input sample rates , and attenuate jitter 1000 times or more. It is very important to use the best kind of ASRC, because inaccuracy in the calculations will degrade performance. The ASRC in Lead Audio has a 140 dB accuracy on the ouput (0.00001%)
The ASRC converts the incoming signal to a signal without jitter. And to keep the signal without jitter the Master clock source is made up with a discrete Collpitz oscillator,and a perfect 50% duty cycle.Jitter is < 10 ps by design. (We only specify 100pS).
One(big!) benefit in using Asynchrounous SRC, is that the D/A Converter will allways run at the same samplerate, so it can be optimized for this Sample rate.
In Asynchronous USB the D/A converter will have to run the selected sample rate , but this is rarely the case. Allmost all(99%) D/A converters use oversampling (or synchrounous sample rate conversion), otherwize the linear ouput filter will be extremely complicated. But the the DAC will run at different samplerates, when different samplerates are played.
Asynchronous SRC has same benfits (if designed properly) as Asynchrounous USB. It remove jitter and audio data are correct.
The main advantage over Asynchronous USB, is that the D/A conversion and the neccesary outputfilter will allways run at the D/A converters fixed sample rate(192khz). So the analog buffer design can be more optimized.
Another Advantage is that all other Digital inputs (S/PDIF, TOSLINK), will have the same jitter removing advantages.
The disadvantage is that it is more expensive.
The use of an Asynchronous SRC is only one of the reasons why our DAC's sound is so extraordinary. First we make sure our data are correct, then we clean up the jitter. And the Output is optimized at a fixed sample rate. We have chosen to use the same design in all our USB products, although it is more expensive than Asynchronous USB. Asynchronous SRC has proven its worth, but take care, there are many low precision ASRC solutions on the market.