USB audio and why there is no error correction...
Posted by: PureHifi on 24 March 2010
I thought the following would be of interest to those seeking further info on USB interfaces and the use of that standard for streaming audio/video.
HEREOn the face of it it gave me the answer I was looking for..i.e. Audio devices using USB do not employ error correction due to the isochronous data transfer in use, but it also states that data transfer errors are estimated at 1 error per month of 24/7 streaming of 44.1 audio.
Nothing that anyone would notice, but the error rate estimated may be a lab test of the technical standards in a completely different test environment to prove the technology - real life use means we use cables to connect the 2 devices and that is possibly where a better or worse cable will be susceptable to causing errors in the data stream between 2 devices...
It might also explain why data storage USB sticks can sound different when used for audio playback...the lack of error correction may then be susceptable to the quality of the controller & memory chips on the stick.
Posted on: 24 March 2010 by ferenc
It is a bit outdated somehow now as for example HiFace is not using the audio mode, it sends data to the USB-SPDIF converter. This is why it can handle 192kHz and not limited 96kHz. The reason why USB sticks and SD card can have jitter problems are well summarized by Charles Altmann, who wrote this in an email few weeks ago:
"With the cards it happens that each time you have to address a new cluster, or jump back to the FAT, there is a sudden and quite huge current draw happening (the chip flushes the data that already is in the pipeline and has to fill it up with new data). This sudden outrush of current causes a voltage drop which generates the jitter. The jitter is directly correlated to the frequency of addressing a non-concurrent cluster. So it is also a question of how the data (of the music file) is distributed over the memory. There are workarounds that try to minimize FAT reads ..."
If you use a DSP (like Naim does) and not computer OS to read the external storage, you can minimize the power supply related jitter while reading the FAT formatted storage device. It can be the reason why the Nam DAC works so well with the USB memory reading wav files: oversized and not very current sensitive power supply and DSP - based reading and decoding. It is the way for the future even with the very simple navigation.
So yes, clever reading of the FAT formatted memory cards can be a very good way to get jitterless file-based music playing.
Posted on: 24 March 2010 by likesmusic
PureHifi - do you believe that a USB stick transfers data to the NAIM DAC isochronously?
Posted on: 24 March 2010 by PureHifi
Even reading the DAC whitepaper and data sheet for the Blackfin DSP I am no further enlightened as to the answer....maybe you know....if so, please share.
However, my reason for posting was not to take such a narrow view of USB as it is implemented in the Naim DAC, but to promote further information being shared (such as feranc has offered) and discussion for those who are interested.
Posted on: 24 March 2010 by likesmusic
OK.. so, disregarding the NAIM DAC, which DACs do you know of that use isochronous transfer to access data on a USB stick?
Posted on: 24 March 2010 by pcstockton
Who the hell uses FAT32 formatted drives these days? Archaic.
Posted on: 24 March 2010 by Aleg
quote:
Originally posted by pcstockton:
Who the hell uses FAT32 formatted drives these days? Archaic.
USB sticks are! Aren't they?
-
aleg
Posted on: 24 March 2010 by 0rangutan
Yes. USB sticks use FAT format as it is the most universally compatible.
Apple's HFS isn't readable on Windows.
Windows NTFS is readable on a Mac but isn't (normally) writable.
Hence FAT.
Posted on: 24 March 2010 by ferenc
If you do not want operating systems to handle your memory stick or usb drives, practically the only possibility is to use FAT32 formatting with the storage device.
