lordgibbness wrote: > Anyone else know why the version of Linux on the Touch has issues with > USB Audio Class 1 DACs at 96kHz but is fine with Class 2 at 192kHz? > > FYI, I don't currently own any audio files at this frequency (all of my > files or 16/44.1 FLAC and MP3), but in the future as 24/96 becomes more > popular (it surely will) I would like to see if the Touch could > potentially be patched to fix this issue? > > I know there is a workaround available to add a USB Hub in between the > DAC and the Touch, but I am trying to keep electrical noise to a minimum > so don't really want to go down this route. > > thanks. > Rob.
This is my understanding of the situation: The Touch has a USB 2.0 port, the MDAC has a USB 1.0 port, some of the low level USB information is different in these two standards. From now on I will refer to this as 1.0 packet and 2.0 packet. The system deals with the difference between these with something called a Transaction Translator (TT). Most systems have the TT implemented in hardware as part of the USB port. The software driver running on the host computer talks to the hardware port with 2.0 packats, if the port knows it is connected to a 1.0 port, it's TT will do the translation, so 2.0 packets go to host driver and 1.0 packets go to the device on the other end of the cable. Everybody is happy and the system works smoothly. BUT for some reason (presumably cost) the makers of these highly integrated processors (which include the USB hardware in the processor chip) have left out the hardware TT, thus the driver has to take on the responsibility of the TT functionality. The default driver that comes with linux for these devices seems to be able to handle the TT functions well as long as packets are going unidirectionaly. But Async USB interleaves packets going back to the host in the middle of the rapid stream of packets going to the DAC, the software TT can't seem to handle this. Triode figured out how to hack the driver so it could at least handle this situation, but the processor is not fast enough to handle this interleaving at sample rates faster than 48KHz. The external hub includes a hardware TT which does the translation so the 1.0 port on the MDAC sees 1.0 packets and the 2.0 port on the Touch sees 2.0 packets so the software TT is not needed on the Touch. BTW this is only an issue for async 1.0 DACs, adaptive DACs don't send packets back in the middle of the stream from the host so they don't have a problem with the software TT and of course DACs with 2.0 ports (async or adaptive) are already talking 2.0 packets so the translation is not needed. So the USB hub is really your only choice if you want to go with sample rates faster than 48KHz. John S. ------------------------------------------------------------------------ JohnSwenson's Profile: http://forums.slimdevices.com/member.php?userid=5974 View this thread: http://forums.slimdevices.com/showthread.php?t=94512 _______________________________________________ Touch mailing list [email protected] http://lists.slimdevices.com/mailman/listinfo/touch
