24 bit audio over USB

Pooh Bear · 2006-06-11T20:05:01+00:00

Hi,I'm interested in passing stereo audio over USB in both directions.I believe that Windows already has a 16bit driver for this but I want 24bit.Does this mean one has to write one's own ? It's slightly outside my area ofexpertise. I'd aslo be interested in how Macs support this function.Any general advice about implementing this, recommended chips etc is verywelcome. The quality needs to be high-end / professional rather than consumerbtw which I suspect means using separate A/D and D/As.Graham

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Andy Peters 20 years ago

You're confusing "USB 2.0" with "high speed USB." While USB 1.1 had two speeds (low, at 1.2 Mbps, and full, at 12 Mbps), USB 2.0 adds a third speed (high, at 480 Mbps), but a USB 2.0 _device_ does not have to support high-speed transfers. However, note that a USB 2.0 _host_ must support high speed.

"High-Speed USB (USB 2.0) and Full-Speed (USB 1.1) compatible The UA-101 is the first multi-channel high-speed USB audio interface to offer basic compatibility with USB 1.1. When connected through USB 1.1, the UA-101 will function as a stereo audio interface at 44.1 or 48kHz with any Win98 or later PC or Mac OS 9 or Mac OS X computer. When connected to Windows XP computer running USB 2.0, the UA-101 offers the full set of I/O options to your computer."

What Edirol is saying is that if you connect the UA-101 to a USB 2.0 host (which, as noted above, must support high-speed transfers), then the device will do multiple channels at 96 kHz, as the device will enumerate as a high-speed device. However, if you plug it into an older USB 1.1 host, which doesn't support high-speed transfers, then the device enumerates using a full-speed configuration which only supports 48 kHz stereo. This happens because the USB spec allows a high-speed device to support an "Other_Speed_Configuration," the idea being that it's better that a device work with some reduced functionality in a lower-speed system than to not work at all.

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Andy Peters 20 years ago

24bit.

of

consumer

the

Yes. In the device firmware, you'll have a structure that holds the device's configuration and device descriptors. The firmware responds to the USB requests that ask for those descriptors as part of the enumeration process.

have to

If using the TAS1020B, then playback and record do not have to be synchronized. Both chips support using a clock generator PLL to synthesize the clocks (the TAS1020B has two synthesizers). This eliminates the need for an external oscillator, but jitter and such may not be good enough for your application. Both devices support using an external oscillator, but you'll have to carefully think through how that will work.

I'll go

You should also download the firmware development kits, as the data sheets barely skim the surface of how these chips work.

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Mike Rivers 20 years ago

I'm not confusing anything, I'm just not familiar with the terminology. I only know USB versions 1 and 2, and 2 is capable of much faster transfer rates. Since it's backward compatible, might as well use it. I know that disk drives connected to a USB2 port have a much higher throughput than when connected to a USB1 port. That's good enough for me (as a user). Designers may, and probably do, see it differently, as do the bean counters.

This is similar to my laptop situation. It has only USB1.1 ports. When I plug a disk drive in a USB2 case into the computer, it runs slow enough so that I can only record about 6 tracks at 44.1 kHz without it getting behind. If I plug the same drive into another computer with a USB2 port, it works just like an internal disk drive.

Bottom line - sufficient speed for 96 kHz duplex stereo is available through USB. You just need to implement it properly, which is what Graham is trying to find out how to do.

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Andy Peters 20 years ago

Ahhh, OK. You are correct -- the terminology _is_ confusing, and some manufacturers take advantage of the confusion by offering "USB 2.0" products, insinuating that the devices are high speed when they are not.

Good -- that is exactly how it's intended to work.

And the rub here is that the TAS1020B chip is USB 2.0 compliant, yet is only capable of full-speed operation. Therefore, if he needs full-duplex stereo 96 kHz operation, the TI chips are not the solution.

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Mike Rivers 20 years ago

Am I still confused? This sounds like there will be plenty of speed for full duplex (four channels) at 96 kHz - more than necessary. If he wants backward compatibility to work at low speed and transfer four channels at 48 kHz to a device that's only set up for USB1.1, that sounds like it won't work with this chip.

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