Re: troubles with midisport : solved
Re: troubles with midisport : solved
- Subject: Re: troubles with midisport : solved
- From: Mike Kent <email@hidden>
- Date: Tue, 29 Apr 2003 12:28:57 -0700
>
There are two speeds in standard USB. I think that some hubs, like
>
the ones in the keyboards, only run at slow speed (which would be
>
deadly for MIDI or audio). Also, sharing the bus with slow speed
>
devices (like keyboards) can also cause problems.
The available speed of transfer on USB is not the cause of this problem.
Here's some background on USB speeds:
USB 1.1 supports 1.5Mbs and 12 Mbs devices - Low Speed and Full Speed
USB 2.0 supports: (with typical device examples)
1.5Mbs - Low Speed (keyboard, mouse)
12Mbs - Full Speed (audio, MIDI, printers, many others)
480Mbs - High Speed (drives, multichannel audio, video, etc.)
As far as I know, all currently shipping USB MIDI and Audio devices are Full
Speed devices. All USB 1.1 hubs support low speed and full speed. So the
speed of a hub should never cause a problem with existing USB MIDI or Audio
devices.
But what should be and what reality is might be different. Some hub chips
are designed better then others.
I run our USB MIDI and Audio interfaces in my office on hubs on Windows and
Mac systems. I have been responsible for official compliance testing for our
products. That requires running at many levels of hub, even at he end of a
chain of 5 hubs. I experience no hub related problems.
However, a hub can indeed cause a problem on the basis of power supply. Each
USB port can supply power to an attached device. A USB hub can be "Bus
powered" by the upstream port (such as the host). But if the hub is powered
by the upstream port, it cannot supply full power to all of its downstream
ports. So most hubs are "Self Powered"; They have their own power supply so
they can send power downstream to devices attached to the hub. Some devices
will not work on a bus powered hub because they need the full power.
back to transfer speed:
There will come a time when transfer speed on a hub is more critical. That
is when using USB 2.0 High Speed devices. High Speed audio devices are
coming soon (we already announced our first).
A High Speed Device needs to be connected to a USB 2.0 host. If there is a
hub in between, it must be a USB 2.0 hub. Only a USB 2.0 hub supports High
Speed mode. All USB 2.0 hubs I have used seem to work as per the spec.
A note for USB 2.0 hub buyers:
USB 2.0 hubs have a "Transaction Translator" (TT), a component that
communicates with downstream connected devices that run at low speed or full
speed and converts the data to high speed for upstream communication with
the USB 2.0 host.
Most USB 2.0 hubs have 1 transaction translator. That means that all devices
on that hub that require low/full speed share one common 12Mbs bus which is
merged into the 480Mbs stream to the host.
However, some hubs have multiple Transaction Translators. Both Cypress and
SMSC make hub chips with 4 Transaction Translators (maybe others make those
now too). So a 4 port hub could connect 4 Full Speed devices, each with it's
own dedicated 12Mbs bus, each merged onto the High Speed bus.
So there you go, more info than you asked for. And I'm not sure how most of
this post as USB 2.0 info relates to and Apple/Mac mailing list today.
Regards,
Mike.
-----------------------------------------------
| Mike Kent, Manager of Technical Relations |
| Roland Corporation |
| Email: email@hidden |
| Tel: (604)543-2973 Fax: (604)543-2983 |
-----------------------------------------------
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