Computer Audio Asylum

RE: iPad and external DACs

72.49.109.51

Gordon Rankin

M

In Reply to: RE: iPad and external DACs posted by soundchekk on August 17, 2012 at 01:57:52

SC,

In the enumeration process the connecting unit tells the host computer how much current it draws. So not matter what you are doing the problem will exist because the iPad is seeing that the device draws X amount of current and if X > max limit then the iPad sends the shut down command and ignores the connection.

The only way is to put a smart powered hub in front of it so the iPad see's that the unit is not drawing any current directly.

Thanks
Gordon
J. Gordon Rankin

Follow Ups:

RE: iPad and external DACs - soundchekk 00:12:06 08/18/12 (2)
"...."

That's what I figured already.

The question is if I can make the USB audio interface chip (probaly depends on brand and firmware) announcing two different modes (external/USB powered) with related power consumption.

If the USB audio device is externally powered it should be able to announce
zero consumption. Obviously many devices fail on this feature.

Cheers
--------------------------------------------------------------------------------------------
::: Squeezebox Touch Toolbox 3.0 and more ::: by soundcheck
- RE: iPad and external DACs - rick_m 06:44:26 08/18/12 (1)
  "Obviously many devices fail on this feature."
  
  Feature? Does the device in question have a separate power input besides the USB connection? In that case it at least could do it. However some designs use both. I think Ayre does for instance and runs the USB transceiver and some other glop from the host power then opto-isolates that from the self-powered chunk.
  
  If not it would be unable to devine that the user has hacked up his interface to not draw power from the host which I believe was your test case.
  
  Regards, Rick
  - RE: iPad and external DACs - soundchekk 06:58:09 08/18/12 (0)
    My device allows to switch bus vs. extern.
    
    I'd expect all devices with external PS - switchable or fixed - to announce 0 current consumption to the host. That's not the case.
    
    My cut-the-line test had shown that the host doesn't measure the actual current draw.
    My guess that somehow the client firmware must get involved was confirmed by Gordon.
    
    Cheers
    --------------------------------------------------------------------------------------------
    ::: Squeezebox Touch Toolbox 3.0 and more ::: by soundcheck

Post a Followup:

FAQ

Post a Message!

Forgot Password?

Moniker (Username):
Password (Optional):
	Remember my Moniker & Password (What's this?) Eat Me
E-Mail (Optional):
Subject:

Message:	(Posts are subject to Content Rules)

Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:	Automagically notify you when someone responds.

To view your new posting or follow-up, click on the RELOAD or REFRESH button on your browser.

This post is made possible by the generous support of people like you and our sponsors:

Schiit Audio