27" iMac kills built-in audio streams when switching in external monitor mode

Number:rdar://8958598 Date Originated:2011-02-04
Status:open Resolved:
Product:iMac Product Version:iMac 11,1
Classification:Data loss Reproducible:yes

The 27" iMac has the ability to act as an external monitor and speaker for another computer attached via the Mini DisplayPort interface. When the Mini DisplayPort cable is connected, the iMac switches the display over to act as a second monitor and as optional external audio interface passing through the audio to the built-in loudspeakers.

However, when the iMac switches over in external monitor mode, programs running on the iMac reading audio from the built-in audio interface suddenly get interrupted and stop receiving audio. This is reproducible with the built-in audio receiving either analog or digital audio (optical S/P-DIF).

Steps to Reproduce:

1. Boot iMac with Mac OS X
2. Attach either analog or digital audio input to the built-in audio port of the iMac
3. Start QuickTime Player and select "New Audio Recording" and select "built-in audio" from pop-menu
4. Start recording
5. Boot other Mac with MiniDisplayPort interface
6. Connect other Mac to iMac via MiniDisplayPort cable

Expected Results:

QuickTime Player should continue recording even when the iMac acts a second monitor for the other Mac as the built-in audio in port is not associated with the external computer. Streaming Pprograms like Nicecast should continue streaming as if nothing happend.

Actual Results:

The QuickTime Player stops recording sometimes with, sometimes without an error message stating that the audio stream has been interrupted. Nicecast also no longer gets any audio and transmits silence.


The problem is 100% reproducible and happened under every single OS release since the iMac came out.



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