Right, but there must be video for those data islands. It is actually a serious desire of many to have a lower-bandwidth audio-only high-res out, but I am not aware of any devices that support this, even in a non-spec implementation of the HDMI physical interface (as there is with for instance Denon-link which is a proprietary implementation of the physical ethernet interface for audio only).
If you look at section 5, you don't see any mention of support for signaling that doesn't incorporate Video, Data, and Control periods. It does make mention of "while transmitting video, at least one Data Island shall be transmitted during every two video fields" but there isn't clear support that I see directly of audio-only (or Data & Control only) operation.
And again, there definitely have been many requests for people hunting for devices that can do an audio-only output, and AFAIK this is simply not possible with HDMI, hence why no devices that support such an option. As I mentioned before, there are processors that have what purports to be an "audio-only" HDMI output, but they simply replace the active picture with all black frames but still must transmit video.
And the video rate also does determine the bandwidth available to audio, and if I recall correctly lower video rates such as for 480p etc cannot support the full range of high-res audio like HD resolutions at higher clock rates do.
This is described more in section 7.3.3:
Available audio bandwidth depends upon the TMDS clock frequency, the video format timing, and whether or not content protection re-synchronization is needed.
Further, a representative of the HDMI organization answered this question directly in this thread:http://www.avsforum.com/avs-vb/showthread.php?t=795465
Originally Posted by HDMI_Org It is true that the HDMI signal does require a video signal (namely, it requires the video clock signal) in order to send audio (which sits in the blanking period of the video signal).
However, even a blank/dummy pattern (such as all black) video signal would do the trick, so the audio does not have to be linked with a specific HD video signals.
HDMI is more expensive than than AES/EBU or S/PDIF over XLR/Toslink/Coax at the moment for pure audio applications so i doubt many transceivers have been built for audio only, but thats not a limit of the HDMI spec but rather how designers have chosen to implement it.
I believe you are mistaken about this, as reiterated by HDMI itself.
I don't know of any audio only products but again thats not because HDMI wouldn't allow it, rather that no one has chosen to implement it as it doesn't have any advantage over current options.
Its added speed only matters for lots of high bit rate channels, which means your watching a movie and need video also anyway. If anyone came up with an audio only application that required more bandwidth than AES/EBU or S/PDIF could deliver you may see some audio only HDMI devices.
It would have huge advantage for distributing high-res audio to a device much farther away which is a major obstacle for the huge bitrates of HDMI because of video. If you could extract the audio alone, and send that over an audio-only interface or an audio-only implementation of HDMI then you could go much farther with the signal with much greater ease. Many people have asked for this, and it is not possible, otherwise it would have been done already.