Can an HDMI output be converted into something a USB input would accept?

When you go down the non-standard rabbit hole, don't be surprised if all you find at the bottom is rabbit turds.

External DAC's accept DSD only in two ways:

DSD over PCM (DoP) for S/PDIF and USB. Limited to 2x DSD

Direct DSD via USB 2.0. My Brooklyn plays at least quad DSD this way just fine. 

But to get here I have to buy DSD files I'm afraid. 

You may find more useful data here:

https://www.stereophile.com/category/music-round

Thanks for the responses, folks -- I'm afraid this ship won't float.  @whart you may be right, but the Cambridge tech support people said it was a "DSD" signal, so I assumed they meant unencrypted DSD -- you know what happens when you assume.  It sounds like there is no simple conversion, so I'll just continue to listen to my SACD's the old fashioned way (which sounded good, it's just a bit inconvenient).  Happy listening to you all!

@cheeg  
hi, i was in your same position many years back but not with Cambridge products. i search everywhere even at DIY forums but to no avail. The HDMI to coax/optical converter doesn't work in our case because it ONLY WORK FOR consumer video players that output HDMI standard signals (video & audio) protocol.

Audiophile products are just using the HDMI connectors out of convenience to stream DSD signals, definitely not the same signals from common household bluray player. There isn't a agreed standard between various manufacturers regarding the pin configuration within the HDMI connector too.

you need to look for a DAC that accept DSD signals through HDMI connector that corresponds to Cambridge HDMI pin configuration, otherwise you are stuck with using a Cambridge DAC,

in addition, even if you manage to find a converter that work with your USB input, IMHO, i think it isn't that worthwhile since the additional conversion would cause more jitters , plus additional digital cable & its connections would totally degrade your DSD signals. 
l'm sorry that maybe hard to accept. 
-phil  

Isn't the reason that, at least when this was on an SACD format, it was encrypted so all hardware had to match the 'closed system' protocol with no digital output that could be accessed outside of that closed loop? E.g., like original and Blu DVD? 

Thanks Steve -- I tried that too, but no luck...