I examined Spirit debug logs from a rooted Indian GT-I9500, T-Mobile SGH-M919 & Vodaphone UK GT-I9505.
Spirit appears to work, as it does on so many devices with no usable FM, but RSSI is always 0 on GT-I9500, and on SGH-M919 & GT-I9505 it was 0 or -17.
This is on the Broadcom BCM4335 FM/combo chip that all S4 variants have. The FM antenna pin is likely grounded / disabled and unusable.
And, other than a tiny clue in the kernel source, there's no indication of a dedicated FM chip on the PCBs, like the SI470x chips on previous International GS1 / GS2 / GS3 / Player / Note / Note2.
Hypothetically, there could be a dedicated Silicon Labs FM chip, or the PCB space and traces for it, hidden on the GT-I9500 PCBs. But Samsung would be crazy to waste a dollar on a dedicated FM only chip when FM is not officially supported. Even if there was a space for a chip.
Hypothetically, FM might just be missing, or disabled, in the bcm4335.hcd "patchram, almost firmware" kind of file. But I've never seen this happen before.
Hypothetically, there could be a software controlled switch somewhere that magically connects the FM antenna pin to headset, but I've never seen this before either.
So, I consider this case closed, for now at least, unless someone comes up with some interesting new observations.
My predictions were wrong, whatever, keeps life interesting and educational.
EDIT: May 20:
I had thought the existence of FM specific voice command text in ROM dumps indicated that FM was planned at some point.
But the GS3 GT-I9300 has voice commands in the latest stock JB leaked ROM, and it includes support for FM. See http://forum.xda-developers.com/show...postcount=6078
I will be interested to eventually see the service manual schematics. Like those for the Nexus4 and Optimus G, I wouldn't be surprised to see evidence that an FM antenna connection to the BCM4335 had been planned, then scrapped. Then again, the kernel GPIO symbols imply an SI470x dedicated FM chip, so the schematic might show some dangling connections for that.