Spirit FM Transmit is a real FM transmitter application.Spirit Transmit works ONLY on ROOTED late 2012+ to early 2014 Sony devices that use Qualcomm FM chips, where the "Internal" FM antenna pin is used.
Stock "store bought" phones that "regular" non-techie people buy can not run this app.
NOTE Feb 21, 2015: If Spirit Transmit v1 was working for you, but this redesigned 2015 v2+ does not, please email me at firstname.lastname@example.org . It has been working fine for me on my Z and Z1 running CM11/FXP and CM12 since late December 2014.
Confirmed working: Xperia Z, Z Ultra, Z1, Z1 compact, T, SP. (I have a Z and Z1 for testing, mostly on CM11/FXP and CM12.)
Confirmed or likely not working: Xperia L, M, Z2, Z2 tablet, Z3 and newer.
Free version v1 here, to verify if your Sony device can transmit: http://d-h.st/ytE
Spirit Transmit v2 is now based on the Android 5.0+ Lollipop re-architecture of Spirit2.
More info here: http://forum.xda-developers.com/showthread.php?p=54601208#post54601208
The power is low so range is not very good. It works best as a method to send music from your phone to a car or home radio that is very close. Sound quality is best with the transmitter antenna (wired headset cable) very close to, or even wrapped around the receiver antenna.
I use Spirit Transmit regularly myself, for the extensive testing of the Spirit2 FM receive app on the 13 Android devices I own which Spirit2 supports.
Time I invest adding features is proportional to support from purchases.
Like the Spirit1 & Spirit2 FM receiver apps, buying & supporting Spirit Transmit is not just a one time purchase. It's ongoing technical support and feature updates for at least a few years.
The Spirit2 FM Receive app now has a Free & Open source GPLv3 "Spirit2 Free" variant with 90% of the code of the full paid Spirit2 app. This code includes a more limited transmit mode than this paid Spirit Transmit app.
So, support for Spirit2 & Spirit Transmit is also support for open source. Source here: https://github.com/mikereidis/spirit2_free and more info here: http://forum.xda-developers.com/apps/spirit-fm/app-spirit2-free-source-real-fm-radio-t2951455
Start Spirit Transmit and grant SU when requested. (If you don't know what "SU" is, Spirit Transmit is not an app that you can run.)
Spirit Transmit Version 2+ (2015) supports RDS from music player metadata. For this you need to enable "Spirit Transmit RDS" when presented with the System Security -> Notification Access screen.
As of Feb 21, for RDS, you may need to manually set one or more of the experimental RDS settings under Menu-Set-Tuner-QCOM: Transmit RDS PI, PT, PS & RT.
If wired headset is plugged in, it should start. Tune as needed. Tap on frequency, or use up, down, or presets.
Music and most other audio (except FM receive and phone calls) should be transmitted at the indicated frequency whenever Spirit Transmit is on/running. The app sets volume to maximum for best results and disables headset output at the end of each song.
You can use most of the same features as with the Spirit2 receive app. There is a homescreen widget and a notification and lockscreen display. Media buttons are not handled, under the assumption that a media player will handle that.
Range is limited. Get transmit and receive antennas as close as possible. Wrap them together along their lengths if feasible.
Thanks for your support... !Mike.