RDS Encoder Audio

RDS Encoder Audio
Application Note:
14. IV. 2012
Emitting RDS signal using commercially available MP3 transmitters
This document is for guidance only! Application is on your own risk!
We do not supply any parts or provide any support in relation to this document.
Almost anyone making RDS development and testing takes a note that there's no suitable
low cost FM modulator available on the market with small output power, wideband
modulation input and stable frequency.
It has been discovered that – with some limitations – commercially available mp3
transmitters can provide the same functionality, without need of any modifications.
The solution described has been successfully tested with PIRA32 and MicroRDS on
Belkin Tunecast II transmitter (made before 2011). It should work with any other
similar transmitter that is based on BH1415F chip.
The solution cannot be applied to device based on Silicon Labs chip (SI47xx).
Important notes:
The solution does not provide broadcast quality results (as no BH1415F based transmitter does). Originally the
BH1415F chip does not allow emitting RDS at all!
The transmitter provides audio inputs only; we will use them for RDS signal injection. Internal limiter and stereo
encoder causes considerable reduction of RDS signal quality injected that way. Pilot and RDS synchronization is
omitted. Due to poor quality of the integrated stereo encoder there's a strong 3rd pilot harmonic at 57 kHz,
interfering with the RDS signal. Thus the performance is given in advance and some limitations apply. To avoid
erratic RDS reception the FM deviation caused by RDS signal must be 4 kHz or more (see appropriate signal
levels, pp = peak to peak).
Variant 1 - Broadcasting both RDS and audio signal
Feed the RDS to both audio channels and feed audio to both audio channels using sum resistors. Their value is
about 1 kΩ. It is important to use only low audio levels to avoid internal limiter action.
The audio level from the player should not exceed 200 mV pp in each channel, resulting in 50 kHz total peak
FM deviation.
The RDS level on the RDS encoder should be set to about 150 mV pp (1/10 of the max. level).
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2x stereo jack socket
RDS Encoder
Variant 2 - Emitting RDS only
Feed the RDS signal to both audio channels Right and Left. On the RDS encoder set the RDS level to a value of
about 50 to 100 mV pp (less than 1/10 of the max. level).
RDS Encoder
RDS Receiver
(C) 2012 Pira.cz
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