RDS Decoder Add-on ?

[radioman]

I was wondering if there is a simple way to make an RDS display add-on for an FM radio that doesn't have this feature ?
I assume that you'd need to extract the sub-carrier required, process it in some way then use e.g. an Arduino or similar to decode the data and send this to a 16x2 line LCD.
Either this or a module that does it all for you.
I know you can use a computer running the appropriate software plus an RTL dongle but I'm thinking about a smaller, low-power solution that could be added to an existing FM receiver.

Andy

[Mike Watterson]

A PIC, the old philips RDS chip and take FM decoder signal where it goes to Stereo decoder. Not possible on modern "all in one" radios that have stereo decoder inside RF chip.
Without the HW chip which takes 57kHz in and gives TTL 1170 bps out, it's not possible without serious DSP on a much higher power ARM. The basic Ardunio is an Atmel, now owned by Microchip who do PIC. I have the decoding code for the PIC and also the LCD panel code.

The Ardunio like boards that take the same shields and have decent ARM are not as good as Raspberry Pi and would work out more. For any DSP solution you'd want at least 192 kHz sampling (96kHz is Nyquist unless doing I & Q which needs two 90 degree clocks, two signals or 384kHz) and an older radio where there is access to signal at input of the Stereo Decoder. Of course the newer all in one FM chip radios usually do RDS also, anyway.

Only the bit to get the 1170 bps data from the modulation on the 57kHz carrier (approx?) needs DSP. After that it's just implementing the public RDS spec. The old Philips DIL IC has an internal switched capacitor filter for 57kHz, a sort of hybrid Analogue / Digital signal processing maybe first used by Mitel in DTMF decoders that were single chip and outperformed previous C.O. systems. I think equivalents still made. MT8870?
I think there may also have been some sort of programmable filter using the technique, which power wise out performs an FPGA (hardware DSP) or a decent ARM doing DSP in software. Volume designs based on the FPGA take little power and can be made directly from the design file, if you have $500,000 to spare. But then if you sell 10 million gadgets the cost might be $2 each inc the $500,000 NRE, whereas FPGA is in comparison maybe 100x cost and 100x power usage. CPU DSP in software is very limited in speed x complexity compared to FPGA and can use more power than an FPGA at higher frequencies or complexity.

[jimmc101]

Not quite what you're looking for, but the Si4703 is a complete FM receiver and RDS decoder on  a chip (There are some cheap Chinese modules using this chip).
Needs a Arduino of similar to control it.

As an aside the Si4713 is an FM transmitter and RDS encoder on a chip.  Again needs a Arduino of similar to control it.

Jim

[radioman]

Thanks for the replies.
It seems that for what I want, I am about twenty years too late(!) as most of the RDS decoding chips required are obsolete...
I found on-line references to SAA6579 / SAA6581 (Philips) and TDA7330B (ST)
Some people have managed to do what I had in mind but have either not shared their code or else used now-obsolete microcontrollers...
A useful web-page I found is here :-http://www.tekmanoid.com/rds.shtml

Michael, I have to admit that some of your reply was 'over-my-head' : although I would be interested to know which particular PIC and RDS chip you used in your project, so I can check on current availability.

It appears that things have moved on and that available modern IC's like the Silicon-Labs Si4703 'all-in-one' chip (as Jim has mentioned) will not do what I want due to their high integration.
Also, I don't know how good the audio quality of these DSP based devices is ..

Andy

[Murphyv310]

Hi.
A raspberry Pi and Hifiberry DAC + ADC would be capable of doing it. Have a look on Git Hub, there may be something there that will work.
https://www.elektormagazine.com/labs/pir...i-160520-1

https://github.com/windytan/redsea

[Mike Watterson]

I'll look out the schematics and export images from Eagle. I've no idea if the current free version of Eagle (if there is one with the current owner) can read my ancient schematics.
I used the TDA7330 with a 4.332MHz crystal.  No, it's not the PAL subcarrier frequency.
Managed to get to work on  a TDA7088 clone as well as real radio. I also added a TDA7040 stereo decoder. I use little DIL headers for the SMT ICs. The TDA7088 needs it's 70KHz IF filter components adjusted a little for RDS and Stereo. Both worked better on the "data" connector of the Yaesu FT817ND. I found it had FM de-emphasis TWICE. I remove the 1st cap that was on the FM detector. The rig uses a regular FM Radio part for WBFM Band II but a different IF and detector for FM on all other bands. The "data" connector has unfiltered audio.
Just about any midrange PIC18F will do. Depends on what else you want. The 18F4550 is easy to have a USB slave (just add connector, a capacitor and SW for HID, Serial or Mass Storage emulation).  I mostly used JAL, but some Assembler. Can be used inline too. Software serial data reading used for the TDA7330 1170 bps. Much easier on a PIC in JAL or Assembler or C than on Windows/Linux/Mac OS.

Version 2
   

Not shown is the PIC and its display.

I don't recommend the TDA7088 or clone for Stereo and/or RDS, but it can work.

I've added the TDA7040 SM IC to some valve tuners that had a now unavailable MPX option. Powered off the 6.3AC via a rectifier and regulator. A pot added at the input to set level. Doesn't take long to make up on stripboard and the DIL adaptor. Regular size iron and reflow the solder.

I'll email a pack to Murphy310 later in the week.

I got some standalone PIC (add your own resistors and stripboard) from "PIRA" to implement TTL Serial to 57kHz RDS encoding.

[Mike Watterson]

The software to unravel the 1170 bps serial data from the TDA7330 is simple. More complicated to drive the LCD panel if you don't have a library.
Right now I'm not sure where that is.

Here is the 7040 on its own for valve tuner.
   

Here is a €4 FM Tx PCB (bought locally) showing where RDS is fed in via a suitable resistor / capacitor from a PIC based encoder.
   
The pins of the IC are labelled. Also the channel / frequency selection switch and unused lines to select other channels. This IC also does channels in the Japan 76 to 90+ band.

The more common CE marked FM TX with an LCD and up/down arrows does the entire FM band, but the FM TX IC is controlled by the separate CPU via I2C that manages buttons and screen. Those can be got for about €12. There is no added quality with more expensive models. Also fed from Satellite, 256K MP3 or CD the quality of the cheapest ones is better than most local FM who have bad Optimod settings.
Some models only have space for one cell. Yet the IC is recommended for 2.75V to 3.5V! Better ones have two cells and 3.3V regulators. The ones with an auxiliary USB out (charge) need 8V to 9V on the power jack. No heatsink, so 12V in and using the USB charge is optimistic!

[radioman]

Mike, this is all great - thanks a lot. Your circuit diagram is very informative to me.
I have an old version of Eagle (6.2.0 Light Edition) on one of my PC's and I also have a PicKit3.
However, this will be academic if I'm unable to obtain an RDS demodulator chip.
Practically ALL of the common ones have become unobtainium within the last two years.
The only RDS demodulator I can find for sale in the UK is this :-https://uk.rs-online.com/web/p/modulator...s/8051286/
and the package is so small I doubt I'd be successful at soldering it.
Ho-hum.

Andy

[`FRANK.C]

Hi Andy
You can get SSOP to DIL adaptor boards as an example
https://www.ebay.ie/itm/2x-Adattatore-SO...SwvRhdg-48
Which should make it easier for you to use that chip.
Soldering them is not as difficult as you might think.

Frank

[radioman]

Hi Frank,
Thanks for pointing out these boards, I forgot that the SOIC-16 adapter boards I bought a few years ago have the smaller version on the other side... I've never tried soldering an IC that small before but I'm willing to give it a go.
I'll need to try and find out how similar/different the communication protocols between all these RDS demodulator IC's is now.

Andy