Written by Editor’s note: This is a guest article written by fmdxing, an WTFDA member from Australia’s east coast who has a DX blog at http://fmdxing.wordpress.com/.
By fmdxing
Long distance FM enthusiasts are always watchful for better ways to authenticate station reception to cement the credibility of their loggings. Radio Data System (RDS) is one way to do this, providing the received stations are equipped with an RDS encoder. Sadly, not all FM broadcasts feature RDS, illustrated below. Prevalence varies with the continent the listener resides in.
Some FM receivers offer better RDS sensitivity than others. For example, the Blaupunkt Digiceiver series of car radios (e.g. Casablanca CD51, Daytona MP53 & Las Vegas DVD35) provide significantly improved RDS sensitivity compared to the Sony XDR series of component tuners (consisting of the XDR-F1HD, XDR-S10HDip & Sony XDR-S3HD).
It is worthwhile for an enthusiast to try and maximize RDS performance. Mark Hattam explains:
Even brief bursts of a distant FM Radio signal can contain sufficient readable RDS data for that signal to be identified. So for any propagation mode, being able to monitor and decode RDS data is an invaluble tool for an FM dx-er. Using a PC to continuously log incoming RDS data is an additional asset.
How can I make RDS work better?
For those enthusiasts who own a component tuner with an RDS decoder Integrated Circuit (IC) and a portable laptop, netbook or tablet loaded with a modern Windows operating system (Windows 98 or above) there is a simple way to get the very best RDS efficiency.
This article aims to demonstrate how easy it is to set up software decoding RDS directly from a component tuner (pictured below) into a computer. This technique will beat any component tuner’s existing RDS sensitivity.
What is needed?
- ‘Vintage’ RDS tuner (without HD or DAB capability) that features an RDS decoder IC.
- Quality computer sound card with a Line Level Input.
- Long shielded stereo RCA cable.
- 2 x RCA Female to 3.5mm Stereo Mini Jack Adaptor.
- Soldering iron and solder.
What is the procedure?
For this project, the test RDS tuner is the Sherwood TX-5090RDS. The test laptop is running the Windows XP Professional operating system with a Centrino 2 processor. The sound card is an Realtek ALC883 Azalia. Realtek specifications for analogue recording suggest a maximum 96kHz sampling rate and 85dB signal to noise ratio.
The chosen freeware software is RDS Spy. Another is Esslinger RDS Decoder for Windows. A Java 6 based alternative is the open source software RDS Surveyor, illustrated below.
Remove the screws and lid from the tuner. Find the RDS decoder Integrated Circuit (IC) by looking over the tuner’s circuits. Some common RDS demodulators include:
- the TDA7330 (pictured below) made by SGS Thompson Microelectronics &
- the SAA6588 & SAA6579 made by Philips Semiconductors.
Once the appropriate IC has been found, search for the GROUND, CLOCK and DATA pins on the chip. The bare wire is to be attached to these pins.
A schematic for the tuner may be available on-line to provide further assistance. If the pins are not marked on the tuner’s Printed Circuit Board (pictured below) as DA and CL, refer to the datasheet for the particular RDS IC, available online.
The Sherwood TX-5090RDS tuner features the common TDA7330 B IC. The datasheet (extract below) indicates that the wires should be soldered to pins 5, 12 and 13. This tuner has these marked on the front and rear of the Printed Circuit Board so the datasheet reference is not required in this instance.
Cut off one end of each RCA cable to reveal bare wires, pictured below.
Solder the CLOCK and DATA to the CENTRE conductors (these may be coloured red and white) on the freshly-‘skinned’ RCA cable. The photographs below show the appropriate soldering to pin outputs marked on the Sherwood TX-5090RDS tuner. Further, solder the GROUND to the SHIELD of the RCA cable. This is pin 5 output on the TDA7330 B IC. The soldering to this connection is not illustrated.
Turn to the computer for a moment and download RDS Spy. Plug in the RCA male plugs into the Stereo Mini Jack adapter and slide the adapter plug into the socket marked Line Level Input on the computer, as shown in the photograph below.
Execute the RDS Spy software in Windows. Proceed to the Configure menu in the software and navigate to Select RDS Source. Ensure Data And Clock are selected. Other settings such as Polarity, Rising Edge or Falling Edge settings do not apply to the chosen decoder IC.
Return to the tuner. Plug the tuner into a rooftop antenna socket and tune to a strong local station which features RDS.
Once functional, ensure the other RDS broadcasts can be decoded. If RDS functionality is not possible in RDS Spy, please repeat the above configuration step until there is uniform decoding on all stations.
Have it working?
Congratulations! Replace the screws and lid back onto the tuner. Now stations may be more easily logged using a PI code that appears in RDS Spy.
Can’t get it working?
Whilst coverage is beyond the scope of this article, please be aware that differences between sound cards may prove problematic. For troubleshooting, please refer to Jan Kolář’s RDS Spy instruction manual and the comprehensive user forum. Another suggested reference is the step-by-step guide to using RDS Spy with the Sony ST-SB920 tuner written by William Kitching, the inspiration for this article. Lee J Freshwater has a similar guide for the Denon TU-1500 tuner.
Importance of testing
Excessive use of 80 kHz and 50 kHz narrow ceramic filters may prevent stereo decoding and therefore make RDS decoding impossible. It is prudent to ensure the FM tuner is optimized to receive RDS signals sufficiently (with the best quality rooftop antenna available) before proceeding with such a project. Please try to be patient when checking the local signals with the software. The author knows first hand exactly how exhausting it can be, but the time invested to get it working properly will surely pay dividends later with weak signal reception!
Need some ‘harder liquor’?
Component tuners and portable radios without an on-board RDS demodulator may also be used if one ‘brews their own’ RDS decoder. For example, Associate Professor Christophe Jacquet, writer of RDS Surveyor software has tapped the Multiplex Output on a Sangean PR-D1 portable receiver. The writer of this article has built the RDS decoder circuit described by Puskas Barnabas, and recommends it for those who like to have fun building circuits!
Simpler alternatives
An audiophile-grade sound card such as Creative Labs’ E-mu (above) supports analogue recording at 192 kHz sampling rate. Thus, RDS decoding from a vintage (non-RDS) tuner’s Multiplex Output or even a Line Level Output may be possible! The E-mu sound card may be purchased secondhand at modest prices between $40 – $120 depending on whether a USB or PCI model is required. For more information, again the best resource is Jan Kolář’s instruction manual and the forum.
Please consider the risks involved with modifying a tuner before proceeding with any project. The author shall not be liable for any loss or damage whatsoever (including human or computer error, negligent or otherwise, or incidental or consequential loss or damage) arising out of, or in connection with any use or reliance on these instructions.
FM Radio DX 