Stereo Tool

I am the field engineer for a nation wide radio network in the US. http://www.rejoice.org

We currently have 42 translators and 1 soon to be 2 main stations. Your software is impressive and might be of great use to us for a special project I want to do. I want to include RDS is my broadcast but the encoders I use for my satellites does not have a side channel input for RDS so I need to find a way to send the data without having an encoder at every station.

I am working out all the numbers now but I need more information on how your software works. Does it output the RDS properly in stereo so if I send both channels to the satellite encoder and it can handle the bandwidth that your signal will use then in theory it should transfer up to the satellites and be received. I have a lot of research to do as this chain goes through a lot of equipment before it reaches the crown transmitter at the other end. I know this might be a long shot for everything to work out but it would be the cheapest way to implement RDS over my entire network at one time.

If the signal can be output in stereo on a sound card then I might be in luck. Also I am assuming you put the RDS on the signal at the proper offset from the carrier as the transmitter might and if I feed that signal straight into my transmitter and out it should go on the air.

Has anyone tried this before?

I'm not sure that I really understand what you are doing exactly - I'll give it a shot and try to explain how things work, and answer your questions.

Reading your question I'm afraid that it's not going to work the way you are describing it now. But there might be another way to do it which may even be simpler...

Basically, the RDS signal consists of:
- a 19 kHz pilot tone
- The RDS signal at 57 kHz +/- 2.4 kHz (so you need to be able to transmit frequencies up to 60 kHz, or a sampling rate of 120 kHz).

Important: The 19 kHz pilot tone is the same pilot tone that is used for the stereo signal. So the RDS and stereo information must be encoded at the same time. For your setup, that would mean that:
- You need to use both the stereo encoder and the RDS encoder from Stereo Tool.
- You need to transmit the encoded signal to the satellite
- The 40 transmitters must receive the signal from the satellite and broadcast it unchanged. So there must be no pre-emphasizer, stereo coder etc. at the transmitter side.

If I understand your description correctly, you currently transmit 2 different channels to each transmitter - which means that each transmitter is performing the stereo encoding. To make this work, you would have to turn those stereo coders off, and use the stereo encoder of Stereo Tool instead.

The good news is that the combined MPX signal that Stereo Tool outputs is only 1 channel. So you needs half the bandwidth that you would need for two channels. The bad news is that you are now probably using 2 32 kHz channels, and you need 1 at-least-120 kHz channel - so you probably do need more bandwidth than you are now using. (depending on whether the signal is analog or digital etc.)

Back to your questions:
Does it output the RDS properly in stereo so if I send both channels to the satellite encoder and it can handle the bandwidth that your signal will use then in theory it should transfer up to the satellites and be received.
See above. You get one channel that contains the mono signal (L+R) at 0-15 kHz, the stereo difference signal (L-R) at 23-53 kHz, the RDS signal at 54-60 kHz, and the (phase locked) pilot for both the stereo and RDS signal at 19 kHz.

I have a lot of research to do as this chain goes through a lot of equipment before it reaches the crown transmitter at the other end.
That does not necessarily have to be a problem. I'm currently working on a new version that will contain a calibration filter to accommodate for changes that occur between Stereo Tool and the transmitter. See the 2nd posting in this topic: viewtopic.php?f=14&t=83 . Of course, if any of those other devices removes the high frequencies, it won't work.

If the signal can be output in stereo on a sound card then I might be in luck.
Well, it will be one channel, but that one channel can be output by a sound card. Just make sure you use a really good quality card (especially if you're going to feed 40 stations with it!). Most importantly, don't use a Creative (SoundBlaster / X-Fi) card, they really ruin the signal.

Also I am assuming you put the RDS on the signal at the proper offset from the carrier as the transmitter might and if I feed that signal straight into my transmitter and out it should go on the air.
As far as I know I have implemented the RDS specification correctly, so yes. I have received measurements from people with professional transmitters that measured an offset of 1 degrees - other big commercial stations were between 0 and 4 degrees with the same measuring equipment. Small offsets can occur due to the sound card, length of the cable between the sound card and the transmitter, and the quality of the transmitter. As mentioned above, there's now a beta version where you can correct these offsets.

If you have any more questions, please let me know - having Stereo Tool run on a big setup like this would be really cool


The set-up that I would suggest would be:
- Use Stereo Tool at one location to perform the Stereo and RDS encoding. The good news is that you can also use things like the composite limiter - and if you like it, the audio processing itself.
- Send the single MPX channel to the satellite. Make sure that it is received unaltered (so don't use lossless compression etc.), and that frequencies up to at least 60 kHz are supported.
- The 40 transmitters can now be extremely easy: They just need to receive the MPX signal from the satellite and broadcast it. No further processing is needed at this point - including stereo encoding, pre-emphasis etc. (you might want to hard-limit the signal directly before broadcasting it to make sure you don't break any broadcasting regulations).

The good news is that you do all the processing at one location, and use extremely simple (and cheap) equipment at the 40 transmitters.
The bad news is that you probably need more bandwidth on the satellite, which is probably expensive (?). This depends on the encoding type that is used by the satellite.


If you are going to perform test measurement, please use the beta version. There was a bug in previous versions which could lead to peaks above the maximum volume. This has been resolved in the beta version.

Well looking over my system it is most likely not going to work for the entire network. I might get it to work with my main station STL but the translators are another problem. The audio path is crazy.

Analog audio leaves a DA
Enters a DAC700 and is encoded to MPEG 2
Leaves the DAC700 and enters a CM701 which puts the MPEG 2 data on a carrier for the satellite
that is sent to the satellite and is then uplinked on the C-Band and Downlinked by NPR in Washington DC
This is then uplinked to the KU-Band and downlinked to the translators who receive the digital signal and decode it to stereo and send it to the transmitter to be encoded again and placed on the carrier for broadcast.

I knew it was a long shot but the more I look into it the more it seems that if it did work it would be a miracle.

however I am still investigating the my STL to see if it can handle it and then it should work for my main station and thats ok with me the rest of the network was just a bonus anyway.

thanks for the Info

Too bad The MPEG compression would completely remove the RDS and stereo info. So it's indeed not going to work - unless you could broadcast a raw signal.

Quick Question that just came to mind:

Can I have the software encode the RDS data I send it and not send any audio to it?

I want to convert my system to AES but could use this to send just the RDS data through the STL link without audio and send the audio through AES

Thanks again
Jonathan Bowen

Probably only if you want to broadcast in mono. As I mentioned before, the RDS and stereo signal need to be encoded at the same time, because they are both locked to the 19 kHz pilot tone.

Possible solution: There are some hardware RDS encoders that can connect to an already present pilot tone (otherwise you couldn't use separate RDS and stereo encoders) - maybe there are stereo coders that can do the same thing. In that case it would work.