Stereo Tool

Hi there, first time poster, but been lurking for many years.

I have a question, though the answer could be more to do with FM, the TX or RX rather than ST. But just in case it is something I have miss-set, I thought I would ask..... (I have a FM Pro licence with declipper & advanced dynamics)

ST is great and put it to good use over the summer with a short term FM licence in the UK to run a scout radio station. I set up ST on a RaspberryPI (4, 8MB) & HiFiBerry in order to generate an MPX. It was to feed a Lucoro TX50, bypassing its built in audio encoder and processing. I think I set the tilt okay on a scope, and all the levels were good and peaks nicely under 75KHz deviation, frequency measured and spot on (I think!). Preset based on Greek 5&7. So I hope I set it up well.

Sounded great to be honest, both in testing TX into a dummy load and once deployed. The only setting that was not the best possible was latency in the Pi. I set this as low as I dare as I needed reporters in the field to use off-air as a cue feed. Was around 25ms, not overly easy to use, but was the compromise between delay and good enough stable audio.

However, the issue being every time the audio level went low (music fades, gaps, overnight carrier on), the background RF noise picked up. Quite a lot. If you were doing speech or had gaps, it would be hard to listen to. With music and fast talking or background sounds, it sounded good, which was the majority of the time.

I didn't notice this in testing, just when on-air. It could be a feature of the TX or the RX I used, or perhaps an FM anomaly, or something in the settings of ST I missed. Has anyone else ever come across this anomaly?

Andrew.

Have you had any luck fixing your noise problem on FM? I'm having the same issue. Thanks Brian

If it only happens during silence, it could be that your sound card is picking up noise from the nearby transmitter, and the AGC, multiband etc are boosting it. If it's always there but masked by louder sounds, then it's more likely that it's something in the output or the transmitter.

Things to try:
- Turn off the pilot. If it disappears, you know it's in audio above 19 kHz. (You can also just set your receiver to mono if it suports that).
- If that doesn't affect it, turn Stereo Tool off. If that fixes it, it's in the input, otherwise it's after Stereo Tool or in the transmitter
- If turning off the pilot does make it go away, try feeding it with some other device that can generate a 19 kHz tone (you could even just play a 19 kHz tone on a phone if it has a headphone jack) and check what that does. That will tell you if it's the Pi or the transmitter.