Radio AdBlock 227 Feb 2016
In the previous post I described the process of creating an ad-blocker for the polish radio station “Trójka”. It uses cross-correlation and fast Fourier transform to detect the ad jingles in the internet radio stream, silences the commercials and outputs the result. In other words: it plays the internet radio stream without ads. Creating the app wasn’t my final goal, though. I wanted to bring the same feature to my home amplituner (Yamaha R-N301), which receives “Trójka” in a traditional way via the FM broadcast.
So, I already have the algorithm, but I’m missing 3 elements required to run it in the real world of my flat:
- a device on which I can run the analysis,
- the broadcast, which will be used as a source for the analyzer,
- a way to turn the volume up and down on the amplituner.
That’s the easy one. I already have Raspberry Pi 2, which works great as a home media center. It should be able to run the Java analyzer.
the broadcast source
The internet broadcast is delayed by about 30 seconds, so I can’t use it to analyze the signal and silence the FM tuner. No, I should analyze the real-time FM broadcast. How to receive it on a Raspberry Pi? Well, it will cost about $20. There’s a lot of cheap DVB-T USB sticks on the market and apparently (because of the RTL-SDR library) they can be used to receive all kinds of signals, FM radio stations included.
After plugging-in the stick, following command will receive the broadcast on 89.5Mhz:
rtl_fm -f 89.5M -M wbfm
The stream will be redirected to the standard output as a 16-bit, little-endian, 32k, 1-channel PCM data, perfect for the Java analyzer.
controlling the amplituner
Yamaha calls R-N301 a “Network HiFi Receiver”, which means (among other things) it can be controlled from an iPhone app. Apparently, Yamaha network-enabled devices exposes a RESTful interface, which can be called with
curl. Following command will set the volume level to 55:
curl -d '<?xml version="1.0" encoding="utf-8"?><YAMAHA_AV cmd="PUT"><Main_Zone><Volume><Lvl><Val>55</Val><Exp>0</Exp><Unit></Unit></Lvl></Volume></Main_Zone></YAMAHA_AV>' 192.168.1.101/YamahaRemoteControl/ctrl
What’s more, it’s possible to get the current amplituner state, so we can launch the analyzer only if the tuner is enabled and plays the appropriate station.
I’ve used a few bash scripts to detect the current amplituner state, run the analyzer and turn down the volume during commercial blocks. Scripts can be found on my github.
This is how the adblocker works in practice. The terminal tails the analyzer log file: