In broadcast, lots of people are still using dedicated analog lines to connect remote sites. These operate like old telephone systems: you call up the operator and request to be patched through to a specific site. They’re also rather expensive.
For a hospital radio station, [Marc] wanted to replace the old system with something less costly. The result is his Raspberry Pi STL in a Box. Inside the box is a Raspberry Pi, PiFace display, a pair of meters, and some analog hardware for the audio.
On the software side, the system uses LiquidSoap to manage the stream. LiquidSoap uses a language to configure streams, and [Marc] has a write-up on how to configure LiquidSoap for this application. On the hardware side, SSM2142 ICs convert the signal from single-ended to balanced. The meters use the LM3915 bar drivers to control the meters.
The Python script that controls the box is provided, and could be helpful for anyone needing to build their own low-cost audio link.
7 thoughts on “Raspberry Pi Remote Audio Link”
HaD “you call up the operator and request to be patched through to a specific site”
Well no. The key word is “dedicated”, a direct wired connection. It’s normally a 600-ohm balanced pair (or quad for stereo these days) that you hire from the local phone provider that goes from the studio rack room to the transmitter site rack.
These generally have no connection to the actual phone systems which incidentally allows common mode to be used for remote signaling and monitoring of the transmitter, although they may be fitted with dedicated passive EQ to flatten the line bandwidth to broadcast spec.
A word of caution for new players; the LM3915 Line-of-Light VU IC is a great little device, but care must be taken with power supplies and ground to avoid injecting “zipper” noise into the signal lines as the LED’s switch.
OP shouldn’t be worried about “messy” – it’s neater than a lot of other one-off rack room residents. Well done.
OP here. Usually the lines are dedicated and have all the equipment for flattening EQ. However, one of the features of the lines we use is that they can be re-patched at the exchange and were often used to temporarily take feeds during OBs. It’s not all done on BT jacks but gets the image across. ;)
As the other lines we had have since been ceased (we use POTS codecs for OBs now), it’s fairly permanently wired up. Especially so since the stretch between two exchanges was cut by accident and replaced with an EPS circuit.
I’ll look into the noise side of things. I’ve used an “off the shelf” PSU from another bit of broadcast kit that’s fairly low noise. Either way, I’ve not heard any noticeable noise on the output since some of the earlier tests.
This is very cool. First thing that comes to mind is a remote pirate radio station…
Nice. A mini-ITX would be an option but would cost more in parts but might have cost less in labor. I love the build as is and looks great.
The fire extinguisher on stand-by is a nice touch!
Hey OP, good job!
Now, you should consider using an external USB sound card, as the raspberry audio out have some noise.
Those found on ebay for $1.66 would work just fine (search: usb sound 7.1)
Also check out this site for a list of supported DACs and other info related to the raspberry and sound:
I work for Stoke Mandeville Hospital Radio, We are looking to use this to decode our shoutcast stream. I keep getting empty
Line 4, char 6: Empty token!, any ideas?
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