[grc] [Stubblefield] Using a digital stream as STL to LPFM transmitter site

[David Klann]

I set up a pair of purpose-built PC's running liquidsoap as the digital
transport between the studio and transmitter site for WDRT, Viroqua, WI.
The connection has been running for a just shy of six years without
interruption except for accidental cable cuts (twice) of the fiber
between the two locations. But liquidsoap reconnected when physical
connectivity was restored. With appropriately-sized UPS these boxes can
run until the electronics give up. The "uptime" on the transmitter end
of WDRT's STL is currently:

 21:20:20 up 2145 days, 22:07,  1 user,  load average: 0.59, 0.48, 0.39

(The studio end has less reliable power and has a lower "uptime".)

It didn't take much to build the operating system (linux) to do this.
There's nothing running that does anything automatically except for the
liquidsoap process that transports the audio from studio to transmitter.
In the (unlikely) event of a power interruption, the boxes simply
restart and pick up where they left off.

I've since refined the operating system and have an appliance that I'd
be happy to share with anyone that's interested. Most of the work is on
the github site at https://github.com/opensourceradio/stl

  ~David Klann
   Broadcast Tool & Die


On 09/29/2016 07:21 PM, Danny via grc wrote:
> The Ubiquiti has always been our preferred option. But we’re building out both a LPFM and associated translator in tandem, with the CP for the latter expiring first, so we already have a very full plate. Plus, the hills are public parks, which greatly complicates that option. Our focus right now is saving both CPs and then making it all pretty later when we have more time and people will have a clearer sense what we’re up to.
> 
>  
> 
> From: Stubblefield [mailto:stubblefield-bounces at lists.prometheusradio.org] On Behalf Of Bill Putney
> Sent: Thursday, September 29, 2016 7:35 PM
> To: stubblefield at lists.prometheusradio.org
> Subject: Re: [Stubblefield] Using a digital stream as STL to LPFM transmitter site
> 
>  
> 
> If you can find a place up on the hill at someone's house or business to host a couple of Ubiquiti 5 GHz radios that would make a nicer STL transport than public Internet. Public Internet connections are prone to drops and outright failure. Not really something you want your listeners to have to put up with. If the Internet where you are is really good, it will work fine but a lot of times it isn't an it just makes everyone's life miserable. 
> 
> As for using an Icecast server at the studio end, that sounds fine. Use something like 256 Kbps AAC or another modern good sounding codec. Full CD rate PCM would be best and with a Ubiquiti link you have the bandwidth. I'd stay away from MP3 even at higher bit rates. People are going to want to bring in their crummy MP3 music to play on the air. A bad idea on several levels but if you have to skimp on the STL bandwidth and use an aggressive codec you really need to start worrying about the cascading codec's making a mess of the audio. It won't turn up in everything that gets played but when it does it'll like fingernails on a chalkboard.
> 
> The Raspberry Pi has plenty of horsepower for the job. The audio interface on the Raspberry Pi isn't the greatest. It does have a USB port. Ask your software person if they can use a Pro-sumer USB to balanced audio interface with the Pi. If your transmitter has an AES input, you can use a really inexpensive USB to S/PDIF interface that will sound better than any analog interface. I ran our automation through a $35 module I bought on eBay when we started. Bits in/bits out. If it sync's up you can't go wrong. Bits to an analog waveform is a much harder thing to do right and it costs money to get it right. 
> 
> I don't think the Pi has S/PDIF out (heither the coaxial or optical-TOSLINK). It does have HDMI so the software does know most of the stuff it needs to know about S/PDIF. AES and S/PDIF are nearly identical protocols and the electrical differences are close enough to work if the connection is under a few feet.
> 
> Bill Putney - WB6RFW
> Chief Engineer
> KPTZ - Port Townsend, WA
>  
> PP-SEL/A&P-IA
>  
> "...you know me to be a very smart man. Don't you think if I were wrong, I'd know it?" -Sheldon Cooper
>  
> 
> On 9/29/16 4:09 PM, Danny wrote:
> 
> Our LPFM transmitter site is 6+ miles from our studio without a clean line-of-sight because of hills. So, to save time and money, we’ve been expecting to use a pair of Barix boxes to move a digital stream from our studio to our transmitter.  But a couple of our key people think we can skip using one or both Barix boxes.
> 
>  
> 
> Specifically, the person who maintains the Icecast server that hosts our webcast feels confident he can create a second private stream of much higher quality that can take the place of the need for a Barix box at the studio end and another person thinks we can use a “headless internet radio player” on a Raspberry Pi instead of a Barix box at the transmitter site. He’s looking at https://www.raspberrypi.org/forums/viewtopic.php?t=43108  especially noting this setup has auto-run upon reboot, which is what we want. This is a key person of ours with lots of experience using the Raspberry Pi.
> 
>  
> 
> Are one or both of them missing something with these suggestions? Will these be viable and reliable ways to avoid paying $1,200 for a pair of Barix boxes?
> 
>  
> 
> Thanks!
> 
> Danny
> 
> WSPJ-LP Syracuse NY
> 
> 
> 
> 
> 
> 
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