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SDR Audio Mentor Needed

Posted: Sat May 07, 2016 3:36 pm
by CapCouillon
Glad to find this forum, and hoping to find a Mentor to hold my hand thru setting up a regular audio feed via a RTL-SDR receiver. I am a net controler on the Boot Key Harbor Cruisers Net (Marathon Fl) and we have been discussing the possibility of at the very least archiving the daily 0900 Cruisers Net and possibly streaming it live in the future if we can find a net controler with enough bandwidth.

The net runs on VHF (marine) Channel 68... 156.425Mhz I have purchased a RTL-SDR.com kit and since my "spare" VHF antenna is bork, have been running the included antenna at the aproximately correct length to get a decent signal via GQRX running on a linux laptop (Navigatrix/Ubuntu) Succesfully recorded a partial session this morning to a .wav file using the option to record in GQRX then later converting the .wav to an .mp3 using Audacity for a smaller file size.

Not really a linux weenie (although not scared of bash scripts and the command line) and know just enough about SDR to get in trouble. Given tbe above set up (single freq, set broadcast time, duration 10 min to 1 hr depending on season) does anyone have any suggestions as to an optimal way to do this? Possibly a command line interface piped to a wav/mp3 conversion? Current setup would work as is, but a lot of goofing around to end up with a mp3 for the days net we can upload to the server.

TIA for any help, suggestions, and support

David
Onboard S/V Solitaire
Boot Key Harbor

Re: SDR Audio Mentor Needed

Posted: Sun May 08, 2016 5:23 am
by rtlsdrblog
I think the simplest solution would to use an Audio Recorder program with a scheduler like http://www.totalrecorder.com/MainFeatures.htm. Just set SDR# to continuously listen to your signal, and use the scheduler in total recorder to record to an MP3.

Re: SDR Audio Mentor Needed

Posted: Sun May 08, 2016 11:33 am
by CapCouillon
tks for the suggestions, however neither SDR# or the suggested "Recorder" program will run native on a linux box. SDR# will run using Mono, but like using Wine to run windows apps it eats processor time. Remember this system is being run on a small saillboat anchored in the mkiddle of the harbor. Solar panels will only go so far in powering the laptop. Will probably (initially anyway) be manually started and stopped as the freq is used by fishing boats and other local traffic outside the morning net operation.

Again, Gnu-Radio installed with most of the available command line options that will work with it. Just having a bit of an issue getting my head wrapped around a basic plan. GQRX works well, but don't need to be using all the CPU time to run the grafix and UI and its associated processor time. (old IBM t-42 aint too smart).

Command line interface to set freq/filters/demod(narrow fm)/offset/gain then output piped to a recorder (Audacity?) which will convert and save as mp3 (one tenth file size of .wav). Just not sure how to get there yet. Will keep investigating.

---EDIT---


Looks like the command line utility rtl_fm (under installed rtl-sdr package) will do what I need on the rcvr/demod side, now just have to find an appropriate piece to record resulting output as an MP3. The search continues... Will try rtl_fm piped to aplay when this mornings net comes up and post results later for general reference. Still looking for suggestions. Will post final setup in "projects" forum when (and if) we get there.

Tks again for the input
David
S/V Solitaire
Boot Key Fl

Re: SDR Audio Mentor Needed

Posted: Sun May 08, 2016 10:33 pm
by rtlsdrblog
Ah I misread your initial post, I thought not being a Linux weenie meant not wanting to use Linux.

If Linux and starting it manually then yes it should be fairly easy. You can pipe the output of rtl_fm into sox, which will encode the file into an MP3. Have a look at the Network Streaming heading on http://kmkeen.com/rtl-demod-guide/. That should give you an idea how to use Sox. Then instead of piping it to the network, you need to read the sox documentation on how to write it to an mp3 file instead.

To make it automatic you could probably put the command into a bash script that runs at the right time and automatically closes. A raspberry pi might be a better choice if running it on solar.

Re: SDR Audio Mentor Needed

Posted: Mon May 09, 2016 1:01 am
by CapCouillon
Thanks for the help and confirmation.... Spent the morning reading thru man rtl_fm and man sox as well as the rtl_fm overhaul docs... (Great minds think alike?) And will start fooling about with them in the morning.

The rasberry pi will mean learning some new hardware, but the power savings alone would be worth it. Leave on 24/7 with a cron job to start the rtl_fm/sox script. Unattended operation would be really nice, as I am usually, but not always in the harbor. Have to figure a way to shut the script down as the length of the net is entirely based on the days traffic. Can run an hour in the season when we have a few hundred boats in the harbor, down to the record length of 57 seconds last summer. No one breaking squelch for 45 sec or so would be a good trigger. Might be a bit of a kluge, but gotta be a way.

Lots of people heading back north for the summer have expressed an interest in seeing the net streamed or at least archived (eventually both) and looking forward to this project. Something to keep the "Keys Disease" away for me. Helps make sure I have not killed off all my braincells with the sun and the rum. If we can set this up with a pi, and a small enough physical footprint, might even get the City Marina to let us put the antenna on their tower, power from the mains, and a fat bandwidh internet connection. Headless with remote admin. they would never know its there.

New antenna arrives Thur and in the meantime will use local NPR station to start playing with rtl_fm and sox. Thanks again for your suggestions, and support. Let me know if you think this would be appropriate for your "Projects" sub-forum, and I will start a thread there with history, hardware, software, triumphs and failures. Might be helpful?

Tks Again
David

Re: SDR Audio Mentor Needed

Posted: Mon May 09, 2016 10:08 am
by rtlsdrblog
Definitely start a thread in the projects section if your willing. Always good to have some experiences shared for future readers.