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PostPosted: Wed Jan 22, 2014 3:16 am 

Joined: Sun Apr 01, 2012 2:27 am
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You may remember my post from some time ago about wanting to create a hybrid analog/digital system for FM, basically "HD Radio" based on open standards. Well, I am delighted to say that 1 month and a BladeRF later, I have done it. Using DREAM to modulate the digital signals, and Stereo Tool to generate the FM composite, You get something like this.

Image

Now, the filters need to be tuned, and I think the filter of a typical FM radio is 200khz correct? That's what the design is based on.

Anyway, what does this offer? Well, like HD Radio, it allows you to simulcast your analog station in superior digital quallty. For instance, my digital setup has 3 "channels" spread over the two digital sidebands. The leftmost features a signle program, a simulcast of the analog feed - without any processing of course - at 64kbps. The second sideband carries a 40kbps secondary music program, and a 23kbps news/talk feed.

A digital signal being received. (This is the left sideband)
Image

Those bandwidths seem low, but with the Opus audio codec, they are optimal. 64kbps sounds somewhere between CD and DVD audio, whist 40kbps sounds like a fantastic FM signal with perfect stereo separation and no processing. The 23 kbps is in mono and sounds like a 5khz AM signal with perfect reception. All of this and the analog signal is still there. Complete with the usual Stereo and RDS.

I have also worked on another method which places a signal 20khz digital signal inside the FM envelope where the SCA signals would be. (Kind of like FMPlus). Though, a receiver for that would be absurdly complicated.

Tests so far have been fantastic, and I will be releasing the flowgraph, and documentation on how to get it all working by this weekend or early next week.

Any questions or comments, feel free to send them my way. :p


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PostPosted: Wed Jan 22, 2014 7:21 pm 
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Cool!! Hint: enable Stokkemask clipping (assuming that you are using the composite clipper) - that will make the normal FM signal less wide with very little impact on the audio.


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PostPosted: Wed Jan 22, 2014 7:24 pm 
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How does the modulation of the IBOC signal work? Afaik you cannot send the total composite signal through a sound card right?


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PostPosted: Wed Jan 22, 2014 11:54 pm 

Joined: Sun Apr 01, 2012 2:27 am
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Quote:
How does the modulation of the IBOC signal work? Afaik you cannot send the total composite signal through a sound card right?
Haven't tested output through a 192khz soundcard so I can't say on that yet. The modulation is OFDM. The two signals are inserted with a center of 149khz placing their LSB at roughly 159khz, thus the total bandwidth is about 230khz. I will likely extend the guard band slightly so the IBOC center would then be 160khz (with 175khz on the LSB) Meaning ti would not be ~250khz total.

I don't think I have the license for the declipper or Natural Dynamics so I'll just stick with my 3-year old sts. :)

Still deciding whether Pulseaudio or Jack for transporting Stereo Tool and Dream's outputs to the graph, JACK is looking to be the winner right now.


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PostPosted: Thu Jan 23, 2014 1:19 am 
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3 year old?!?! NOOOOO!!!! Since then nearly EVERYTHING in Stereo Tool has been rewritten completely from scratch. The difference in sound quality is REALLY big - and the new version with composite clipper gives you a might cleaner signal with better reception under multipath conditions etc. - which will also much better protect the IBOC signal. Really, at least try out the new options - you'll get some spoken texts every few hours but the difference is huge.


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PostPosted: Thu Jan 23, 2014 4:46 am 

Joined: Sun Apr 01, 2012 2:27 am
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Quote:
3 year old?!?! NOOOOO!!!! Since then nearly EVERYTHING in Stereo Tool has been rewritten completely from scratch. The difference in sound quality is REALLY big - and the new version with composite clipper gives you a might cleaner signal with better reception under multipath conditions etc. - which will also much better protect the IBOC signal. Really, at least try out the new options - you'll get some spoken texts every few hours but the difference is huge.
Heh, I should be clear, it is 3 years old but I have tweaked it and it has moved through the newer versions, hell it even had Natural Dynamics when it was still in testing and sounded beautiful Still does. I'll for sure have to get the declipper though because anything that lessens the need to expand the IBOC guard band is a good one.


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PostPosted: Thu Jan 23, 2014 2:35 pm 
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Quote:
Quote:
3 year old?!?! NOOOOO!!!! Since then nearly EVERYTHING in Stereo Tool has been rewritten completely from scratch. The difference in sound quality is REALLY big - and the new version with composite clipper gives you a might cleaner signal with better reception under multipath conditions etc. - which will also much better protect the IBOC signal. Really, at least try out the new options - you'll get some spoken texts every few hours but the difference is huge.
Heh, I should be clear, it is 3 years old but I have tweaked it and it has moved through the newer versions, hell it even had Natural Dynamics when it was still in testing and sounded beautiful Still does. I'll for sure have to get the declipper though because anything that lessens the need to expand the IBOC guard band is a good one.
Not the declipper! The composite clipper!

Edit: I'm curious where this is going. PM me your license name, then I'll upgrade it.


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PostPosted: Thu Jan 23, 2014 8:49 pm 

Joined: Sun Apr 01, 2012 2:27 am
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Quote:
Quote:
Quote:
3 year old?!?! NOOOOO!!!! Since then nearly EVERYTHING in Stereo Tool has been rewritten completely from scratch. The difference in sound quality is REALLY big - and the new version with composite clipper gives you a might cleaner signal with better reception under multipath conditions etc. - which will also much better protect the IBOC signal. Really, at least try out the new options - you'll get some spoken texts every few hours but the difference is huge.
Heh, I should be clear, it is 3 years old but I have tweaked it and it has moved through the newer versions, hell it even had Natural Dynamics when it was still in testing and sounded beautiful Still does. I'll for sure have to get the declipper though because anything that lessens the need to expand the IBOC guard band is a good one.
Not the declipper! The composite clipper!

Edit: I'm curious where this is going. PM me your license name, then I'll upgrade it.
D'aaaw thank ya kindly. :D

Anywho, for anyone curious as to what the release will contain:
- GRC Flowgraphs for: BladeRF, USRP, and HackRF.
I will need some help testing the USRP version as I only have a BlaeRF.
- Documentation explaining usage guidelines and best practices.
- Sampler baseband WAVs to calibrate your SDR and test the system.
- A baseband WAV for use in SDR#, Dream, etc with a sample signal.

It will all be contained in a 7z archive and be hosted on both my Google Drive, as well as Sourceforge.

Again, more info as the release time approaches.


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PostPosted: Sat Feb 01, 2014 6:49 am 

Joined: Sun Apr 01, 2012 2:27 am
Posts: 36
Some updates. Finally managed to figure out why dumping a sample to WAVs wasn't working and finally got a look at an FMD signal in SDR#. This is what it looks like:

Image

The sample-rate is slightly off, hence why the digital signals are a bit "larger" than they actually are. However, the digital feeds decoded fine in Dream, and the FM decodes fine in SDR#. Problem?

That FM comes from my GNURadio FM modulator, and not ST, Why? Because ST doesn't output a an envelope, it outputs the signals prior to the add phase, (Mono, pilot, Stereo, and RDS) I have tried various schemes and have not been able to "add" them as separate signals, the only way I can see doing that, is splitting it with bandpass filters and remixing it, which is kind of pointless. Is there a way for Stereo Tool to output a nice envelope with the individual signals combined? I sure hope so.

Regardless, all that needs to be done now is to set up efficient filters and cut as many corners as I can to bring CPU usage down to practical levels. Right now it sits at ~80% utilization on 8 cores of my Intel i7 4770k. (4.0ghz) which is unacceptable.


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PostPosted: Sat Feb 01, 2014 2:42 pm 
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Can you yell me exactly what signal you need?


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