Stereo Tool

Feedback, questions, settings and more...
      Home         Download         Donate         Register         Forums         Help   
It is currently Fri Dec 15, 2017 1:35 am

All times are UTC+01:00




Post new topic  Reply to topic  [ 29 posts ]  Go to page Previous 1 2 3
Author Message
PostPosted: Wed Jan 04, 2017 6:49 pm 

Joined: Thu Mar 19, 2015 12:24 am
Posts: 74
Hmmm, not sure why you would introduce a third transmitter to keep the other 2 in sync.

In terms of a perfect overlap, there is no such thing.

Have you configured SFN in production before ?


Top
   
PostPosted: Wed Jan 04, 2017 7:43 pm 
Site Admin
User avatar

Joined: Mon Mar 17, 2008 1:40 am
Posts: 9024
There already is such a clock: GPS!


Top
   
PostPosted: Sun Jan 22, 2017 1:42 pm 

Joined: Mon Sep 23, 2013 11:17 am
Posts: 5
PLLs in transmitters all need a reference frequency to keep the transmitter on the correct frequency this is almost always a Crystal Oscillator or it can be an external reference of the same source if you're synchronizing two transmitters on the same frequency.

As an example the NRG PLL Pro4 uses a 6.4Mhz crystal which is divided down to 12.5khz. The RF is also sampled and divided down according to the frequency set on the 4059 divider IC, the result should also be 12.5khz for the RF and the two are fed into an ExOR gate for comparison and to generate a correction voltage by taking the average by using a loop filter, this voltage drives the varicaps which keep the carrier on the correct frequency.

To synchronize two transmitters, both carriers *must* have the same phase angle and offset or you'll get fm capture affect and or multipath distortion. This requires that the PLL reference on both transmitters also has the same phase angle/offset. GPS is one way to do it, however you can also use any other reliable reference that can be received at both transmitter locations. Local medium wave stations can be a reliable source for a reference, I've even heard of the 19khz fm pilot tone being used from analogue stl's using the tv bands.


Top
   
PostPosted: Tue Jan 24, 2017 8:03 pm 

Joined: Thu Mar 19, 2015 12:24 am
Posts: 74
That's a lot of drama to synchronize signals.... I've looked at another broadcaster's configuration in an alternate location and even with all the best tech they could buy, the cross-over point was still rubbish.

I was able to successfully configure a cross-over point between 2 towns in between 2 mountains where normal reception was going to be pretty average anyway, so we masked the sync point that way and adjusted power at the remote site until it sounded right.

Now whenever you drive between the two locations it's pretty good.... it will never be perfect but blended stereo sound between to big mountains is better than the awful sound we previously had. At least now you can listen.

Our competitor has worse coverage through the mountain because their primary signal is about 200w EIRP lower and their remote site is further away so they don't have a cross-over point, instead opting for different frequencies. Initially I thought that approach would be easier, but what we discovered was when you are driving through mountains on winding roads, it's best to keep your eyes on the road rather than finding the alternate frequency for a radio station (AF is a challenge because even after switching to the new frequency coverage is again affected by the next mountain).

Running SFN between the two locations works well. If the config was between 2 sites out in the open, I would probably not run a SFN as I think it would be too difficult to obtain a decent cross-over.

/My2c.

BeDazzler.


Top
   
PostPosted: Wed Jan 25, 2017 2:04 am 

Joined: Mon Sep 23, 2013 11:17 am
Posts: 5
It all comes down to having both carriers perfectly synchronized, and it can be done but is very difficult.

Each transmitter antenna must be phase coherent with the others also or you'll get offset resulting in multipath, each feed line to each antenna in the SFN must be the same length because through coaxial cable you've got a velocity factor to deal with, i.e. RF doesn't travel at the speed of light over the cable and you'll get delay on one if its longer than the other etc...


Top
   
PostPosted: Wed Jan 25, 2017 3:21 am 

Joined: Thu Mar 19, 2015 12:24 am
Posts: 74
Yep, it ultimately does work better with perfect synchronization, however in a real world scenario it highly unlikely that will be achieved.

The StereoTool SFN feature worked pretty well in our case and I wouldn't over-complicate a SFN setup as there will always be an element you can't control, so just do your best.


Top
   
PostPosted: Thu Jan 26, 2017 1:17 pm 
User avatar

Joined: Thu Jul 01, 2010 12:42 pm
Posts: 94
Location: Australia
Excellent thread, well done to all that contributed to the education of those needing filler SFN transmitters.

BeDazzler, it would be great if your able to provide a basic map of the transmission areas, with the goal of assisting those in design & implementation?


Top
   
PostPosted: Fri Mar 03, 2017 9:23 am 

Joined: Thu Aug 06, 2015 4:01 am
Posts: 27
Location: Iraklion
I am very happy that I contributed to finding a solution for that kind of problem. It's true that I searched for a long, long time for a solution. I first thought of audio delay and phase reversal back in 2003, when I came across the problem when we had to use the same frequency for 2 transmitters having a large overlapping area.

It's true that the exciters should be in perfect phase accordance in RF with the appropriate phase delay to have modulation wavefronts arrive with extreme sync precision at the target area.

It's also true that multipath distortion can ruin all efforts regardless the cost of the system, when energy reflected off the mountains reaches at the target area in levels greater than -20db of that of the carriers.

In general, since we remain on the analog domain there are no perfect remedies to the problem. Even if multipath is miraculously absent in the target area, the optimised stripe can have a maximum width of 7 km's and for a single spot, ie overlapping areas at other angles could have the interference effect worsen rather than optimised. In proper design, one would try to allocate those areas in rural areas, like mountaintops or forests.

The SFN implementation in Stereo Tool is the cheapest way to achieve SFN modulation correction, but the proper adjust of modulation indicies in analog STL's and analog input transmitters is mandatory and should not be overlooked.

Best practice includes

>Stereo tool instance controlled remotely from target area
>finding a spot in the target area where audio is muffled mainly by main transmitter interference rather by multipath (I always do it in my car, I move around till I can see both transmitters, then I am sure that the signal I gather belongs to main front and not to reflections off hills around). I turn one of the two modulations completely silent and scan around for a location that has both signals at the same level, so I can get audio muffled by the unmodulated carrier.

> Enable the 2nd modulation and set proper delay as calculated by the distances (see youtube video descriptions)

Opt for "invert" if I have STL's and exciters that are of different models and observe the effect in audio. There is a concept of positive modulation (positive modulation voltages cause increase in carrier frequency) and negative modulation (positive modulation voltages cause reduction in carrier frequency). That is not standardised by broadcast standards, so each manufacturer could have either positive or negative modulation even on different models.

Listen for "chirps" that are acceptable in non-DDS exciters -

( they are due to differences in carrier frequency that are in the order of 3 ΚΗz or less. Can be mended a bit by aligning both exciter's pll at exactly the center frequency. most pll's have a stability of less than 500Hz as per broadcast standards, so if the temperature is constant at the transmitter site if you align pll crystal reference once that should remain w. less than 200 hz/year. you'll definately need a good frequency counter if you opt for the procedure, preferably an oven-stabilised one w. 1Hz precision at 200MHz.
I haven't performed that procedure yet on the RS of the video. )


If one can get the "chirps", then the right phase btw. 0 and 180 degrees is selected, (invert on or off accordingly)

Then enabling a test tone of ~440HZ and adjusting both modulation indicies and trim delay +-2 sample around the sample delay value calculated (To cope for propagation delay at the transmitter site and all that transistor phase reversing inside the transmitters), to have the tone as clear as possible while maintaining proper modulation levels in order to be legal.

Then double the frequency and repeat in more fine increments. And once again.

Revert to normal program and observe the effect with and without SFN correction.

Note the levels for both transmitters down to paper, disable modulation on one transmitter and seek for a spot where the other can be cleary heard. (The unmodulated carrier of the interference is below 20 db of the useful signal) Measure modulation index for that transmitter. Then reverse silence on the transmitters, seek for the other transmitter to be heard clear and measure the other transmitter modulation, they must match and be at legal maximum. If not, adjust them as per the noted value at 1st SFN alignment by adding or removing fraction's of dB's as calculated.

Then for a final time, Return to that spot you found first that has the worst signal overlap, and do some check with and without SFN corrections (enabling and disabling the feature. It should have a big improvement on sound at that spot.


When I'm done with all these, I perform a celebrational victory lap around the overlap area to observe and enjoy the effects of rectification and to congratulate me of some good work done, and maybe capture a video to show the results and upload it to youtube for my fellow stereo tool users to check! :P


Last edited by makrisj on Sat Mar 04, 2017 1:06 am, edited 1 time in total.

Top
   
PostPosted: Fri Mar 03, 2017 9:46 am 

Joined: Thu Aug 06, 2015 4:01 am
Posts: 27
Location: Iraklion
Quote:
It all comes down to having both carriers perfectly synchronized, and it can be done but is very difficult.

Each transmitter antenna must be phase coherent with the others also or you'll get offset resulting in multipath, each feed line to each antenna in the SFN must be the same length because through coaxial cable you've got a velocity factor to deal with, i.e. RF doesn't travel at the speed of light over the cable and you'll get delay on one if its longer than the other etc...

Well, don't worry that much about that. Those details regarding the antennae regarding delay (solutions of wave interference) refer to a single point in space at a given distance from both transmitters. While that would indeed be mandatory in adaptive optics scenarios as in passive radars, laser experiments and such, it has no actual meaning on a *moving* observer. That observer would indeed observe signal levels rising and falling every 3 meters @ 10^8 Hz, but destructive interference down to zero in "real" environments almost never occur (maybe one of the few cases that natural law mathematical imperfections work on our side) and FM radios for cars work their discriminators really deep into amplitude saturation to cope with multipath anyway, so actual path delay at RF wave equations have negigible real world effect and can be omitted totally in FM radio SFN considerations, and the same applies for DVB-T SFN's but for slightly different reasons*.
This is due to the fact that "real" earth does not behave like free space - interference patterns won't occur in theoretically predicted valley depth due to nearby reflections at objects like buildings, trees or even sidebars of the highway at the order of several tenths of meters around the receiver.

*for those interested for a brief description of what dvb does over the air to achieve SFN, here are the important parts:

the same "compressed time" data packet (think of it like having 1 second of program packed in 0.25 sec real time, and then emiting that 0.25 second packet 4 times, the actual times measure in μSecs) is emitted by the transmitters for quite some time, thus widening the proper reception overlapping area and digital receivers seek for clues pre-injected inside the signal by the transmitter to clear it constructively (gap interval - after a packet, wait a bit for the receiver to gather all that there is in the air and then start sending the next packet) , so multipath actually helps there rather than doing damage.


Top
   
Display posts from previous:  Sort by  
Post new topic  Reply to topic  [ 29 posts ]  Go to page Previous 1 2 3

All times are UTC+01:00


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Limited