Synchronizing FM over IP?

FM does it with frequency!
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AnabolicHippo
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Synchronizing FM over IP?

Post by AnabolicHippo » Wed Sep 05, 2018 11:21 am

Good Morning,

We are putting in a high power site and it turns out that the coverage is going to overlap with two LPFM sites that we have. I'm under the impression, that this is going to create a situation where we need to find a way to either a) synchronize the three sites, so that there is no interference between sites or b) decide to run a multi-frequency network.

I spoke with someone at Nautel about a year ago and they had proposed a GPS solution to be tied into the high power transmitter, but was told that it would not be necessary; this is now the exact issue that we are dealing with. So, here we sit and I'm balancing between trying to think up how to sync up three sites, which are fed through a VPN and Barix boxes, or accept that we are going to have to communicate to our listeners the different frequencies. I was doing some preliminary research and came across this:

https://www.bswusa.com/Codecs-GatesAir- ... 12338.aspx

and

http://www.gatesair.com/documents/slide ... rocast.pdf

I don't really know how to balance all of this, we are looking at $7k usd ($10 billion cdn) or to create the potential for listener confusion by running multiple frequencies. If anybody has any thoughts, I'd be interested to hear them.

DaveSt
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Re: Synchronizing FM over IP?

Post by DaveSt » Wed Sep 05, 2018 1:21 pm

The synchronisation you need is very high, so you really would have to use something like GPS. The modulation needs to be identical at all sites and that is far from easy. An Italian manufacturer published some research on this quite recently. You also have to worry about the delay between the different transmissions. The easy way out is to use different frequencies. Listeners have coped with that pretty well everywhere else.

Lee_Wheeler
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Re: Synchronizing FM over IP?

Post by Lee_Wheeler » Wed Sep 05, 2018 2:54 pm

I have some experience with synchronous boosters and it is not easy. I don't know that it can ever be done reliably when the audio is fed via IP codecs since the transmission delay through the Barix codecs is variable depending on the network connection quality. No matter how robust the network is there will be bad packets.

Step 1 is to assure that the actual carrier frequencies are absolutely identical. In the modern world GPS is the best and most practical way to accomplish that task.

Step 2 is to assure that the audio is synchronized. No matter what you do there will be interference when you are filling in a signal with a co-channel station. Once the audio is stabilized you can to some extent move really objectionable interference from one place to another by introducing microsecond range delay to the stabilized composite audio. There is no free lunch though and it is a game where you sacrifice a less populated area for an improvement in a more populated area. In many cases it is an area as small as in intersection of two roads.

An alternate channel fill in translator is much easier to deal with and you never hurt the main signal in the process. I had a couple of situations where I had a main station and then the owners bought the nearest 1st adjacent stations and simulcast them all. In that case syncing up the audio helped all of the stations. The distant stations were fed audio via a KU band VSAT network and I introduced a delay via a pair of Yamaha digital delays at the mother station to equal the sampling time and transmission delay to get the audio to the "affiliate" stations.

...Lee

TPT
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Re: Synchronizing FM over IP?

Post by TPT » Wed Sep 05, 2018 3:52 pm

We experimented feeding a booster with a pair of Comrex Bric Links. First, in order to synchronize the audio, you would need to find some way to delay the main transmitter audio to match the transmission delay to the relay. We were finding a variable delay time which then adds to the complexity.

Had terrain allowed enough isolation between the main station and the booster coverage area, it might have worked--inside the town the booster swamped the main signal so the delay was not obvious. However, there was too much of an overlap area along a main road where the booster and main transmitter fought for dominance.

Unless you can feed the relay transmitters by STL or off air to a long cable (using terrain isolation) I would go with separate frequencies.

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Deep Thought
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Re: Synchronizing FM over IP?

Post by Deep Thought » Wed Sep 05, 2018 8:38 pm

About the only viable way to do this is to sync the RF via GPS and deliver the audio via satellite or private ethernet/fiber so each path is essentially identical and consistent. You will never be able to keep the audio stable over the Internet. A few tens of milliseconds shift in the path delay will destroy the audio in the interference zone.
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DaveSt
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Re: Synchronizing FM over IP?

Post by DaveSt » Wed Sep 05, 2018 9:00 pm

The delay needs to be kept within microseconds. The article I mentioned earlier is available here: https://www.waveart.it/leggi-news?id=51. It is pretty good as they have understood what is needed well and have verified it with tests. To keep the modulation identical, they also say that the composite signal must be digitised and distributed rather than distributing the L/R audio. This is also correct. The best advice for a SFN is that you should only ever do it if there are no other options.

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Re: Synchronizing FM over IP?

Post by Deep Thought » Wed Sep 05, 2018 9:51 pm

DaveSt wrote:
Wed Sep 05, 2018 9:00 pm
The best advice for a SFN is that you should only ever do it if there are no other options.
Indeed. I think I would opt for moving the LPs instead, or if the coverage does indeed overlap, just turn in the licenses.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

PA_TUNE
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Re: Synchronizing FM over IP?

Post by PA_TUNE » Thu Sep 06, 2018 11:40 am

I've done a fair amount of work with synchronous boosters, and the only real successful solution I've found is the Intraplex Synchrocast system. Not cheap, but it works.

http://www.gatesair.com/documents/produ ... ocast3.pdf

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kkiddkkidd
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Re: Synchronizing FM over IP?

Post by kkiddkkidd » Fri Sep 07, 2018 4:56 pm

And I too made the mistake of suggesting a synchronous booster to a client that was desperately seeking an answer to his self-inflicted coverage problems.

It solved a bunch of problems but created 2x more. It was a disaster.

With both frequencies synced identically via GPS on identical gear with carefully synced audio... The interference zone was unusable and larger than anticipated.

Use different frequencies, publicize those freqs creatively and the listeners will adapt IF they want to listen to your programming.

IF you have a huge amount of terrain shielding you might make it work but is an exercise in futility IMOO...

Regards,
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Re: Synchronizing FM over IP?

Post by dbuckley » Sun Sep 09, 2018 2:01 pm

If you do have translators on multiple channels, then make sure you list all the frequencies in the AF section of the RDS data, and then radios will automagically select the strongest signal.

We are about to experience the fun that two non-syncronised translators on the same frequency with some overlapping coverage area entails, so it'll be interesting to see how it works out.

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Re: Synchronizing FM over IP?

Post by RodeoJack » Sun Sep 09, 2018 8:35 pm

In our area, this has just been turned on:

https://www.rbr.com/a-bustos-media-fm-b ... ting-plan/

One of these boosters is on a tower I share with another station and a wifi service. It includes a Gatesair transmitter, one of those Intraplex codecs and an IP product they're getting from Comcast. It's said to include all of the synchronization bells and whistles that are all mentioned in this thread. Yes, the transmitter is GPS synched. It's possible the Intraplex or Comcast box also is.

The antenna design has something to do with this as well. So does the transmitter location and antenna direction. Apparently, which way the signal is directed, relative to the incoming main signal, is a factor they work in to the directional antenna & power mix. At my tower, there are 4 log-periodics, Arranged in a spaced vertical-horizontal configuration. I know next to nothing about that array, but it seems clear that a lot of thought went into what was put up the tower. If you're interested enough to look the patterns up in CDBS, you can see these things target very specific areas.

In practice, I think the network has helped the situation a lot. I haven't listened enough to decide to what extent it's "seamless", or whether (in my opinion) the net result might be worth the cost of 6 boosters and all the tech involved in keeping them synched. However, the areas they're talking about are real problems for the 3 rimshot stations that are looking at this, and this solution does seem to be an improvement.

I do not think a solution involving a Barix box or a Bric-Link would work, merely because there's no way to accomplish micro-second synching when you have a codec that uses unreferenced variable buffering to keep a constant stream going over the public internet. As is noted here; lose a packet and all your work is down the chute.

This may have something to do with why a Barix box costs a couple hundred bucks, VS an Intraplex that gets $2,700+ (no comment re: the source).
Last edited by RodeoJack on Tue Sep 11, 2018 1:36 am, edited 1 time in total.

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NECRAT
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Re: Synchronizing FM over IP?

Post by NECRAT » Mon Sep 10, 2018 4:14 pm

RodeoJack wrote:
Sun Sep 09, 2018 8:35 pm
In our area, this has just been turned on:

https://www.rbr.com/a-bustos-media-fm-b ... ting-plan/

In practice, I think the network has helped the situation a lot. I haven't listened enough to decide to what extent it's "seamless", or whether (in my opinion) the net result might be worth the cost of 6 boosters and all the tech involved in keeping them synched. However, the areas they're talking about are real problems for the 3 rimshot stations that are looking at this, and this solution does seem to be an improvement.
They're using this system in Boston at WXRV (which I do listen to on a normal basis) and it works very well. We actually did a "ride along" test with Hal and a bunch of engineers, where we drove a route in one direction with it on, and the other with it off. And there was a noticeable difference. For a little while after the test, early this year, the system was off, but it's all back on. How do I know if it's off or on? On the WXRV system, the boosters have unique static RDS identifying them. (i.e. The Hancock Tower is "WXRV-H"). WXRV has 5. Where you really notice the difference is in areas where the Back Bay Boston FMs (The 7 from the Prudential Tower, all around 21kW per station) cause major overload on the radios. WXRV's downtown and Charlestown boosters really help them get through the "noise", even over the FM only 400kHZ away on the PRU. With the exception of one spot, I can honestly say, it's never been a bad transition when not listening for it, certainly not enough to be noticeable. (Where you do notice this, and as an engineer, is the audio is ever so slightly different, so you can tell the audio change).

WXLO, the rimshot from Worcester has a bunch of CPs to put in 4 of their own.
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