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900 similcast repeater

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desperado
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900 similcast repeater

Postby desperado » Sat Sep 11, 2010 8:43 pm

I have seen several posts talking about LTR or Smartnet trunking on ham, but what about similcast 900?
The biggest issue I see with 900 is the coverage, or lack thereof for ham use.
VoIP has taken the radio world by storm for audio links.
There are a number of similcast system controllers that are either IP or T-1 linked (T-1 linking can be done over IP connections with proper hardware as well)

Granted there are hurdles, finding sites would be the first and finding the money to put it together would be second, but has anyone given this any consideration at all?
Keith
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Bill_G
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Re: 900 similcast repeater

Postby Bill_G » Sun Sep 12, 2010 6:12 pm

You need some method of synchronization, a rcvr voting system, and low latency, stable timing links between the master site and all the transmitter sites with disciplined oscillators at each site.

desperado
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Re: 900 similcast repeater

Postby desperado » Sun Sep 12, 2010 8:25 pm

Right, GPS time standards with 10 MHZ and 1PPS for syncing the carrier and PL if analog voice
and the data stream if digital. The math would need done for coverage footprint for each site and the delay needed so the signals would arrive in the overlaps
with the correct timing so there was minimized amounts of out of phase cancellation of the signal (probably worded wrong)

Actually I know what it would take. Doing a project right now on VHF, 5 sites, commercial stuff.
The point I am making is that 900 is under used, the only equipment for it is repurposed commercial gear that will accept the 10 Mhz reference oscillator input
from a GPS time standard (Quantars's and MTR 2000 repeaters) so it already lends its self to doing this.

The biggest issue would be a controller that would create the proper delay and be reasonably priced for an amateur radio install.
The controllers I am aware of for this sort of thing are from Harris and they cost more than a new Quantar.

Any ideas on a controller?
Keith
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MSS-Dave
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Re: 900 similcast repeater

Postby MSS-Dave » Mon Sep 13, 2010 8:32 am

I used Convex VDL units. They run about $700-$800 list price. I built a 4 site system on 900 using Mastr 2E stations, a JPS voter, Spectracom GPS Freq References and PL tone generators. One of the biggest issues is you have to use stable transport to the sites. radio or dedicated microwave is the choice. It worked really well but I had other goofy issues which I believe could be tied to the fact that 900 is narrowband. Now, I've not been deep into a standard analog or digital simulcast that was wideband so I really can't prove that statement. I ended up decommissioning that project as it wasn't feasable for what we actually needed in the long run but it was an eye-opener.

If you don't have this info already, here is a great source of most all things simulcast..

http://www.simulcastsolutions.com/

Dave

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Bill_G
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Re: 900 similcast repeater

Postby Bill_G » Mon Sep 13, 2010 9:04 am

The two most important considerations in simulcast are (in order) - deviation, and delay. Freq stability is not that important. Modern equipment is frequency stable enough to run without gps disciplining if you stay on top of your PM schedule, and align all oscillators on the same day with the same reference. I used to reference a local FM broadcaster. If you are within 100hz, you're golden.

OTOH, deviation has to be within 5hz or you will get a growl in the receive audio as the two sources beat against each other. PL is especially problematic and slight differences in PL deviation lead to massive audio degradation. if I had my druthers, simulcast would be sent without PL. You also have to equalize the audio curve of each transmitter. They may be right on at one kilohertz, but be way off someplace else in the 300 to 3000hz spectrum. If you can be within 5hz per octave, it will sound reasonable. With xmit PL turned off, it will sound super.

The modern equipment constantly recalculates the required delay per site so that the transmissions launch from every site at the same time. But, if you use fixed time distances - ie: highly stable links - then you can apply a fixed delay to each site, and be reasonably safe for quite a long time. That's how we did it before microprocessor controlled equipment came out. Finding analog delay boxes today is another matter.

If you can apply the correct amount of delay, and get the deviation equal, the overlap will disappear. You'll pick up some zub zub in a vehicle on VHF as you approach and pass 45mph. Nothing you can do about it. You'll experience a similar effect in UHF as you approach 70mph. It's a function of wavelength. With sufficient separation between sites and little overlap, you won't experience it. Specific locations with poor quality because of lucky paths to two or more sites can be addressed with gain antennas aimed at a primary site, and/or attenuation added to knock down the interfering site(s).

Simulcast is engineered chaos, and it does have a dink factor. Once you have an appreciation for the precision required in the alignment, it will work quite well even without some of the modern conveniences. It can be done on the cheap, but you may have to McGuiver some of the bits necessary.

SlimBob
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Re: 900 similcast repeater

Postby SlimBob » Mon Oct 25, 2010 2:33 am

Another thought is to move beyond the overlap zone. Then you'll only have "overlap" in cases of band openings. If there's at least 6dB or greater signal difference, the capture effect should rule.
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