Are you actually in the primary service area of the system you're having difficulty with? Reliable demodulation of LSM requires attention to details such as local oscillator phase noise. Frankly, I have yet to see a consumer grade scanner with a reasonably clean synthesizer that's actually up to the task. In a professional grade receiver, a great deal of attention is given to LO purity and the demodulator sections. If the same attention was given to scanners, the complaint wouldn't be that they don't work well, the complaint would be that they're unaffordable by hobbyists. Engineering is always a compromise - you can get it done right, you can get it done cheap, and you can get it done fast. Pick two.
So what is the technical issue with decoding simulcast?
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Are you actually in the primary service area of the system you're having difficulty with? Reliable demodulation of LSM requires attention to details such as local oscillator phase noise. Frankly, I have yet to see a consumer grade scanner with a reasonably clean synthesizer that's actually up to the task. In a professional grade receiver, a great deal of attention is given to LO purity and the demodulator sections. If the same attention was given to scanners, the complaint wouldn't be that they don't work well, the complaint would be that they're unaffordable by hobbyists. Engineering is always a compromise - you can get it done right, you can get it done cheap, and you can get it done fast. Pick two.
kayn1n32008
ØÆSØ Say it, say 'ENCRYPTION'
I don’t have answers to those questions. I’m not uniden or whistler.
The SDS and Unication use I&Q SDR receivers to properly decode the various wave forms used in modern simulcast systems.
What I can tell you, is that these wave forms are used to allow a better delay spread, or in laymen’s terms, allows better symbol recovery when 2 or more signals are received slightly out of phase(different Time of Arrival).
Difference of time of arrival is caused by being at unequal distances from 2 or more simulcast sites.
You can ask Whistler they gave up after almost a year after they were not having any luck building a scanner like the Uniden SDSxxx series.
a29zuk
Member
Been battling simulcast issues, and complaining about it here, since 2006 with a 396T. The 325P2 performs better but it is not stellar.
Jim
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a29zuk
Member
The southern third of the lower peninsula of Michigan is simulcast or moving in that direction. This is where most of the population lives.
When vacationing in the northern lower peninsula and upper peninsula there are no simulcast systems. I wouldn't be surprised if Traverse City, Escanaba, or Marquette changed over to simulcast in the near future, though.
Jim
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My area got a Phase 1 simulcast system 8 years ago and people were complaining on RadioReference about the inability to get a clear decode back then. Since then, several other suburban and rural jurisdictions in the region have acquired digital simulcast systems, sometimes joining with other jurisdictions to split the cost. Simulcast issues aren't something that has sneaked up on the scanner manufacturers within the last three years or so.
No, but the issues involved in economically demodulating it correctly still stand.
Who wants to pay $1000 for a scanner, raise your hand.
Some are paying thousands for commercial radios
Yes they are. And you don't usually hear those guys complaining about the price.
So what is the similarity with multipath and QPSK? will a QPSK decoder eliminate multipath also? I am having trouble decoding when driving so I don't know if it's because of movement or just my scanner.
QPSK (quadrature phase shift keying) is the type of modulation, transmitting 2 bits at a time. Multipath distortion and simulcast both make it harder to decode QPSK, because of multiple overlapping signals. I/Q receivers are able to distinguish between the multiple overlapping signals, and decode from the clearest one. All other receiver designs choke, and decode with high error rates.
I like to think of the simulcast problem this way (I may be off on the finer details here, but I think the gist is pretty close):
Digital transmissions are essentially a series of 1's and 0's, expressed in the radio signal as on/off spikes. To provide enough data to represent analog sound, the number of digits per second flying through the air has to be extremely high. When these rapid-fire digits are transmitted from a single location the older radios receive these 1's and 0's in succession and are able to make sense of them, simultaneously transforming the data stream into analog audio. When the stream of 1's and 0's comes from two or more different locations, the digits won't arrive at the radio at precisely the same time (because they have travelled different distances). So, they 'intermingle'. So instead of 1-0-0-0-1-1-0, the sequence may appear to be 1-1-0-0-0-0-0. The older radios just weren't capable of separating the streams and deciding which to use. The newer ones, like the SDS and Unication, can.
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Digital transmissions are essentially a series of 1's and 0's, expressed in the radio signal as on/off spikes. To provide enough data to represent analog sound, the number of digits per second flying through the air has to be extremely high. When these rapid-fire digits are transmitted from a single location the older radios receive these 1's and 0's in succession and are able to make sense of them, simultaneously transforming the data stream into analog audio. When the stream of 1's and 0's comes from two or more different locations, the digits won't arrive at the radio at precisely the same time (because they have travelled different distances). So, they 'intermingle'. So instead of 1-0-0-0-1-1-0, the sequence may appear to be 1-1-0-0-0-0-0. The older radios just weren't capable of separating the streams and deciding which to use. The newer ones, like the SDS and Unication, can.
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kayn1n32008
ØÆSØ Say it, say 'ENCRYPTION'
Not entirely. While the difference in time of arrival is part of the problem, the bigger issue is the modulation type used. ‘LSM’ modulation is used because it allows for a greater time of arrival difference, while allowing the radio to properly decode. This is called the ‘delay spread’. 4FSK has a very low tolerance to differences in time of arrival, a very narrow delay spread, where ‘LSM’ and the various types of PSK used for simulcast have a wider delay spread, or are tolerant of greater differences in time of arrival. These modulations, coupled with arriving slightly out of phase are not properly recived with a simple discriminator tap radio(scanners built before the SDS series)
The reason I was asking was Simulcast and Multipath sound the same on my radio; garbled garbage. Back before I found out about LSM and the fact that they were transmitting it on purpose I (like thousdands of others) believed my scanner was capable and it was just not properly decoding the multipath. Then I found out the manufacturers were producing units incapable of decoding and selling them anyway.
They are different causes for the same effect--multiple copies of the same signal on the same frequency, with some time interval between them. An I/Q receiver can separate out the multiple copies of the signal with fancy math, and get a clean decode from the best one, when older receiver designs choke and spew out garbage.
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