I don't understand the entirety of LMR 2way or P25

[scanmanmi]

My cell phone sounds like someone is sitting right next to me, plays music, movies, opens Autocad drawings, sends files on email, has a GPS, web browser, calculator, compass, stopwatch, I can trade stocks, make movies etc... Now when I listen to my scanner it's choppy, can't understand half of what's said, mobiles go digital, dispatch asks to repeat, coverage is a constant issue, on a system running something like 9600 baud. If somebody said he had a 9600 baud modem I would laugh at him. So I believe these systems only exist becasue of the manufacturers who sell them. Why in the world do they keep polishing this turd?

 

[buddrousa]

Sounds like you need to build a radio system and sell it.
Just remember the 1st Digital cell phones would do the same thing as todays radios and still do where I live.

 

[kayn1n32008]

My cell phone sounds like someone is sitting right next to me, plays music, movies, opens Autocad drawings, sends files on email, has a GPS, web browser, calculator, compass, stopwatch, I can trade stocks, make movies etc... Now when I listen to my scanner it's choppy, can't understand half of what's said, mobiles go digital, dispatch asks to repeat, coverage is a constant issue, on a system running something like 9600 baud. If somebody said he had a 9600 baud modem I would laugh at him. So I believe these systems only exist becasue of the manufacturers who sell them. Why in the world do they keep polishing this turd?

Your cell phone uses radio channels that are 5MHz wide, minimum, a LMR radio uses channels that are up to 12.5KHz wide. What do you expect? Unless LMR moves to data formats like LTE, in spectrum that has 5MHz or wider channels, it’s not going to change.

I’m not sure I understand why you think LMR digital would ever sound like a cell phone.

It’s like wondering why a lime does not taste like a watermelon. Or why a 3 cylinder Geo Metro does not haul *** like a Ferrari.


Sent from my iPhone using Tapatalk

 

[cmjonesinc]

If emergency services charged each user ~50 bucks a month just to use the tower, and had as many users as cell companies, we could probably have some pretty sweet 4g radios. Would you rather have a Motorola phone or Motorola radio in a burning building or when cell towers go down?

 

[RFI-EMI-GUY]

My cell phone sounds like someone is sitting right next to me, plays music, movies, opens Autocad drawings, sends files on email, has a GPS, web browser, calculator, compass, stopwatch, I can trade stocks, make movies etc... Now when I listen to my scanner it's choppy, can't understand half of what's said, mobiles go digital, dispatch asks to repeat, coverage is a constant issue, on a system running something like 9600 baud. If somebody said he had a 9600 baud modem I would laugh at him. So I believe these systems only exist becasue of the manufacturers who sell them. Why in the world do they keep polishing this turd?

Get a better scanner.

30 some years ago you could buy a conventional analog FM radio system that used wide band channels and would put the audio quality of your smartphone to shame.

A lot has changed with LMR since the 80's, some of it good, some if it bad. The worst of it, the narrowbanding of VHF and UHF resulting in severe performance degradation for analog FM. Most of these decisions were driven by corporate greed, though the engineers had good intentions.

As far as your scanner, it is an imperfect gauge of simulcast P25 systems. Though in my opinion the audio quality of wide band FM LMR is far superior to P25, even comparing the best P25 equipment.

Try using you smartphone in some rural areas. It won't work like someone is next door. My sister's iPhone consistently sounds like Crap and she calls from Cincinatti. My elderly Mothers Doctor called me on his phone regarding very critical medical related information, and I could barely make out what he was saying. He was driving on a major thouroughfare in Naples Florida. I hate to think he might phone in a prescription or procedure for my mom using that phone.

I listen to callers phoning in on a local radio show and the cellphone audio is so awful I switch the channel. Smartphones suck. They are not designed for telephone voice. Test messaging perhaps, but making phone calls no . They suck big time.

Truth is, your fancy pants smartphone and the planned nationwide FirstNet system will never replace LMR for mission critical communications.

Sent from my SM-T350 using Tapatalk

 

[CQ]

Mobile quality is still crappy, especially mobile to mobile. People just put up with it. Give me a landline any day.

 

[zz0468]

Blame Nyquist.

When the phone company first digitized telephone trunks, it took 64kb of data to faithfully reproduce the voice waveform. That was when Sprint advertised that "you can hear a pin drop".

That "pin drop" audio quality was based on Nyquist theorem requiring a sample rate of twice the highest frequency to be sampled. 8 khz sample rate of a 4 khz voice band, at 8 bits per sample. Add mu law encoding to artificially increase the dynamic range, and you have a very high quality voice grade channel for cheap. But 64 kb data modulating an RF carrier would be a few hundred khz wide.

LMR radios get either 12.5 or 6.25 khz per voice channel, so the upper frequency is more like 3 khz, and the resulting data gets heavily compressed. The resulting received waveform is a poor mathematical recreation of the original waveform that occupies a whole lot less spectrum than 64 kb would.

The algorithms used to squeeze the data have gotten quite good, but it's used to get more voice channels, not improve voice quality. Cellphones are adaptive, so a lightly loaded cell might have noticably better voice quality. Digital LMR is fixed and always sounds bad. A high bit error rate will make it sound worse.

We're approaching the theoretical limits of how much you can squeeze the "information rate" on a digital voice circuit. The mathematical algorithms sometimes leave very interesting artifacts behind. The "information rate" available in the transmission path can get very low, but with a low BER. The syllabic rate of speech along with the frequency spectral content can exceed the "information rate" that's available, and the algorithm pieces together something that sounds like undistorted human speech, but you just can't quite make out what's being said. Merely talking slower can make it perfectly clear, because the information rate is again narrower than the pipe carrying it.

The problem used to be noise. Then it became bit error rate. Now, it's outpacing the compression algorithms. Early adaptive PCM multiplex circuits would put two voice channels in that 64kb data slot, and it sounded great! But a fax machine wouldn't work on it. That's an early manifestation of what cellphones do today.

 

[chief21]

...and how many cell sites do you have in your area versus how many LMR or public safety tower sites?? They are totally different animals with very different funding.

- John

 

[RFI-EMI-GUY]

Your cell phone uses radio channels that are 5MHz wide, minimum, a LMR radio uses channels that are up to 12.5KHz wide. What do you expect? Unless LMR moves to data formats like LTE, in spectrum that has 5MHz or wider channels, it’s not going to change.

I’m not sure I understand why you think LMR digital would ever sound like a cell phone.

It’s like wondering why a lime does not taste like a watermelon. Or why a 3 cylinder Geo Metro does not haul *** like a Ferrari.


Sent from my iPhone using Tapatalk

Simple math says you require 52 dB more signal to convey a 5 MHz bandwidth than 12.5 KHz. A lot is made up in CDMA processing gain, but still orders of magnitude more signal required for cellular than LMR. Then you have antennas to consider. The last smartphone I looked at had an antenna that was 15 dB worse than a dipole and buried inside a half metal structure.

Sent from my SM-T350 using Tapatalk

 

[Project25_MASTR]

My cell phone sounds like someone is sitting right next to me, plays music, movies, opens Autocad drawings, sends files on email, has a GPS, web browser, calculator, compass, stopwatch, I can trade stocks, make movies etc... Now when I listen to my scanner it's choppy, can't understand half of what's said, mobiles go digital, dispatch asks to repeat, coverage is a constant issue, on a system running something like 9600 baud. If somebody said he had a 9600 baud modem I would laugh at him. So I believe these systems only exist becasue of the manufacturers who sell them. Why in the world do they keep polishing this turd?

What you don't realize about cellular devices is the latency when it comes to voice. When looking at two phones utilizing the same tower, you are looking at a 2100 ms delay (voice encode to voice decode). That is simply unacceptable for public safety. Some thirty years ago, APCO set some standards for latency's and access times. This was known as Project 16. Today, part of the requirements of Project 25 are that radio to radio contact through a repeater cannot have a latency of more than 350 ms encode to decode and the latency through a linked interface cannot be more than 500 ms encode to decode. With software DSP, cellular just can't match that (hardware DSP drives the price of the handsets into the same price range as existing P25 subscribers).

As already suggested, the move from wideband (5 kHz deviation) to narrowband (2.5 kHz deviation) initially hobbled systems leading to up to 50% coverage losses. Going digital allowed agencies to make up for that loss in coverage (not fully at all times though) at the sacrifice of audio quality (but there is no noise to listen to).

If emergency services charged each user ~50 bucks a month just to use the tower, and had as many users as cell companies, we could probably have some pretty sweet 4g radios. Would you rather have a Motorola phone or Motorola radio in a burning building or when cell towers go down?

How do you think large scale P25 systems operate and generate revenue? They charge airtime fees (typically $20-$35 per radio per month).

Blame Nyquist.

...

But 64 kb data modulating an RF carrier would be a few hundred khz wide.

Not exactly true. 64 kbps is currently doable with a few devices in a 12.5 kHz channel (using 64QAM modulation). The problem is it is not very robust and requires a much higher SNR than a 9600 bps data stream in the same channel which would effectively reduce coverage in a mobile/portable environment by a factor of 3.

 

[RFI-EMI-GUY]

Simple math says you require 52 dB more signal to convey a 5 MHz bandwidth than 12.5 KHz. A lot is made up in CDMA processing gain, but still orders of magnitude more signal required for cellular than LMR. Then you have antennas to consider. The last smartphone I looked at had an antenna that was 15 dB worse than a dipole and buried inside a half metal structure.

Sent from my SM-T350 using Tapatalk

oops power ratio, 26 dB, still a lot of db's!

 

[zz0468]

Not exactly true. 64 kbps is currently doable with a few devices in a 12.5 kHz channel (using 64QAM modulation). The problem is it is not very robust and requires a much higher SNR than a 9600 bps data stream in the same channel which would effectively reduce coverage in a mobile/portable environment by a factor of 3.

*sigh*

I suppose I should have specified a mode suitable for mobile/portable operation. But as you said, 64QAM isn't (currently) suitable, but it does work very well in point to point microwave service with sophisticated adaptive and transversal equalizers.

But this will have to be the trend, increasingly complex modulation methods coupled with increasingly complex compression algorithms to get more bandwidth per hz of occupied spectrum.

Have any hope they'll improve voice quality while they're at it? I don't... That would reduce profits.

 

[RFI-EMI-GUY]

*sigh*

I suppose I should have specified a mode suitable for mobile/portable operation. But as you said, 64QAM isn't (currently) suitable, but it does work very well in point to point microwave service with sophisticated adaptive and transversal equalizers.

But this will have to be the trend, increasingly complex modulation methods coupled with increasingly complex compression algorithms to get more bandwidth per hz of occupied spectrum.

Have any hope they'll improve voice quality while they're at it? I don't... That would reduce profits.

I wonder if anyone has pondered that a premium could be charged for better vocoder and bandwidth?

Why should the rich and famous be subjected to the same voice quality limitations as we "Common Folk"?

No doubt Kanye West and Kim Kardashian both have a zircon encrusted gold iPhones, but do they have to suffer the indignity of muffled calls? What about George Clooney, must he always sound like Poindexter when he calls Brad Pitt?

 

[zz0468]

I wonder if anyone has pondered that a premium could be charged for better vocoder and bandwidth?

Probably, but I'd bet that the math doesn't work. It would have to be priced significantly higher to keep everyone from buying it, which would negate the point. Not enough would buy it to make development of the chipsets profitable.

The cellular companies sell higher priority service to government, but personally, I think it's a scam. I think the only things getting throttled are YouTube and Netflix. And the voice quality still blows goats.

Why should the rich and famous be subjected to the same voice quality limitations as we "Common Folk"?

Because they put their pants on one leg at a time like I do?

No doubt Kanye West and Kim Kardashian both have a zircon encrusted gold iPhones, but do they have to suffer the indignity of muffled calls? What about George Clooney, must he always sound like Poindexter when he calls Brad Pitt?

If there truly was a God, their voice quality would be worse.