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Old 04-14-2014, 9:38 AM
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Default Digital Tones

Hi,

Wasn't sure where to post this, but I have an issue with our Motorola P-25 digital system that recently went on the air and I wanted to see if anyone out there has any suggestions. Our service folks say digital tones are not that great compared to analog, and they cannot come up with any solutions. So, I will ask here.

Issue- We transmit one mono tone for a few seconds from the dispatch console over the active T/G to alert fire companys of an incoming alarm. Frankly, the tone sounds terrible. It has been dubbed "we are getting a fax!!" because of the way it sounds. Very shrill and uneven in its audio quailty.

We did this same operation when we operataed in analog and the tone was much cleaner and even.

Radio service says there is no adjustment that can be made to improve the sound quality of the tone on the receiving end. With that said, I have listened to other agencys using the same P-25 equipment and their tones used to sound like ours, but now they do not. So, they had to have adjusted or changed something...

I listen to MetroSafe in Lousville, Ky and MECCA in Marion Co. both of which use Motorola P-25 digital systems and their alert tones are much cleaner and even than ours.

Anyone have any suggestions on what we can try?

Thanks very much.
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Old 04-14-2014, 10:16 AM
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My former agency implemented a "quasi" P25 "wide pulse" system using the IMBE vocoder. Alert and marker tones were used traditionally, and these did come through, but sounded "wobbly" for the lack of a better word. Some of the product engineers referred to it as "gollywobbles." I have not listened to the same with an AMBE +2 vocoder to see if it were the same or different. We also had siren noise sound weird and wobbly. It took some getting used to.

A lot of this comes from how the vocoder breaks up the audio it hears, references it to a table of sounds that resemble it, selects that digital code, and sends it. On the other end, it's referenced against the table again and then reconstructed into audio. That table is optimized for speech, not for tones -- although that may have changed with AMBE +2 vocoder.

I guess it might be like a Star Trek transporter that's tuned to certain objects, but doesn't reconstruct other objects as well. In this case, a vocoder is designed to carry speech. I think the updated vocoder has considerations for tones, too

So, I don't know for sure, but the systems may be using different versions. They should be retrocompatible, but one is more up-to-date and noise resistant than the other.

Just my theory.
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Old 04-14-2014, 1:24 PM
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Thanks very much 902. Just an ole dumb fireman here when it comes to this stuff... is this something I should ask our radio service about? The AMBE vocoder and different versions?
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Old 04-14-2014, 3:30 PM
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Originally Posted by pumpercaptain View Post
Thanks very much 902. Just an ole dumb fireman here when it comes to this stuff... is this something I should ask our radio service about? The AMBE vocoder and different versions?
NO SUCH THING as an ole dumb fireman, Cap! That's how I started in public safety radio - because the ones I had to take into the projects on EMS runs didn't work. I started the same way, asking questions. (But I'm a lot like Lou Costello's character - I went to school stupid and came out the same way, LOL.)

Yes, you should ask about what vocoder your current equipment uses, and ask your radio vendor and sales rep about the AMBE +2 vocoder - especially if you are preparing a bid spec or about to procure equipment. The reason you should is because of the noise on the fireground. It's been well researched and documented by the IAFC and IAFF, as well as other professional organizations, NIST, and industry. The P25 people went back to the drawing board - this time with some input from stakeholders - and recommended a different vocoder that handled noisy environments better.

Some manufacturers also built in additional digital signal processing or noise reduction to cut down on noise and things like PASS alarm noise so you can hear a mayday if it were sent on a channel that operates in digital mode.

Be safe!
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Old 04-14-2014, 4:29 PM
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Some tones will not pass thru P25 without obliteration....

VOCODER problem. IIRC some Alert tones had to be changed, frequency, to pass thru.
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Old 04-15-2014, 6:57 AM
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Are they all using the same consoles or vaious versions ?
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Old 04-15-2014, 7:44 AM
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Originally Posted by kb2ztx View Post
Are they all using the same consoles or various versions ?
It's not so much the console but the radio equipment controlled by the console that matters in this case. It's how the audio generated by the console is handled by the radio that has changed so much.

As mentioned above, some alert tones pass through digital radio systems better than others, so if a tone change is in order, that is almost always done in the console. But then most modern consoles can be programmed to do whatever is needed.
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Old 04-15-2014, 8:32 AM
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Just be aware that the vocoder that provides audio to the dispatchers can lie in several different places depending on system configuration.

In a Motorola system, it's in the DIU (digital interface unit). A repeater system might not demodulate the data, just pass it to this DIU, either locally through a cable or through a network (or dropped down to a DS0 TDM slot in a multiplexer through a modem). Or, the repeater can go back to a simulcast controller and voter, THEN to the DIU.

In some other configurations, the vocoder is in an over-the-air control station. My former system had both DIUs to the consoles at dispatch (installed in the CentraCom 2 Gold Elite console rack, not at the comparator site), and control stations that received off the air, not through the dispatch console system at the EOC.

The vocoder in a scanner does not hear what the dispatcher hears, regardless. Most times that's a good thing.
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