Anyone with in-depth knowledge of PRE-DIGITAL LAPD Radio????

[d119]

Just a shot in the dark out of curiosity's sake, but I'm interested in the PRE-DIGITAL-CUTOVER infrastructure of the LAPD.

If you listen to old recordings of LAPD in the late 90's/early 00's, back when they were carrying Astro Sabers that ran MODAT, you hear a very unique squelch noise when the unit unkeys. It's obviously the sound of a voter, but it doesn't sound like a SpectraTAC or DigiTAC or any AstroTAC that I'm familiar with.

Instead of smooth clicking as the notch filters for channel status tone come online when a unit unkeys, you kinda hear this combination of a squelch burst and a voter "cheep" when the unit unkeys.

Audio samples of what I'm talking about are here: http://www.mediafire.com/?sharekey=4d55 ... 78c4de0fc1

This is obviously before they made the switch to ASTRO operation, but it was still while they were using ASTRO Sabers in analog mode.

Anyone know what the infrastructure equipment that handled the voting was, and what the configuration was? I admit, I like the sound, and want to see if I can set up my equipment to do the same thing (I run a DIGITAC).

Is this an early-generation AstroTAC? I know today's AstroTAC's don't do this, they don't make any voting sounds, they're transparent in analog and digital.

[Will]

Status tone and wire line keying tone leaking thru into TX.


Spectra Tac voting receivers...

[d119]

I would expect SpectraTAC's to have cleaner squelch tails than that, considering they are MICOR based. What was the comparator?

Chris

[Jason]

SpectraTAC comparitors also, that is all that was available in those days.

I'd say the distinctive chirp is a combination of the status tones from the comparitor keying + analog microwave link.

[d119]

Analog microwave eh... Tell me more... Why would that make a difference?

What I'm curious about is when LAPD made the digital switch from analog, was it simply subscriber and infrastructure reprogramming, or did they overnight flip the old analog infrastructure off, and bring the new digital infrastructure online?

IOW, before the switch to digital, were they running on Quantars and Astrotacs in analog mode, or what?

[Will]

LAPD went P25 in June 18, 2001.

[d119]

After looping a sample and analyzing it, it's roughly 1950Hz. Was this an SP configuration???

[Q2222A]

Chris,

The LAPD voters used just before cut to ASTRO were a mixture of GE Voters and SpectraTaccomparators which can both use 1950 Hz Status tone. During the cut from analog to ASTRO a Raven switch was used to allow Analog audio to be patched to ASTRO and to the City dispatch consoles. It also allowed ASTRO audio to be patched to Analog channels and to the Console. It also allow the Console to have PTT/Audio priority to Analog and ASTRO simultaneously. There was a decoder box that was connected to the analog audio source and to the DIU that allowed the dispatcher and CAD to see unit IDs from bith ASTRO and MODAT signals. The keying of the Analog system was via E&M leads through the microwave for the Analog and a PTT signal to the DIU for ASTRO.

After all analog radios were changed over to ASTRO, the Raven switches were removed. The new ASTRO conventional system operates via digital microwave and V.24 paths on SRU cards in TeNSr channel banks.

When the new dispatch centers were built, the DIU's were moved from City Hall East to Mt. Lee. The majority but not all of the ASTROTAC 3000 comparators reside at Mt. Lee.

Hope that helps,

Q2222A

[d119]

Sure does. Sounds like that sound is coming from a GE voter most likely. What kind of receivers were they using on the analog side, Q2222A? Sounds like you've got pretty in-depth knowledge of the system, I appreciate your response.

[GEMOTO]

Wow, interesting someone brought this up, I have long been facinated by the little clicks the different comparators make. I have noticed that in some systems the sound is more pronounced than others.

I have a system that consists of AstroTAC receivers, a DigiTAC Comparator and an MSF5000 transmitter. Before the clicks were not too noticible but now since I have replaced the transmitter with a Quantro, the clicks are much more pronounced. I kind of like the noise as I'm a bit of an audiophile and am interesting in that sort of thing.

My question: How would one enhance the these particular sounds or in the case of someone who would rather not have them, decrease them?

I noticed on the old SpectraTAC comparators, if you slid the Notch filter switch on the command module to the right, the click sould be a bit more noticible.

[d119]

Well the "NOTCH OUT" removes the notch filter from the comparator, thus you hear a fraction of the status tone, or all of it if the "MONITOR" switch is on.

Quantar/Quantro stations do their notching in DSP, so thats why the notches are more pronounced than in MSF 5000 stations. I currently run a bunch of SECURENET SpectraTAC receivers into a DIGITAC connected to an MSF 5000 on one of my amateur stations, so I'm familiar with what that sounds like.

At work I've got an MSF 5000 with a GE Receiver Voter connected to it. I'll have to play around with that to see if I can get the old LAPD sound out of it.

[d119]

I can't figure it out - they must've had some sort of highly SP products in that system.