Wanted: Solution to buffer (delay) a VHF transmission

[bv156]

Here's the current situation:

We are a city with an 800MHz Astro 25 system. Being digital, it has a delay between when you talk into the radio and when someone hears it on the other end. This delay is a little less than one second.

We are in a county that uses a county-wide VHF fire channel to alert Minitor pagers with quickcall tones.

When our dispatchers page our fire department, they simulcast the broadcast over the VHF channel to alert the pagers, as well as on our trunked system to our own users.

The problem: When firefighters are sitting in their firetruck, with their minitor pagers turned up, and the vehicle's 800MHz radio turned up, the dispatch is nearly inaudible because of the echo effect... they hear the same thing twice, offset by about a second.

Other dispatch centers that monitor both the county VHF frequency and our fire talkgroup are equally overwhelmed by the echo.

The solution I'm looking for:

I want something that will buffer or delay our dispatcher's transmission over the VHF frequency, something that we can adjust to the same # of milliseconds so that, as the end result, the VHF is delayed the same amount as the digital 800, eliminating the echo for end-users during a simulcast.

Suggestions? Does anyone else do this?

The technical info: Our dispatch centers use Centracom Gold Elite consoles... the VHF resource transmits over what is basically a base/mobile radio and hits a repeated VHF system. We do not have direct access to the repeater for that frequency. So we wouldn't be affecting the repeater or anything like that, just delaying our own transmission to sync up to the 800MHz.

[Andy Brinkley]

Just my .02 cents worth -

Rack mount digital audio delay devices are used in large public address systems in order to eliminate the "echo" from speakers spaced long distances apart. IIRC the delay was set up by using a "clicker" at the microphone and adjusting the time delay until the the "click" was heard at the same time from all speakers.

The other issure to consider is that you will need a delay on the PTT so you can match the audio to the transmitter key up.

[xmo]

A neighboring system had this exact same problem. The fire guys were very unhappy whenever dispatch would broadcast a call simultaneously on the trunking system and the paging system.

Motorola's solution is a box from Raven Electronics. This subsystem is installed between the Centracom CEB and your conventional channel resources. Its primary function is to enhance the console's capability as an interoperability gateway. As a mater of fact, we were able to acquire one of these units for that purpose through a homeland defence funding grant.

Inbound audio from your conventional channels is passed through the Raven where it is delayed long enough for positive VOX detect to be applied to the channel's BIM [COR Aux I/O]. That allows the console to key the patched trunked talkgroup and/or any other conventional channels and have the audio path all set up when the incoming audio arrives. This avoids the initial audio truncation so common with VOX based patches.

It can also be connected into the outbound conventional channel audio so that your pages can be time aligned with the digital voice on the trunked system. Audio delay can be set up to one secone in 1 ms. increments. Works way cool.

Raven M4. Talk to your Motorola rep if you want them to do the project. They have used this product in numerous systems.

If you are planning on doing the project yourself, I can give you contact info for the guy we worked with at Raven [small company - very nice folks]

[bv156]

Thanks for the replies. This looks like exactly what we'll need.

Our MSS is working on a quote right now.

[nmfire10]

NHRC makes something for you as well.
http://www.nhrc.net/nhrc-dad/