FDNY/NYPD radio system/ how does it work

[mdsxfire]

Not sure if this is posted somewhere I'm looking for some detailed explanation on how the system works ..how the mobiles can be heard even while the dispatch is talking, I read that the cars can be talking and not heard on the repeater till the dispatcher injects that audio into the repeater. Anyone point me in the right direction??

 

[CaptDan]

The dispatcher would actually transmit on a different frequency than the field units. The field units transmit on the repeater input frequency while the dispatcher actually transmits via the repeater - on the repeater output frequency. This allows the dispatcher to hear a field unit that might transmit an emergency message.

I am talking about a conventional repeater radio system. Trunking can be a completely different operating system.

In the old days they actually taught the cops that is they had an emergency - do not wait for the dispatcher to end a lengthy transmission - just put the emergency message out because the dispatch center can receive even when the dispatcher is transmitting.

 

[N2SCV]

"New York City Fire Department Fire Dispatch

The FDNY dispatch frequencies are not repeaters in the traditional sense, instead it's a full duplex system with voted receivers and a mixer..." See New York City, New York (NY) Scanner Frequencies and Radio Frequency Reference

 

[FrankRaffa]

To oversimplify it, field units use a transceiver which cannot transmit AND receive at the same time. In our offices, the transmitter and receiver are separate units that are operated independently of each other.

 

[blaze]

Just to be clear, FDNY and NYPD are two different systems, with different features.

In both systems, the mobile units transmit on one frequency (called "talk-in") and receive on another ("talk-out"). The dispatcher transmits on the talk-out frequency and receives the mobile audio from receivers on the talk-in frequency. The base station also re-broadcasts the mobile units on talk-out so other mobiles can hear them (but see below for a special difference with FDNY).

This is similar to a repeater, but there are some differences. First, the dispatcher is directly wired in to the base transmitter, and so never has to transmit on the talk-in frequency as they would if it were a simple repeater (the dispatch audio is just patched directly in on talk-out). Secondly, there's not just one receiver, but an entire network of them spread around the coverage area. The receivers are also called "voters"; their signals are sent to a central "comparator" that picks the receiver getting the strongest signal for rebroadcast by the base on talk-out.

Both FDNY and NYPD have some special features not found in many other systems.

For FDNY, the main feature is that the mobile re-transmission on talk-out is not automatic. The dispatcher has to switch it on manually after a mobile calls the dispatcher and off again when the exchange is finished. FDNY refers to the manual mobile retransmission mode as the "mixer", which I believe is terminology unique to FDNY. I'm not aware of many other departments that have the manual mixer feature; in most systems (including NYPD), the retransmission is automatic, as with a standard repeater.

The NYPD system is huge, perhaps the largest scale police system in the country. Some of the citywide channels have close to 200 voter receivers spread around the city, and even a local "zone" dispatch channel serving a few precincts may have 10 or 20 spread around the zone. They need this many receivers to provide good coverage in the urban geography of NYC, especially with portables inside buildings.

FDNY also has voters, but doesn't need as many as NYPD because of the way they use the system. Generally, NYPD offiicers portable radios to contact the dispatcher, while FDNY dispatch is primarily to higher power mobile units installed in the vehicles. (Individual firefiighter radios generally don't communicate with the dispatcher directly). Because of the higher power of the mobile transmitters, fewer voter receivers are required for good coverage.

A special feature you may not notice on the NYPD system mixes audio from the mobile units while the dispatcher is talking, allowing units to break in and be heard by all if they have an emergency. (In most systems, the dispatcher has priority over the mobile units).

Both systems are very impressive, given their scale and reliability.

 

[Danny37]

Both systems are really impressive and the coverage is great. With a good portable scanner and a good antenna you can basically listen to every area of the city with no problems depending on where you are and elevation your at. However if your scanning FDNY it can get really annoying because there tends to be an open Carrier from the dispatch console and your scanner will lock in to that channel with just a bunch of dead air. I find TEMP LO to be very useful for this issue.

 

[R8000]

Not seeing a technical write up or equipment list, I can assume the "mixing" is happening in the voter itself. Console priority can be configured a couple of different ways, as well as "RPT ON/OFF" can be controlled via function tones.

In console priority setup , you can choose if you want the dispatcher to stomp on reepater audio, or if you want it mixed with repeater audio.

The Motorola Spectra TAC had a few jumper settings to allow this to be setup, and the newer JPS voters can be dip switch configured.

 

[blaze]

Not seeing a technical write up or equipment list, I can assume the "mixing" is happening in the voter itself. Console priority can be configured a couple of different ways, as well as "RPT ON/OFF" can be controlled via function tones.

In console priority setup , you can choose if you want the dispatcher to stomp on reepater audio, or if you want it mixed with repeater audio.

The Motorola Spectra TAC had a few jumper settings to allow this to be setup, and the newer JPS voters can be dip switch configured.

Right - I believe the current audio mixing is in the comparator. In the older system (when it was dispatched from 1PP), I'm pretty sure it was done in the actual consoles with an SP feature NYPD got Moto to put in, but that was a long time ago.