Strange MDC-1200 Continous Chirping Signal

Status
Not open for further replies.

freska2x

Member
Joined
May 25, 2020
Messages
10
Does anyone have any idea what a channel frequency that broadcast nothing but a signal that sounds like the MDC1200 chirp repeating continuously about 144 times a minute would be used for? The organisation this frequency is assigned to uses an analog NFM system on several other frequencies and the staff carries around motorola radios.
 

freska2x

Member
Joined
May 25, 2020
Messages
10
This is not really strange this has been on the air for a few decades. It is the data for the MK I Syktrain system in Lower Mainland.
I wonder what data is being transmitted? It sounds like it is going around 1200bps will take a closer look at the signal to see if raw data could be decoded into a hex output at least. I know the control for the trains are not transmitted through the airwaves but rather slippery coax cable. Its those two wires running along the travel rails on either side of the middle lim rail crosses over every 20 metres.
 

freska2x

Member
Joined
May 25, 2020
Messages
10
Found out a bit more about the signal done a bit of analysis on the signals. Seems like the signal is some sort of Radio ID.

Approx figures are:

Burst Length: 107ms
Raw Bit Rate: ~1430bps
Approx Data Per Burst: 152bits
Modulation: Probably BFSK

The same data burst can be heard sometimes after a voice transmission like conversations or station announcements on the other frequencies. The length of the data burst is exactly the same, but the pauses between the bursts can be different. I have heard two or three data bursts after a voice transmission, of which the data contained in them are different.

The data after voice transmission seems to have a preamble binary sequence of "01011" (or inverse thereof) repeating 9 times, while the repeating bursts on 410.7875MHz has the same preamble repeating 10 times with the data appearing inverted compared to those after the voice transmission.

I have attached an image of these packets after pulling out the signal by doing a low pass filter at 1625Hz (Midpoint between the peaks of 1480Hz and 1770Hz).

Any idea of what brand / model of radio system this might be or why there would be a need to transmit constantly on 410.7875MHz?
 

Attachments

kb4mdz

Member
Joined
Apr 28, 2003
Messages
306
Location
Cary, NC
OK, haven't scanned this portion for a long time, but near Raleigh NC there was some sort of FSK signal down in the low 450MHz region, near the allocation for broadcasters use. I never bothered to DF it or anything, but I was told by some older local scannists that it had to do with telemetry of a transmitter, going back to the studio building and Engineering Dept.

I'll look into whether it's still there, or has been superceded and suspended.
 

freska2x

Member
Joined
May 25, 2020
Messages
10
I think you are right regarding the telemetry. I found out a bit more about the system - the repeater input is on 415.7875 MHz. On the input you only hear chirps every so often some may be 10 seconds to over a minute, and these chirps gets repeated onto 410.7875 MHz. They have a system called TRIMS or "Train Radio Information System" that is used for PA Announcements, Train Emergency Intercom, Silent Alarm Activation on the train, transmitting train Fault Codes (FC), and change the Platform LED Message Displays (PLEDS).

I am guessing from this information the chirps are broadcast in some sort of timed sequence from each station and/or train and contains data of the broadcaster eg. station ID or train ID along with some sort of status code or command. This would make perfect sense since listening on the repeater input is simplex and only the transmission local to my radio receiver is heard, but since the chirps out the output is continuous it must mean the rest of the chirps are coming from different places along the train route. If anything it is likely some sort of a ping saying something like "Radio ID 1234 with FC 0 is alive" back to the control centre.

At the end of PA announcements you would hear two chirps, one of them might be coming from the head end and the other from the train or station, like <End of Announcement Packet>, <Acknowledgement Packet>. Although PAs are on a different frequency, the data format of the packets are similar suggesting they are part of the same TRIMS system. The frequency pair in question is dedicated for data.

It would be really interesting to pull up a datasheet if the manufacturer of such hardware could be determined.
 

devicelab

Whacker Extraordinaire
Joined
Jul 18, 2016
Messages
1,119
Location
Nowhere in WA
AWOS data from a nearby airport.

 
Status
Not open for further replies.
Top