Pro-96 Random Garbled Transmissions

[agentdss]

Hey,

Occasionally during a converstion on a voice channel, the voice is all digitally-garbled and then returns to normal after a few moments. What is up with that quirk? I'm pretty sure it is not related to encyption as the channels I have listened to are not know to be encypted.

 

[scannerfreak]

Are you too far away from the system? Hold on the Control Channel and see what your decode rate is. If it drops from high to low, theres your problem.

 

[mlevin]

Sometimes it could be the units operating that are having trouble transmitting. You can confirm this, because when it happens the dispatcher will ask then to 10-9 (repeat) Often times it's you being in a bad spot. i.e. basement, Cell tower etc. etc.

 

[agentdss]

scanner_freak: I'm very close...around 99% most of the time, so thats not an issue

mlevin: That happens sometimes, when disp. has to 10-9 the officer, but most of the time when these garbles come on...a moment later, another officer or disp says direct, so they clearly heard it, though I didn't.


I'm just thinking its a quirk in the decoding process in our scanner.

 

[mlevin]

It's not quirk, it's just digitals version of static. It sounds even worse on the Unidens. The voice just drops off. At least on the 96 it's squeely but if you listen closely you can still make most of it out.

Note:On a analog TRS you get a good decoding rate, but always a little amount of crackle in the background. Here it's just the same thing. The digital just reacts differently.

 

[agentdss]

It's weird because it never happens in the middle of someones speaking, but instead as a response to someone's question or something. I can never make out the garble no matter how hard I try.

 

[rdale]

That would mean the garbled officer is encrypted.

- Rob

 

[mlevin]

Rdale, I doubt it. The pro 96 mutes encrypted comms.

Agentdss, could you get a recording of the comms. And what system and talkgroup are you listening too. If it happens system wide it's definately not encryption, and is most likely what I speculated before.

 

[mikey60]

The Pro-96 mutes encrypted comms on Moto 3600, but not on 9600 P25 or conventional. And on 3600, if the first transmission is not encrypted, and the repeater hasn't dropped, it won't mute the encrypted reply either.

Mike

 

[agentdss]

It's defintely not encypted. I listen to Montgomery County, Maryland's APCO-25 (3600bps) System. Yes, this happens system wide, though as mentioned before, only on short occasion. (1 or two times during a 2 hr. listening session)

 

[dc2wheel]

I'd swear I've heard conversations on the Montgomery County
PD SAT channels in which one officer was in the clear and the
other was encrypted. The officer in the clear could definitely
understand the encrypted officer, but his transmissions just
sounded like a drunk R2-D2 on my Pro-96 [which is how I've
been told encryption sounds]. Should my Pro-96 be blocking
this out?

 

[Forts]

I've heard that as well in my area... There have been a few times where the PD have been doing undercover work, and at least one radio in their group transmits in the clear while all the others are encrypted. They seem totally oblivious to it, so it must just be a setting on the officers radio. It will receive encrypted transmissions just fine, but is transmitting in the clear for some reason.

I have also experienced the garbled transmissions on my Pro-96 and heard the dispatcher say 'You were 10-1... Please repeat', so it wouldn't seem to be an issue with the scanner...

 

[rdale]

"Should my Pro-96 be blocking this out?"

As mentioned just a few posts above - if the repeater doesn't drop between transmissions you will hear the encrypted 'garble' traffic while the officer listening will hear just fine.

- Rob

PS I will also confirm the statement that the Pro96 does not block encrypted comms on 9600 systems, but you aren't suffering from that -- but the repeater timeout.

 

[agentdss]

dc2wheel: I think a lot of the times I have this issue on SAT channels, like you mentioned. But if one officer was encypted, why would it go on for just a moment. Example, i'm listening to 3 officers on the channel, each have been talking for a while, then for like 10 seconds, I hear the garbled, yet possibily encypted audio, and then it goes back to normal.

 

[mstarn]

I think what is happening is that you are hearing, what is the same as signal drop out or loss in analog. In analog the signal just gets weaker and noisier till you lose it. When the digital signals get "weak" you start losing a syllable or syllables, then a word or words, and finally parts of sentences. After that goodbye to the signal. This is fairly common to hear. It really can sound strange depending upon what and how much is being dropped out of the data. "Two or three folks talking one sounds encrypted and the other(s) sound fine" Probably one of the transmissions is being received weak. :?: :?: :?:

 

[dc2wheel]

It's likely I don't know enough about how encryption works, but
if by this:

"the repeater doesn't drop between transmissions you will hear the
encrypted 'garble' traffic .... "

then would the frequency change also? I'm noticing the frequency will
roll over as usual during a pause between transmission [as is the nature
of a trunked system] and the next user will be encrypted.

Tonight I was definitely hearing a one-on-one conversation between one
encrypted officer and one un-encrypted officer. They were apparently
within line of site of the same "target vehicle", and their location was
less than a mile from my location, so I'm fairly sure it also wasn't a loss
of signal issue.

Agentdss: are you hearing what sounds like encryption for what obviously
seems like one use of the ptt button on the officer's radio, or is part of
an officers single transmission encrypted and part not? Damn, this would
be so much easier to explain in person than in text form.
-dc2wheel

 

[agentdss]

Yea, I'm not mistaking this for a PTT button action...I can tell a sentence or two goes by during this garble fit. It just didn't seem right for a random officer who is encypted to just chime in during a conversation and leave and not to be heard for quite a while.

 

[mikey60]

What I meant by "if the repeater doesn't drop" on a 3600 Baud Motorola system is this. When a system user initiates a call, the controler assigns a repeater for the talkgroup. Repeaters on Motorola systems have a delay from when the last transmission was heard until the repeater is released to allow for a reply from another user without having to go through the process of assigning another repeater for the conversation.

When the call is started by the first user, the talkgroup is set with appropriate status bits to indicate whether the talkgroup whould be encrypted or not (plus others, they have been posted repeatedly so I'm not going to post them here). That talkgroup and status bit setting will remain with the repeater until it is released by the controller.

So two scenarios, where User 1 is encrypted, user 2 is not.

Scenario 1:
User 1 initiates the connection, Since it is encrypted, the status bit in the control data stream is set for encryption, and the conversation is ignored by the Pro-96 until the exchange has completed and the repeater is released. You won't hear either user.

Scenario 2:
User 2 initiates the connection, since the user is not encrypted, the encryption status bit will not be set, and the Pro-96 will switch to the voice channel for the conversation. You'll hear User 2's transmission, and as long as the repeater hasn't been released by the controller, User 1's transmission will be picked up by the radio, but since it's encrypted, all you'll hear is a series of blips from the radio.

That work for you?

Mike

 

[dc2wheel]

Mikey60, Thanks - that pretty much explains it. I guess the
one question I still have is whether each repeater has it's own
frequency? Would a repeater being dropped necessarily be
accompanied by a change in the frequency displayed on the
Pro-96? I'm noticing a pause and frequency change after
each party ends their transmission.
-dc2wheel

 

[mikey60]

I guess the
one question I still have is whether each repeater has it's own
frequency? Would a repeater being dropped necessarily be
accompanied by a change in the frequency displayed on the
Pro-96? I'm noticing a pause and frequency change after
each party ends their transmission.

Yes, this is standard trunking operation.

Most trunking systems have anywhere from 2 to 28 repeaters in them, depending on load and configuration.

The primary difference between Motorola and EDACS trunking is that Motorola/P25 systems have a delay from the end of the last transmission until the repeater is released (dropped) to allow for a reply. This delay is usually around 2-5 seconds. If that delay expires and the repeater is released, the next exchange of conversations for the same group will generally be on a different repeater (Frequency) in the system

EDACS on the other hand releases the repeater immediately following the transmission, and the next transmission will generally be on a different repeater (frequency) in the system. This has been referred to as transmission trunking.

Both methods use the same basic concept, allowing any system user to use any system repeater in an efficient manor. What used to take 20 separate discrete conventional channels, can be done on a trunked system with 10 repeaters or less with no drop in service.

Mike