Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

How does a cable TV decoder work?

Status
Not open for further replies.

cruxader

Newbie level 5
Joined
Sep 13, 2003
Messages
10
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
148
how does a cable box work

Interested in knowing how a cable TV decoder works. Specifically the Jerrold brand set top boxes. I expect the box to receive commands from the provider but not sure if the box also send info back to the provider. Thanks.
 

klug

Advanced Member level 1
Joined
Jul 4, 2002
Messages
476
Helped
12
Reputation
24
Reaction score
7
Trophy points
1,298
Activity points
4,382
how does cable tv work

There are Tocom decoders on my region. They are similar to some Jerrold's decoders. They are using special data on line after vertical sync (in my region, lines 22 and 334). This data contains a lot of info about subscribers and flags of coding with little bit of data encryption. There is no back channel to provider, as you know these decoders are used on MMDS air reception too. There are a lot of decoders for this SSAVI TV scrambling on Internet. You can find them by google with "3chip" keyword.
 

cruxader

Newbie level 5
Joined
Sep 13, 2003
Messages
10
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
148
cable tv decoder

Thanks for the info.

The system here is not off the air. Strictly cable only and the provider also provides internet services with an add-on cable modem on the same line.

OK, if the special data is on the line after vertical sync, how does the provider 'disable' certain channels?

I am assuming the provider sends commands down the line to a specific residence.

Observation:
1. You can choose different channels to subscribe to but all decoders in the same residence have the same channels.

2. When you sign up for a new plan, they usually enable all channels and then later restrict you to the plan you subscribe to a month later.

3. The channel capability seems to reside with the decoder box (ie. a decoder box from a residence with all channels enabled appears to keep its functionality if brought to a residence with basic channels available).

4. There are alternative decoders on the market but they seems to function differently :-
4a. They rely on a plug-in chip
4b. When a channel has been disabled (but no effect on the Jerrold boxes), you need to get an 'upgraded plug-in chip'.
4c. They do not always decode a channel correctly and often suffer from noise and occasional color loss.
4d. Some decoders even gives a 'blue screen' which kinda makes it difficult to 'fine-tune' the unit when you cannot even see the on-screen commands!

Hence my conclusion and thus my quest to understand the system more.
It would appear that the alternative decoders operate on a different system/approach. Perhaps, my request should also include them. Are these decoders SSAVI decoders?

klug said:
There are Tocom decoders on my region. They are similar to some Jerrold's decoders. They are using special data on line after vertical sync (in my region, lines 22 and 334). This data contains a lot of info about subscribers and flags of coding with little bit of data encryption. There is no back channel to provider, as you know these decoders are used on MMDS air reception too. There are a lot of decoders for this SSAVI TV scrambling on Internet. You can find them by google with "3chip" keyword.
 

klug

Advanced Member level 1
Joined
Jul 4, 2002
Messages
476
Helped
12
Reputation
24
Reaction score
7
Trophy points
1,298
Activity points
4,382
how tv decoder works

There is no need for cable-tv provider to have back link from decoder, except for Pay-Per-View subscription. You can imagine, what headache is to make back link: they need special transmitter on decoder - so they also need some diplexer on decoder to avoid interference; next, they need to add back channel amplifiers on every cable amplifier - as result they need to add some directional splitters and simple diplexers on every in and out of every amplifier. Next, they need to avoid collisions from back signal from different decoders. And also they need to add receiving system on the end. Really they need also a big money to teach staff that will make service on that network.

But on real system they avoid that all.

They simply send some messages on that special tv line. These messages are like that: "Decoder #4323541 - Stop your receiving and go to lock state", or "Decoder #3458756 - Unlock and receive channels 1,2,3,4,7,8,9". So, as you see, every decoder has unique name. Also there is unique name for whole network to avoid migration of decoders to other networks.

So, when decoder is locked, it is configured to receive one "main" channel, but do not show TV picture on tv-set. This decoder is staying on locked mode on that tv channel untill it will receive permittion signal to unlock and receive certain channels. Also there are coming blocking messages. All these messages are repeated all the time with 2-5 minutes period.

There are few methods to c rack this system :
1. To make indipendent decoder.
2. To change firmware on decoder.
3. To intercept digital data in input of decoder processor and withdraw blocking messages.

So, as tv-provider knows that subscriber can decode channels by these methods, then provider tries to make it more difficult. They add special blocking RF-filters on network. All TV spectra is divided on few bands :
Let say 1-12 channels (49MHz-223MHz) - is "basic plan" for subscribers,
1-30 channels - is "extended plan"
1-50 - is "primary plan".

Ok, if subcriber is receiving only basic plan then they add some blocking rf-filter that will stop all signals upper than 250 MHz

That is why :
1. You can choose different channels to subscribe to but all decoders in the same residence have the same channels.
They simply have the same subscriber packet.

2. When you sign up for a new plan, they usually enable all channels and then later restrict you to the plan you subscribe to a month later.
They are making some advertistment and "present" to you for some time to encourage you to subscribe next plan, and they need time to change filters.

3. The channel capability seems to reside with the decoder box (ie. a decoder box from a residence with all channels enabled appears to keep its functionality if brought to a residence with basic channels available).

All data messages are the same on whole network, and maybe there are not blocking rf-filter on that new place.

Are these decoders SSAVI decoders?

SSAVI - is a scrambling method, AFAIK. You need to divide "scrambling method", and "data protocol". All these messages are "data protocol", but changing of TV picture is "scrambling method". Different system decoders can have the same scrambling method but different data protocol. All that I have written is based on system used in my region.

Look, you will find additional info here:
https://www.saunalahti.fi/~alamayry/ssavi/ssavi_eng.html

BTW what is model of Jerrold decoder is there on your area?
 

abb652

Member level 1
Joined
Jun 6, 2003
Messages
37
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,286
Location
Uruguay
Activity points
342
decode cable

Hi Cruxader.

You'll find more information about scrambling methods at this web site:
https://ar.groups.yahoo.com/group/DecosArgentina/

The forum is in spanish, but in the "Archivos/Archivos de Patentes" directory you'll find some US Patents documents (in english) explaining the scrambling systems from Jerrold a SA.

See the files:
us004558464.zip
us004928309.zip
us004466017.zip
 

cruxader

Newbie level 5
Joined
Sep 13, 2003
Messages
10
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
148
cable decoders

Thanks klug & abb652!

Your explanation sounds reasonable except the part about RF filters.
I have checked and they do organize the channels into limited groups.
However, channels in each group are all over the frequency spectrum!
Therefore a simple RF filter will not work.

In my country, the provider has two-way communication as the network is cable modem ready.
Although I believe that may be another frequency band.

Also according to your explanation, the decoder addressing cycles every 2-5 minutes.
That is not apparent to other observations.
But it is possible that each decoder has unique ID & service is keyed to specific decoders.
However, that does not explain why they cannot offer different services in the same residence.
You only have option to rent additional set-top box.

Yes, your comments supports my suspicion that the alternative decoders operate on a different system/approach.
It is probable that they are not SSAVI nor data protocol dependent decoders.
Probably has a fixed set of parameters which they try on all channels and select whichever appears to produce good results.
I noticed that:

1. There is a very slight delay before the picture comes through. Barely noticeable.
2. Dependent on signal strength and analog tuning settings.
3. Affected by severe high & low luminance levels (in the picture).

What do you think?
 

klug

Advanced Member level 1
Joined
Jul 4, 2002
Messages
476
Helped
12
Reputation
24
Reaction score
7
Trophy points
1,298
Activity points
4,382
jerrold unlock

Ok, you can simply test if there is back link:

Take some wide band cable tv amplifier and install it before your decoder, you need to be sure that level is not too big in amplifier input to avoid intermodulation. Your amplifier will pass signals only in one direction from cable system to decoder - so you will see if it will work.

But you can simply read what model(s) of Jerrold decoder is used in your network, and make search for it on google.
 

cruxader

Newbie level 5
Joined
Sep 13, 2003
Messages
10
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
148
unlock jerrold

Great suggestion!

Come to think of it, they did install a booster (amplifier?) in my residence because I wanted 8 distribution points! However, since that issupplied by the provider, I am not completely sure that it is a straight amplifier/booster.

Will check it out.

Unfortunately, I have returned my decoders on Friday! :oops:
Terminated the service cos my mother-in-law has returned to her home country! The service was meant to keep her entertained... :p

Let me look up a friend's unit...

klug said:
Ok, you can simply test if there is back link:

Take some wide band cable tv amplifier and install it before your decoder, you need to be sure that level is not too big in amplifier input to avoid intermodulation. Your amplifier will pass signals only in one direction from cable system to decoder - so you will see if it will work.

But you can simply read what model(s) of Jerrold decoder is used in your network, and make search for it on google.
 

Mrrenzo

Newbie level 1
Joined
Jun 27, 2004
Messages
1
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Activity points
8
how does the tv decoder work

Is there a way to code a digital box if u don't have service? :(
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top