Do Digital TV stations hide information in their signals

[TerraForce1]

Hi,

I would like to know if Digital Broadcasters only use their logo to mark their broadcast or does it go deeper like a text file or some header in the PVA stream that marks the stations property?

Could someone inform me?

[Herske]

They broadcast their name, teletext info, EPG and many other things no one cares about :D. They can't hide anything, anyone can parse the PMT, dump the pids from the transport stream, etc.

BTW, PVA is only a recording format for ss1 compatible dvb cards.

[TerraForce1]

Hi, For example I want to record a movie from a station, remove the logo and re-encode it with CCE. is it still possible to say that this material came from the specific station and the subscriber was TerraForce1.

[cheap-red-wine]

Originally posted by Herske
They can't hide anything, anyone can parse the PMT, dump the pids from the transport stream, etc. Have fun trying to decrypt their encrypted data!

[Herske]

Your reply absolutely nothing to do with what I said. Decrypting is one thing, analyzing the PMT, for example, is something completely different and can be done regardless of encryption.


Terraforce: No.

[cheap-red-wine]

You said: "They can't hide anything." That is manifestly wrong. They hide all kinds of things by means of encryption, as well as undocumented user data. Yes, the PMT is in the clear. So what?

[Herske]

Originally posted by cheap-red-wine
[B]You said: "They can't hide anything." That is manifestly wrong.No, it is not if you re-read the original question. There's no provider specific information you can not see.
They hide all kinds of things by means of encryption
A person legally entitled to receive the broadcast will also have a smartcard. Decoding the broadcast is as trivial as inserting the card in the card slot.
as well as undocumented user data.
It's called "private user data" and it's pretty much available to anyone interested and technically able to access it.
Yes, the PMT is in the clear. So what?
This means you can always analyze the complete structure of a transport stream, regardless of encryption.

[TerraForce1]

Hi, so it means that they can know who coded the film.

[SeeMoreDigital]

Just side tracking away from "Do Digital TV stations hide information in their signals"...........

I think it's quite interesting how Gspot (the media info tool) is able to know which application was used to create your encodes!

[DrP]

Not really, most encoding apps actually stamp an ID string into the headers of the files. Even ancient AVI has space for this.

[Kon]

This is a interesting subject :) I have been writing applications for the last 8 years that deal in some way or another with 'hiding' information in the DTV signals, either running at low (and sometimes high) bitrates, or replacing the null packets with data.

In terms of tables and the actual TS stream, the data is in fact 'hidden', since the PIDs it is transmitted on are unreferenced in the TS program stream information (PMT, etc).

Any TS analyzer will find them as unreferenced PIDs though, without any problems.

The data is either custom packet data encapsulated directly into PID packets, or it is IP data which is wrappered in MPE/DSMCC. 99% of this is UDP multicast packets.

To decode you need a card capable of decapsulating IP via a virtual network driver, and a multicast sniffer/listener.

And then you need to figure out the protocol and underlying encryption. If you are unfamiliar with the data it is pretty much a waste of time.

Mostly the data is carouseled more than one time, often uses extended headers (UDP + custom header + data), and the data payload may be encrypted. Software FEC is also popular but since the MPEG layer can handle error correction, it is only necessary to deal with erasure correction.

Some of the data I have worked with transmitting in the past:

Weather Information
Satellite Images
Games
Music/VOD content
Emergency Services
Windows Media 9 streams (and 8/7/..)
Stock Trading Data
... and so on

[timecop]

hm interesting.
no wonder I couldn't find anything that looked like weather data in the streams I capture from a station that had "Data" mode which displayed current temperature/weather info for my area (after entering my zip code in the setup).

Of course, I don't have any fancy TS analyzers, jsut the General Parser someone referred me to a while ago...

And you say its usually encrypted / compressed? I wonder why..

I did come across a couple streams that looked like intermixed XML/HTML data + normal JPEG and PNG images.
But I did not have a tool that would properly demux those streams, so I was limited to only viewing them in a hex editor
(For some reason the demuxer I had would only demux first 4 or 8k of data then insert random garbage, so I couldn't see most of the jpegs, but could get most of the XML/HTML data.

The format looked like it was using it to layout the stuff on the screen (this was from a digital radio station, so they had background music + some changing pictures).

[Kon]

If youre looking at a TS and have any unreferenced PIDs of type 0x05 then its a good chance those are data or carousel PIDs.

There are really only two types of data services - those for STBs, and those for PCs. Digital Radio carousels like the one you found can litter the TS stream. These are usually raw files with simple headers in a low bitrate carousel. ITV apps, sports apps like those on Sky+, Wink, MHEG and so on are all carouseled this way. The current set of STB technology supports XML, HTML, and XHTML. So it makes sense to see that type of data in the TS. The Microsoft TV platform for example is using a lot of 'web' type data.

As for PC-based data services, you will (should) always be looking for DSMCC sections containing UDP headers. If you spot that in the stream, you have a network multicast. :)

Its likely that the binary data you had was encrypted. Sometimes the headers are sent in the clear and the binary data is further packetized to make transmission more reliable.

The encryption is usually there not for content protection but rather to stop people snooping the data. A lot of manufacturers use a symmetric cipher system to encrypt the data - many have the feature as a 'selling point' for their device. The encryption also very seldom matches the CA encryption, the reason being that STBs usually don't have much CPU cycles over while decoding the video/audio services (so they opt for something like simple XOR). PCs don't use the CA encryption because they may not have a smartcard slot, and most CA vendors do not offer PC-based CA solutions (for obvious reasons).