[Bradford] BBC's plan to encrypt HD content management

Wed Jun 29 00:28:54 UTC 2011

Hi Brian,

For DVB satellite and cable networks a pre-configured 'home frequency' is
usually tuned by the receiver (STB, PC etc.). From the DVB-SI found on this
frequency, specifically the NIT (Network Information Table),  the receiver
can determine the frequency of all all the other multiplexes in the network.
Although the same principle could work on terrestrial networks it generally
less reliable for a number of reasons. As such it is common with DVB-T
(terrestrial) networks for receivers to do a frequency scan to find all
multiplexes on the network. For each multiplex that the receiver finds it
scans the SDT (Service Descriptor Table - part of the DVB-SI) to find what
services are available on the multiplex. The services found are all grouped
together and duplicates removed to produce a service list. The problem with
this approach is that if a multiplex moves in frequency then the services on
it disappear from the receiver's perspective. A rescan will find them again.
This is what you are describing.

In order to 'hide' services, broadcasters can do non-standard things like
miss services out of the SDT or not even broadcast an SDT, instead
broadcasting some private data that only 'authorised' receivers can decode.
Utlimately, if the content itself is not encrypted/scrambled, then a
suffciently flexible receiver can tune to it - it's just how much hassle it
is to do it and how much hassle it is when things change.

As a matter of interest, on cable and satellite networks that use a NIT, a
version change of the NIT causes the receiver to re-read the information
contained in the NIT, so picking up any changes in frequency. Obviously if
the currently tuned frequency is moved then the NIT update won't be spotted,
but the user will usually tune to a different service on a multiplex that
hasn't moved and so pick up the new NIT as it is broadcast on all
multiplexes. Alternatively, there are standard ways of signalling a change
in frequency prior to the change happening but these are not commonly used.

Hopefully this is not too baffling - if you've got any more questions on
this I do plan to come to the meeting tomorrow night.

.Steve

On 28 June 2011 12:06, Brian <bradlug at techchico.org.uk> wrote:

> So, Steve, if understand you correctly, the frequency data would not be
> available when a scan takes place?
> Presumably, however, it would be possible to add such information manually
> the the database in, for example, MythTV. I am doing this now because BBC HD
> has changed some information such as 'symbol rate' etc. Now I have to do the
> rest by hand or rescan.
> One thing I am happy about though is, for the first time, I have been able
> tot get HD working properly on all the HD channels - bar the BBC which I
> haven't been able to try yet. I am dreading the introduction of Wayland to
> Ubuntu which will probably muck everything up.
>
> The EPG is not so important to MythTV because the data can be gleaned from
> elsewhere, so that is not any real problem for MythTV. I don't publish my
> grabber programs because I suspect that certain websites would be deluged
> with downloads, for EPG purposes, and those same web sites might  block or
> obfuscate downloads. Having said that the only one I use to any extent is
> one for 'Travel' which uses their web site so they might not object.
>
> Brian
>
> ------------------------------
> *From:* Steve Kerr <steve at cullingworth.net>
> *To:* BradfordLinuxUserGroup <bradford at mailman.lug.org.uk>
> *Sent:* Tue, 28 June, 2011 1:21:07
> *Subject:* Re: [Bradford] BBC's plan to encrypt HD content management
>
> To quote the article: "...it is requesting that it be allowed to encrypt
> the data associated with TV listings without which set-top boxes are not
> able to decode the TV content".
>
> What they are describing there is the encryption (or obfuscation) of the
> DVB-SI and (possibly) MPEG-PSI. The STB (or other client) uses these data
> streams to find the content. The SI (System Information) describes where the
> service (what mere humans call a 'channel' - e.g. BBC1) can be found i.e.
> what frequency and other parameters to use to tune to the digital multiplex.
> The PSI (Program Specific Information) within each multiplex describes how
> to find the component parts of the service within that multiplex, i.e. the
> Packet IDs for the video, audio and other elementary streams.
>
> The 'listings', or EPG data, is held in the Event Information Table (EIT)
> which is referenced as a component of the service in the PSI. Some service
> providers encrypt the EIT data to prevent it being used by unauthorised STBs
> but this doesn't prevent the STBs from receiving and decoding the services.
>
> Freesat did a similar thing to prevent any standard DVB satellite receiver
> from picking up the Freesat services. In this case I think they just used a
> propriety means to describe where the services are (i.e. non-standard SI &
> PSI). The Freesat services could still be received by my media centre
> (non-Linux at the time I'm afraid) by doing a scan. The problem came when
> services were moved - they just disappeared from my media centre's
> perspective and I had to perform a manual rescan.
>
> .Steve
>
>
> On 27 June 2011 20:23, Brian <bradlug at techchico.org.uk> wrote:
>
>> The BBC plan to introduce encryption in regard to their HD channels. It
>> will mean that existing Linux boxes  won't function anymore and that, of
>> course, will include MythTV *(see below).
>>
>> More information:-
>> http://news.bbc.co.uk/1/hi/technology/8259154.stm
>>
>>
>> From which comes:-
>> "Critics of the BBC's request say that open source licenses are
>> incompatible with the regulations because DRM locks down software so that it
>> cannot be altered by the user. "
>>
>> Ofcom:-
>> http://tinyurl.com/66umlcx
>>
>>
>>
>> There is a PDF on that page and in it is an email address which can be
>> used to respond to the BBC's plans via Ofcom.
>>
>> Put simply, as I understand it, the BBC can't encrypt the video but can
>> encrypt the EPG etc.
>> *If this is all they intend to do then, as far as MythTV is concerned, it
>> won't be much of a problem as, presumably, listings can be obtained from the
>> Net. I have written 'grabbers' for channels which aren't in the the standard
>> EPG so the BBC should not prove any more difficult. However, with standard
>> Linux set-top boxes this may prove a problem.
>>
>> Brian
>>
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>>
>
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