Re: Proposal from HbbTV

Dear Bob,

On 2014-09-30, at 18:13 , Bob Lund <B.Lund@CableLabs.com> wrote:
>>>>> [...]
>>>>> 
>>>>> For @id, why doesn©öt the use of PID work.
>>> 
>>>> PID can't work in Europe as we do a lot of forwarding of broadcast
>>>> streams and service aggregation. A service on a cable trunk for instance
>>>> may come from a terrestrial feed, which in turn is sourced from a
>>>> satellite feed, and the cable operator is mixing is some more services
>>>> from another satellite or another cable. The probability that the PID
>>>> will be changed a couple of times along the way is quite high. For this
>>>> reason DVB has invented the stream identifier descriptor, bearing a
>>>> component_tag. Even if the PID changes the tag is retained.
>> 
>>> But why does it matter? The transport stream still requires that PIDs be
>>> unique. So, each HTML track will have a unique id if PID is used. What
>>> else does the track.id need to convey to the Web application?

Ah that's a very good question! Apologies for not having provided more explanation earlier on in.

Beyond unambiguously distinguishing the tracks in the player - for which PID would be sufficient - in HbbTV we are looking at applications referencing tracks. Such an application will typically be authored and provided by the content originator. If the original broadcaster in their app were to use the PID under which they broadcast a particular track, that PID would become altered after the signal going e.g. from satellite into a cable network. But the cable provider downlinking the satellite service will of course not be able to change the JS to accommodate the new PID value. Hence, for HbbTV, we need and end-to-end unique identifier that is independent of the underlying transport, so that a document or JS script can unambiguously reference a particular track no matter what sausage machines the TS has gone through.

I was wondering whether OCAP might not have similar requirements on track identification?

>> For DVB systems, the spec might as well just require that the id be a
>> unique identifier arbitrarily assigned by the UA rather than requiring
>> that identifier to be the PID.

I am afraid that I'm not fully with Nigel here. See my explanations above.

> So the unique identifier assigned by the UA could be the PID, right? It
> would best for interoperability if the the UA set the MPEG-2 TS track id
> the same way in all cases.

As opposed to the Web, the broadcast landscape is segmented into national profiles since governments traditionally very closely regulate their broadcast market. Hence, historically there is a different, national broadcast profile in each and every country. Back in the analogue days, there were eight different flavours of PAL, seven flavours of SECAM, and three flavours of NTSC. Hence, there unfortunately still is no one, true, global digital TV receiver implementation that fits all.

But of course there is a one, true, global UA implementation that ought to fit all. Apparently there is a structural gap to bridge here, and the segmentation of the MPEG2-TS section as we are suggesting is a symptom of this gap.

I am fully with you that a single, global TV standard would have benefited industry and consumers alike. But then - for numerous reasons - we are stuck with the history we have.

Looking forward to your comments,

  --alexander

Received on Monday, 6 October 2014 10:52:02 UTC