Re: In-band track metadata attribute

On 03/17/2014 03:22 AM, Silvia Pfeiffer wrote:
>
>
>
> On Tue, Mar 11, 2014 at 7:15 AM, Brendan Long <B.Long@cablelabs.com 
> <mailto:B.Long@cablelabs.com>> wrote:
>
>     On Tue, 2014-03-11 at 06:41 +1100, Silvia Pfeiffer wrote:
>>     Could you provide some examples of the data that goes there for
>>     mp2-ts?
>     The same data that's currently listed for
>     inBandMetadataTrackDispatchType would work, except without base64
>     encoding:
>>     Let stream type be the value of the "stream_type" field
>>     describing the text track's type in the file's program map
>>     section, interpreted as an 8-bit unsigned integer. Let length be
>>     the value of the "ES_info_length" field for the track in the same
>>     part of the program map section, interpreted as an integer as
>>     defined by the MPEG-2 specification. Let descriptor bytes be the
>>     length bytes following the "ES_info_length" field. The text track
>>     in-band metadata track dispatch type
>>     <http://www.w3.org/html/wg/drafts/html/CR/embedded-content-0.html#text-track-in-band-metadata-track-dispatch-type>
>>     must be set to the concatenation of the stream type byte and the
>>     zero or more descriptor bytes bytes, expressed in hexadecimal
>>     using uppercase ASCII hex digits
>>     <http://www.w3.org/html/wg/drafts/html/CR/infrastructure.html#uppercase-ascii-hex-digits>.
>>     [MPEG2]
>>     <http://www.w3.org/html/wg/drafts/html/CR/references.html#refsMPEG2>
>     Even better might be including the entire "elementary stream info
>     data" <http://en.wikipedia.org/wiki/Program-specific_information>:
>
>>     *Name* 	*Number*
>>     *of bits* 	*Description*
>>     stream type 	8 	This defines the structure of the data contained
>>     within the elementary packet identifier.
>>     Reserved bits 	3 	Set to 0x07 (all bits on)
>>     Elementary PID 	13 	The packet identifier that contains the
>>     stream type data.
>>     Reserved bits 	4 	Set to 0x0f (all bits on)
>>     ES Info length unused bits 	2 	Set to 0 (all bits off)
>>     ES Info length length 	10 	The number of bytes that follow for
>>     the elementary stream descriptors.
>>     Elementary stream descriptors 	N*8 	When the ES info length is
>>     non-zero, this is the ES info length number of elementary stream
>>     descriptor bytes.
>>
>
> Is this once-off information or does it repeat? I was under the 
> impression that, e.g. PID can repeat and change during the course of 
> the video.
The PMT appears in the file several times per second, and it can change. 
I think if we see a new PID, it would generally be associated with a new 
stream/track. I assume the PMT data for a program could change 
mid-stream though, but someone with more familiarity with MPEG-TS might 
need to confirm that. Some people we talked to suggested that changing 
an attribute on the track and then firing a "change" event would be 
easier than dealing with cues, but I'm not entirely convinced there's 
much difference.