Subject: RE: [AVT] Update (-09) of Timed Tex draft (WAS: RE: Commentsondraft-ietf-avt-rtp-3gpp-timed-text-08.txt)

From: "Rey Jose" <[email-masked]>
Date: Wed, 9 Feb 2005 22:42:06 +0100

Jan,
 
welcome in!
 
The reason while this is being discussed is actually to conform with how RTP was designed to forward packets. The modus in which we devised the inband transmission collides with it because (as the example from Magnus illustrates) one cannot make use of redundant SDs even when you have received them, thus making their use meaningless.  Usually you pull out the RTP packets from RTP buffer in TS order (what Colin calls the "RTP model"). 
 
On the other hand, while editing the draft, I realised that it says: if a sample description required by a text sample is not there, ít is a matter of the application what to display. Usually a default sample description can be used instead, this is a matter of the encoder.  
 
So it actually depends on how serious this distortion is, whether we introduce the modification. Otherwise, just rely on the app to conceal it ?? 
 
Looking forward to your feedback,
 
José

________________________________

	From: [email-masked] [mailto:[email-masked]] 
	Sent: Wednesday, February 09, 2005 8:12 PM
	To: Magnus Westerlund
	Cc: IETF AVT WG; [email-masked]; Rey Jose
	Subject: Re: [AVT] Update (-09) of Timed Tex draft (WAS: RE: Commentsondraft-ietf-avt-rtp-3gpp-timed-text-08.txt)

	Magnus, Jose 
	
	Magnus wrote: 
	
	"With the proposed timestamp 
	model, the timestamp of the TYPE 5 unit in packet 3 should have a TS 
	equal to the TS of packet 1, rather than the TS of packet 3 to indicate 
	that it is useful from the TS time of packet 1."
	
	I have problems with this time stamp interpretation for TYPE 5 units, which, if I understand correctly, results from the current notion that the time stamp of a TYPE 5 unit indicates when the TYPE 5 unit becomes available for use. 
	
	In my view, there should be no relationship at all between the TYPE 5 units and time stamps, not even an indication when they become available for use. A packet that contains data from a "timed text access unit" carries the time stamp associated to that  "timed text access unit", but for a contained TYPE 5 unit that time stamp has no meaning. The SD contained in a TYPE 5 unit will be used in future, but when may be unknown at reception. A notion of "when it becomes available for use" has very little meaning in my view. I suggest the time stamp model fully ignores TYPE 5 units; assigning "time" to TYPE 5 units complicates the problem, while I think there is no need for that. 
	
	Best regards,
	
	Jan

Magnus Westerlund <[email-masked]> 

Sent by: 
[email-masked] 

08-02-2005 09:48 

        To:        Rey Jose <[email-masked]> 
        cc:        IETF AVT WG <[email-masked]>
(bcc: Jan vanderMeer/EHV/IPS/PHILIPS) 
        Subject:        Re: [AVT] Update (-09) of Timed Tex draft (WAS: RE: Comments        ondraft-ietf-avt-rtp-3gpp-timed-text-08.txt) 

        Classification:         

	Hi,
	
	I am in support of this change, however as outline below I don't think 
	we have thought yet of all the issues.
	
	Rey Jose wrote:
	> Jan,
	>  
	> this would simply translate into requiring that the SD is assigned the timestamp of the RTP packet in which is contained. That is:
	> 
	> 
	> "TYPE 5 units are an exception: TYPE 5 units receive their timestamp from the RTP packet in which they are contained.  These units can be used to convey sample descriptions that are used by many samples. Accordingly, their timestamp does not represent the instant when they are played out, but instead the instant at which they become available for use.  A sample description conveyed in a TYPE 5 packet MUST NOT be referenced by a unit with earlier timestamp."
	> 
	I don't think it will be this simple. We must consider how we assign 
	timestamps to SDs and when packet losses occurs. Let me give an example:
	
	Packet 0: Type 1: SIDX=1
	
	Packet 1: Type 5: SIDX=2
	          Type 1: SIDX=2
	
	Packet 2: Type 1: SIDX=2
	
	Packet 3: Type 5: SIDX=2 (Repetition)
	          Type 1: SIDX=2
	
	IF you in the above case would loose packet 1, you can only utilize 
	packet 2 if you get packet 3 and its SD. With the proposed timestamp 
	model, the timestamp of the TYPE 5 unit in packet 3 should have a TS 
	equal to the TS of packet 1, rather than the TS of packet 3 to indicate 
	that it is useful from the TS time of packet 1. To make this work we 
	need to possibility to provide the type 5 with a TS even if it is sent 
	very long after the first usage of that sample description.
	
	As I can see it we can get this TS in two ways. First proposal would be 
	to use the existing model, which would mean equipping the TYPE 5 with a 
	SDUR field to provide duration until the next unit. However the 
	semantics is not clear, as the SD has a usage duration, however it may 
	not be known or be infinite, and in this case if would simply be a TS 
	offset field for the next unit.
	
	Alternative 2 would be to provide the TYPE 5 with an explicit TS 
	indication (4 bytes) and maintain the current model for the other types, 
	however that would mean that the RTP TS would reflect the other types 
	timestamp and not the SD at all. This would not follow the usual 
	behavior of RTP TS according to the first content, however it may be 
	advantageous. The reason is that if you have a periodic repeat of any 
	dynamic SD during a long lived multicast session, your timestamp will 
	otherwise make big backwards jump for each packet that contain a 
	repetition.
	
	Opinions on this?
	
	Cheers
	
	Magnus Westerlund
	
	Multimedia Technologies, Ericsson Research EAB/TVA/A
	----------------------------------------------------------------------
	Ericsson AB                | Phone +46 8 4048287
	Torshamsgatan 23           | Fax   +46 8 7575550
	S-164 80 Stockholm, Sweden | mailto: [email-masked]
	
	_______________________________________________
	Audio/Video Transport Working Group
	[email-masked]
	https://www1.ietf.org/mailman/listinfo/avt

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: [email-masked]
Date: Thu, 10 Feb 2005 00:15:38 +0100

Hello Jose,

Pulling TYPE 5 units out in TS order is fine, but for TYPE 5 units itself 
TSs are meaningless, and therefore no relationship between a time stamp 
and a TYPE 5 unit should be defined. I may be wrong, but in my view the 
current problem is caused by assiging a meaning to TSs for SDs (indicating 
the instant in time when the TYPE 5 unit becomes available for use). SDs 
just arrive in TYPE 5 units, and they should in principle be present prior 
to their use. The exact time at which they become available does not 
matter at all, as long as the SD is present prior to its use. So my 
suggestion to move forward is to leave the TYPE 5 unit as is, but to 
change your suggested text below into something along the following lines:

"TYPE 5 units are an exception: though time stamps are meaningless for 
TYPE 5 units, TYPE 5 units receive their time stamp from the RTP packet 
carrying the most recent text sample data received prior to the sample 
description. The TYPE 5 unit conveys a sample description that may be used 
by many samples, and is removed from the play-out buffer upon reception in 
time stamp order. Typically, a sample description conveyed in a TYPE 5 
packet should not be referenced by a unit with an earlier timestamp, 
unless the sample description is currently in use and included for random 
access purposes." 

Hope this helps...

Best regards,

Jan

"Rey Jose" <[email-masked]>
09-02-2005 22:42
 
        To:     Jan vanderMeer/EHV/IPS/PHILIPS@PHILIPS
"Magnus Westerlund" <[email-masked]>
        cc:     "IETF AVT WG" <[email-masked]>
<[email-masked]>
        Subject:        RE: [AVT] Update (-09) of Timed Tex draft (WAS: 
RE: Commentsondraft-ietf-avt-rtp-3gpp-timed-text-08.txt)
        Classification: 

Jan,
 
welcome in!
 
The reason while this is being discussed is actually to conform with how 
RTP was designed to forward packets. The modus in which we devised the 
inband transmission collides with it because (as the example from Magnus 
illustrates) one cannot make use of redundant SDs even when you have 
received them, thus making their use meaningless.  Usually you pull out 
the RTP packets from RTP buffer in TS order (what Colin calls the "RTP 
model"). 
 
On the other hand, while editing the draft, I realised that it says: if a 
sample description required by a text sample is not there, ít is a matter 
of the application what to display. Usually a default sample description 
can be used instead, this is a matter of the encoder. 
 
So it actually depends on how serious this distortion is, whether we 
introduce the modification. Otherwise, just rely on the app to conceal it 
?? 

José

From: [email-masked] [mailto:[email-masked]] 
Sent: Wednesday, February 09, 2005 8:12 PM
To: Magnus Westerlund
Cc: IETF AVT WG; [email-masked]; Rey Jose
Subject: Re: [AVT] Update (-09) of Timed Tex draft (WAS: RE: 
Commentsondraft-ietf-avt-rtp-3gpp-timed-text-08.txt)

Magnus, Jose 

Magnus wrote: 

I have problems with this time stamp interpretation for TYPE 5 units, 
which, if I understand correctly, results from the current notion that the 
time stamp of a TYPE 5 unit indicates when the TYPE 5 unit becomes 
available for use. 

In my view, there should be no relationship at all between the TYPE 5 
units and time stamps, not even an indication when they become available 
for use. A packet that contains data from a "timed text access unit" 
carries the time stamp associated to that  "timed text access unit", but 
for a contained TYPE 5 unit that time stamp has no meaning. The SD 
contained in a TYPE 5 unit will be used in future, but when may be unknown 
at reception. A notion of "when it becomes available for use" has very 
little meaning in my view. I suggest the time stamp model fully ignores 
TYPE 5 units; assigning "time" to TYPE 5 units complicates the problem, 
while I think there is no need for that. 

Best regards,

Jan

        To:        Rey Jose <[email-masked]> 
        cc:        IETF AVT WG <[email-masked]>
(bcc: Jan vanderMeer/EHV/IPS/PHILIPS) 
        Subject:        Re: [AVT] Update (-09) of Timed Tex draft (WAS: 
RE: Comments        ondraft-ietf-avt-rtp-3gpp-timed-text-08.txt) 
        Classification:         

Hi,

Rey Jose wrote:
> Jan,
> 
> this would simply translate into requiring that the SD is assigned the 
timestamp of the RTP packet in which is contained. That is:
> 
> 
> "TYPE 5 units are an exception: TYPE 5 units receive their timestamp 
from the RTP packet in which they are contained.  These units can be used 
to convey sample descriptions that are used by many samples. Accordingly, 
their timestamp does not represent the instant when they are played out, 
but instead the instant at which they become available for use.  A sample 
description conveyed in a TYPE 5 packet MUST NOT be referenced by a unit 
with earlier timestamp."

Packet 0: Type 1: SIDX=1

Packet 1: Type 5: SIDX=2
          Type 1: SIDX=2

Packet 2: Type 1: SIDX=2

Opinions on this?

Cheers

Magnus Westerlund