Subject: Re: [AVT] RE: Carrying SMPTE time-codes in RTP streams, discussion email

From: lazzaro <[email-masked]>
Date: Wed, 23 Feb 2005 13:49:13 -0800

[Magnus and Dave in discussion on SMPTE]

>>> A. Is it acceptable to get the update a bit late? Using RTCP can 
>>> easily result in that the time code update arrives up to a few 
>>> seconds late. However if some latency is acceptable, the RTCP 
>>> bandwidth and the usage of the AVPF profile can ensure that one 
>>> maintain a limit on the latency to send the update.
>>
>>
>>  I *think* so...
>
> This is an important point, so further feedback on this is crucial.

I think the right answer to the question is "depends on the app", and
separates into two questions:

A. For the app, is associating the wrong SMPTE code with the RTP
stream for a few seconds a disaster?

B. For the app, how much advance notice does it have of an upcoming
SMPTE change?

Apps where A="yes" and B="very little if any" are probably not going
to be happy with the RTCP approach ... apps where A="no" will be
fine with the RTCP approach ... the final class of apps is "it depends".

An example of "B=little if any" is when the SMPTE is changing because
an operator is hitting buttons on a tape-deck controller, and is 
expecting
the RTP-controlled rack-mount to respond with 5 ms latency over a local
LAN in a reliable way.

---
John Lazzaro
http://www.cs.berkeley.edu/~lazzaro
lazzaro [at] cs [dot] berkeley [dot] edu
---

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

---

From: Dave Singer <[email-masked]>
Date: Thu, 24 Feb 2005 08:47:01 +0000

At 1:49 PM -0800 2/23/05, lazzaro wrote:
>an operator is hitting buttons on a tape-deck controller, and is expecting

Note that the mapping itself in the RTCP can always be right;  it's 
merely a question of whether it arrives too late for the terminal to 
be able to apply it to the media from the identified timestamp.  If 
indeed the client wants a short buffer, and the source gets no 
advance notice, and it's not possible to send immediate RTCP packets, 
there may be an issue.
-- 
David Singer
Apple Computer/QuickTime

---

From: lazzaro <[email-masked]>
Date: Thu, 24 Feb 2005 07:38:39 -0800

On Feb 24, 2005, at 12:47 AM, Dave Singer wrote:

> indeed the client wants a short buffer, and the source gets no advance 
> notice, and it's not possible to send immediate RTCP packets, there 
> may be an issue.

Agreed, in my description I was analyzing the actions the receiver takes
if it needs to make a time stamp decision under time pressure, not
the accuracy of the RTCP packet once it arrives.

---
John Lazzaro

---

From: Colin Perkins <[email-masked]>
Date: Sat, 26 Feb 2005 13:25:14 +0000

> notice, and it's not possible to send immediate RTCP packets, there 

We might be able to use RTP/AVPF to send early RTCP feedback? Or there 
might be non-RTP signalling which can convey the mapping (e.g. via 
RTSP)?

Colin

---

From: Dave Singer <[email-masked]>
Date: Sun, 27 Feb 2005 19:14:40 +0000

At 1:25 PM +0000 2/26/05, Colin Perkins wrote:
>>>>>>A. Is it acceptable to get the update a bit late? Using RTCP 
>>>>>>can easily result in that the time code update arrives up to a 
>>>>>>few seconds late. However if some latency is acceptable, the 
>>>>>>RTCP bandwidth and the usage of the AVPF profile can ensure 
>>>>>>that one maintain a limit on the latency to send the update.
>>>>>
>>>>>
>>>>>  I *think* so...
>>>>
>>merely a question of whether it arrives too late for the terminal 
>>to be able to apply it to the media from the identified timestamp. 
>>If indeed the client wants a short buffer, and the source gets no 
>>advance notice, and it's not possible to send immediate RTCP 
>>packets, there may be an issue.
>
>We might be able to use RTP/AVPF to send early RTCP feedback? Or 
>there might be non-RTP signalling which can convey the mapping (e.g. 
>via RTSP)?
>

These are sender reports, not receiver.  But yes, RTSP could say what 
the initial mapping was also (just like the NTP association sent now).
-- 
David Singer
Apple Computer/QuickTime