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Re: [rtcweb] Transcoding Delay

Stephan Wenger <stewe@stewe.org> Thu, 16 January 2014 17:04 UTC

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From: Stephan Wenger <stewe@stewe.org>
To: Harald Alvestrand <harald@alvestrand.no>, "rtcweb@ietf.org" <rtcweb@ietf.org>
Thread-Topic: [rtcweb] Transcoding Delay
Thread-Index: AQHPEffzyPg2TegurU2vIKkJtwnEYpqHVPiA//+67gA=
Date: Thu, 16 Jan 2014 17:04:16 +0000
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References: <CAHp8n2kq+_uG=9XwoAGtRgqYU2Asc2Fv6RZ0aCW6cJi-LnhD+A@mail.gmail.com> <10390_1389365676_52D009AC_10390_2407_1_2842AD9A45C83B44B57635FD4831E60A06CBE540@PEXCVZYM14.corporate.adroot.infra.ftgroup> <52D0222F.4010006@bbs.darktech.org> <949EF20990823C4C85C18D59AA11AD8B112238@FR712WXCHMBA11.zeu.alcatel-lucent.com> <CAHp8n2=m3i77SNPZWmJchqVdg1c2WEJCt5g-pFRfmeWA2yV5xw@mail.gmail.com> <949EF20990823C4C85C18D59AA11AD8B114B2D@FR712WXCHMBA11.zeu.alcatel-lucent.com> <CEFAAC25.3F7FD%stewe@stewe.org> <20140115134438.GB8358@verdi> <52D7DA7B.2030503@alvestrand.no>
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Subject: Re: [rtcweb] Transcoding Delay
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On 1/16/14, 5:11, "Harald Alvestrand" <harald@alvestrand.no> wrote:

>On 01/15/2014 02:44 PM, John Leslie wrote:
>> Stephan Wenger <stewe@stewe.org> wrote:
>>> As for the delay introduced by the transcoding engine alone, let me
>>>note
>>> that the single or two frame delay commonly attributed to transcoding
>>>is
>>> realistic only for straightforward PPP type coding.
>>    I agree that seems optimistic...
>>
>>> My understanding is that VP8, in its default encoder settings, uses
>>> much more complex GOP structures for error resilience and coding
>>> efficiency reasons.
>
>I have no idea where this idea comes from.
>
>The dependency structures of VP8 when used in interactive mode are often
>quite complex, with the purpose of minimizing recovery time when frames
>are lost or damaged, but dependencies are always backwards-pointing;
>there is no rearrangement of picture order.

That¹s what I thought, and tried to express, and apparently failed.

Additional delay over straightforward PPP coding comes in only when you
loose something and want to take advantage of your complex GOP structure.
That additional delay is certainly less than the delay you would catch by
coding an I picture and sending that over a bandwidth-limited link, which
is one reason of many why things wrt. GOP structures are as they are in
modern systems.  (That feature, by the way, is the same in modern H.264
implementations and VP8 implementations, and, obviously, it¹s an
encoder/sending system choice.)

Stephan
>
>
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>rtcweb@ietf.org
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