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Re: [payload] new draft - RTP Payload for TTML Timed Text

Thomas Edwards <Thomas.Edwards@fox.com> Thu, 17 January 2019 19:12 UTC

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From: Thomas Edwards <Thomas.Edwards@fox.com>
To: Kjetil Oftedal <oftedal@gmail.com>, James Sandford <james.sandford@bbc.co.uk>
CC: "payload@ietf.org" <payload@ietf.org>
Thread-Topic: [payload] new draft - RTP Payload for TTML Timed Text
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Date: Thu, 17 Jan 2019 19:12:08 +0000
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Subject: Re: [payload] new draft - RTP Payload for TTML Timed Text
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I'm not sure what the author was thinking, but I'd think that in many cases, a TTML RTP flow would be often associated with another RTP media flow (such as RFC 4175/SMPTE ST 2110-20, AES67, etc.) and hopefully the RTP timestamp time would line up...

In RFC 8331 (RTP for SMPTE Ancillary Data), it was suggested they were tightly locked together:
"When an ANC data RTP stream is to be associated with an RTP
         video stream, the RTP timestamp rates SHOULD be the same to
         ensure that ANC data packets can be associated with the
         appropriate frame or field.  Otherwise, a 90 kHz rate SHOULD be
         used."

But that might be less appropriate for TTML which is less directly associated with video fields/frames.

-Thomas

On 1/17/19, 10:53 AM, "payload on behalf of Kjetil Oftedal" <payload-bounces@ietf.org on behalf of oftedal@gmail.com> wrote:

    On 17/01/2019, James Sandford <james.sandford@bbc.co.uk> wrote:
    > I have uploaded a draft specifying an RTP payload format for TTML Timed Text
    > https://urldefense.proofpoint.com/v2/url?u=https-3A__datatracker.ietf.org_doc_draft-2Dsandford-2Dpayload-2Drtp-2Dttml_&d=DwICAg&c=uw6TLu4hwhHdiGJOgwcWD4AjKQx6zvFcGEsbfiY9-EI&r=lekNOOM5noV61zrPH3rwPyhtNnLLWoLEHgd0quQxly8&m=lcOWqoAOZng6eG5ibGyA-k5wqdyjNwxcOsa251X-eLg&s=eGgyZQYCqKg2ArgO0aFCaNFoX8fZTyL7cOP55goWDY4&e=
    >
    
    Hi,
    
    Can you please elaborate a bit on the RTP Timestamping requirements
    outlined here?
    As far as I can see TTML already specifies the timing of the text in
    relation to some epoch.
    Will the processing of the RTP packet require the receiver/sender to
    rewrite the timing data in TTML-information in relation to the RTP
    timestamp? Or written another way: Is the RTP Timestamp considered the
    epoch of the TTML document?
    
    Best regards,
    Kjetil Oftedal
    
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