Subject: [AVT] ptime and frame-based audio payload formats

From: "Link, Brian" <[email-masked]>
Date: Mon, 2 May 2005 17:07:34 -0700

Colleagues,

A question has come up inside the Digital Living Network Alliance (DLNA)
about the preferred way to define an RTP payload format for audio that
uses a frame-based representation. The question applies to the AC-3 I-D,
but hopefully there is a general answer readily available.

Should the optional MIME parameter "ptime" describe the duration of one
audio frame in an RTP packet (regardless of the number of frames in the
packet) or should it describe the duration of an entire RTP packet (even
though the number of frames, and thus the duration, may vary from packet
to packet)?

Thanks for your help,
Brian

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Brian Link
Dolby Laboratories, 100 Potrero Ave., San Francisco, CA 94103
phone: 415-558-0324  fax: 415-863-1373
email: [email-masked]   alt email: [email-masked]

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From: Magnus Westerlund <[email-masked]>
Date: Tue, 03 May 2005 09:31:18 +0200

Hi Brian

> though the number of frames, and thus the duration, may vary from packet

ptime is on packet basis. If you review the definition in RFC 2327:

    a=ptime:<packet time>
        This gives the length of time in milliseconds represented by the
        media in a packet. This is probably only meaningful for audio
        data.  It should not be necessary to know ptime to decode RTP or
        vat audio, and it is intended as a recommendation for the
        encoding/packetisation of audio.  It is a media attribute, and is
        not dependent on charset.

It is clearly spelled out in the first sentence.

Cheers

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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