[AVTCORE] RFC3550: RTP Jitter value calculation

Nataraja Hosahalli <Nataraja.Hosahalli@dspg.com> Wed, 26 March 2014 12:56 UTC

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From: Nataraja Hosahalli <Nataraja.Hosahalli@dspg.com>
To: "avt@ietf.org" <avt@ietf.org>
Thread-Topic: RFC3550: RTP Jitter value calculation
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Date: Wed, 26 Mar 2014 12:55:58 +0000
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Subject: [AVTCORE] RFC3550: RTP Jitter value calculation
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Dear avtcore members,

I have a query on the RTP RFC 3550 which looks be an issue in RFC to me, please help by provide your expert advice.

RFC 3556 specifies 2 formulae for calculating RTP jitter in section 3.8. Couples of the RTCP RFC implementations are following these formulae.

          Section 3.8/ RFC 3550

      int transit = arrival - r->ts;

      int d = transit - s->transit;

      s->transit = transit;

      if (d < 0) d = -d;


One calculation option proposed,

        s->jitter += (1./16.) * ((double)d - s->jitter);

        rr->jitter = (u_int32) s->jitter;


other calculation option proposed
        s->jitter += d - ((s->jitter + 8) >> 4);

        rr->jitter = s->jitter >> 4;

RFC 3550 says both formulae are same. But if I look with sample numbers I see that the second formulae may not be appropriate.
The result of the formula "s->jitter += d - ((s->jitter + 8) >> 4)" LHS always keep increasing infinitely and could not be a reflecting an average value of jitter.

For example, in an hypothetical case if d = 2 each time, Jitter will be 2, 4, 6, 8, ... ever increasing, with the second formula.
Whereas with the first formulae each time it will be corrected towards d by fraction of 1/16. That is If d=2, rr->jitter will  be near to 2 in long run.

I request your opinion on this.

Thanks in advance.

Regards,
Nataraj

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Nataraja Hosahalli

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