Re: [AVTCORE] RFC3550: RTP Jitter value calculation

[Kevin Gross <[e-mail]>]

The formula looks basically correct. You need to look at the settling value
for rr->jitter, not s->jitter (which is a state variable). In your example,
s->jitter will settle at 32 and the correct value (2) will be reported for
rr->jitter.


I think the accuracy of the reporting could be improved by changing the
second line to:

        rr->jitter = (s->jitter + 8) >> 4;



Kevin Gross - AVA Networks


On Wed, Mar 26, 2014 at 6:55 AM, Nataraja Hosahalli <
[e-mail]> wrote:

>  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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>
> Mobile: [phone]
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