PointProcess: Get jitter (ppq5)...

A command that becomes available in the Query submenu when you select a PointProcess object.

This command will write into the Info window the five-point Period Perturbation Quotient, a jitter measure defined as the average absolute difference between an interval and the average of it and its four closest neighbours, divided by the average interval (an interval is the time between two consecutive points).

As jitter is often used as a measure of voice quality (see Voice 2. Jitter), the intervals are often considered to be glottal periods. For this reason, the command has settings that can limit the possible duration of the interval (or period) or the possible difference in the durations of consecutive intervals (periods).

1. The command window

Time range (s)
the start time and end time of the part of the PointProcess that will be measured. Points outside this range will be ignored.
Period floor (s)
the shortest possible interval that will be used in the computation of jitter, in seconds. If an interval is shorter than this, it will be ignored in the computation of jitter (and the previous and next intervals will not be regarded as consecutive). This setting will normally be very small, say 0.1 ms.
Period ceiling (s)
the longest possible interval that will be used in the computation of jitter, in seconds. If an interval is longer than this, it will be ignored in the computation of jitter (and the previous and next intervals will not be regarded as consecutive). For example, if the minimum frequency of periodicity is 50 Hz, set this argument to 0.02 seconds; intervals longer than that could be regarded as voiceless stretches and will be ignored in the computation.
Maximum period factor
the largest possible difference between consecutive intervals that will be used in the computation of jitter. If the ratio of the durations of two consecutive intervals is greater than this, this pair of intervals will be ignored in the computation of jitter (each of the intervals could still take part in the computation of jitter in a comparison with its neighbour on the other side).

2. Usage

The jitter can be used as a measure of voice quality. See Voice 2. Jitter.

3. Algorithm

The five-point Period Perturbation Quotient (PPQ5) is defined in terms of five consecutive intervals, as follows.

First, we define the absolute (i.e. non-relative) PPQ5 (in seconds):

absPPQ5(seconds) = ∑i=3N-2 |Ti - (Ti-2 + Ti-1 + Ti + Ti+1 + Ti+2) / 5| / (N - 4)

where Ti is the duration of the ith interval and N is the number of intervals. If an interval Ti-2 or Ti-1 or Ti or Ti+1 or Ti+2 is not between Period floor and Period ceiling, or if Ti-2/Ti-1 or Ti-1/Ti-2 or Ti-1/Ti or Ti/Ti-1 or Ti+1/Ti or Ti/Ti+1 or Ti+2/Ti+1 or Ti+1/Ti+2 is greater than Maximum period factor, the term |Ti - (Ti-2 + Ti-1 + Ti + Ti+1 + Ti+2) / 5| is not counted in the sum, and N is lowered by 1 (if N ends up being less than 5, the result of the command is undefined).

Second, we define the mean period as

meanPeriod(seconds) = ∑i=1N Ti / N

where Ti is the duration of the ith interval and N is the number of intervals. If an interval Ti is not between Period floor and Period ceiling, or if Ti-1/Ti or Ti/Ti-1 is greater than Maximum period factor and Ti+1/Ti or Ti/Ti+1 is greater than Maximum period factor, the term Ti is not counted in the sum, and N is lowered by 1; this procedure ensures that in the computation of the mean period we use at least all the intervals that had taken part in the computation of the absolute PPQ5.

Finally, we compute the five-point Period Perturbation Quotient as

PPQ5 = PPQ5(seconds) / meanPeriod(seconds)

The result is a value between 0 and 4, or between 0 and 400 percent.


© ppgb, March 2, 2011