Open-Loop Pitch Search (OLP)

Three functions are provided for OLP search. Use of these functions is mutually exclusive, that is, only one is appropriate for an application during any given frame type. The appropriate choice of an OLP search function depends upon the state of the DTX and VAD modules. The OLP search functions should be applied as follows:

Encoder Mode	Appropriate Function
DTX disabled	ippsOpenLoopPitchSearchNonDTX_GSMAMR
DTX, VAD 1 enabled	ippsOpenLoopPitchSearchDTXVAD1_GSMAMR
DTX, VAD 2 enabled	ippsOpenLoopPitchSearchDTXVAD2_GSMAMR

The OLP search functions extract a pitch estimate from a weighted version of the input speech. For 5.15 and 4.75 kbps frames, the search is performed once per frame. For all other modes, the search is performed twice per frame. A unique search is employed for 10.2 kbps frames. The OLP search details are as follows:

If the transmission bit rate is 10.2 kbps,

a. Compute a windowed autocorrelation of weighted speech, that is,

where the sequence sw(n) contains weighted speech, and w is the weighting function given by

In this weighting function, wl emphasizes low pitch, and is defined in terms of the table cw, while wn is a sequence of neighboring emphasis lag coefficients associated with the previous frame, that is

and the parameter T_old is the median filtered pitch lag of 5 previous voiced speech half frames.

The estimated open-loop pitch lag is the value k that maximizes R(k). It is denoted by T_op.

b. Compute the optimal open-loop gain using the relation

In addition,

and

If g > 0, the previous pitch lag buffer and v median pitch lag of the previous pitch lags are updated.

2. For rates other than 10.2 kbps, the following OLP search procedure is employed:

a. Three maximums are found in three different ranges for the correlations given by

For 5.15 and 4.75 kbps frames, length = 160.

For all other bit rates (except 10.2 kbps), length = 80.

b. Normalize the three maximum correlations according to

c. Determine the best open-loop lag using the following rule:

T_op = T₁, M(T_op) = M₁, if M₂ > 0.85 M(T_op);

M(T_op) = M₂, T_op = T₂, if M₃ > 0.85 M(T_op);

T_op = T₃.

Each of the OLP search functions implements the rate-dependent search algorithms described above. Next, details are given for non-DTX, VAD1, and VAD2 OLP search functions.