5.1.8 Time-domain transient detection

26.4453GPPCodec for Enhanced Voice Services (EVS)Detailed algorithmic descriptionRelease 15TS

The HP-filtered input signal including the look-ahead is input to the time-domain transient detector. The HP-filtered input signal is further high-pass filtered. The transfer function of the transient detection’s HP filter is given by

()

The signal, filtered by the transient detection’s HP filter, is denoted as . The HP-filtered signal is segmented into 8 consecutive segments of the same length. The energy of the HP-filtered signal for each segment is calculated as:

()

where is the number of samples in 2.5 milliseconds segment at the input sampling frequency. An accumulated energy is calculated using:

()

A transient is detected if the energy of a segment exceeds the accumulated energy by a constant factor of 8.5and the attack index is set to . If no attack is detected but strong energy increase is detected in segment, the attack index is set to and the frame is not marked as a transient frame.

The energy change for each segment is calculated as:

()

The temporal flatness measure is calculated as:

()

The maximum energy change is calculated as:

()

If index of or is negative then it indicates a value from the previous segment, with segment indexing relative to the current frame. is the number of the segments from the past frames. It is equal to 0 if the temporal flatness measure is calculated for the usage in ACELP/TCX decision. If the temporal flatness measure is calculated for the TCX LTP decision then it is equal to:

()

is the number of segments from the current frame. It is equal to 8 for non-transient frames. For transient frames first the locations of the segments with the maximum and the minimum energy are found:

()

If then is set to , otherwise is set to 8.