5.1.7 Bandwidth decision

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

The following decision logic is identical for CLDFB and DCT versions of energy calculations, except for some constants which were adapted to get similar detection results.

The long-term CLDFB energy mean values for NB, WB and SWB are updated as follows:

()

where the superscript [-1] has been used to denote the value of in the previous frame. The update takes place only if the local SAD decision is active and only if the long-term background noise level, , is higher than 30 dB.

The values are compared to certain thresholds also taking the current maximum values into account, which leads to increasing or decreasing counters for each bandwidth as described below in the flowchart.

Figure 6: Increasing and decreasing of BW counters

The tests in the above diagram are performed sequentially from top to bottom. The BW counters are then used to decide the actual signal bandwidth, BW, according to the logic described in the following schematic diagram.

Figure 7: BW selection logic

In the above diagram, the tests are performed in a sequential order, i.e. it could happen that decision about signal bandwidth is changed several times in this logic. After every selection of a particular bandwidth, certain counters are reset to their minimal value 0 or to their maximum value 100.

Finally, the resulting bandwidth can be upper limited in case the codec performance has not been optimized for it at particular bitrate. For example, at 9.6 kbps, the codec supports coding up to SWB. Therefore, if the detected bandwidth is FB, it is overwritten to SWB at this bitrate. The following table shows the range of bitrates for which the codec performance has been optimized for each bandwidth.

Table : Optimization of the codec performance per bandwidth

bandwidth	bitrate range [kbps]
NB	7.2 – 24.4
WB	7.2 – 128
SWB	9.6 – 128
FB	16.4 – 128