6 Requirements to be assessed by subjective tests

06.773GPPMinimum Performance Requirements for Noise Suppresser Application to the AMR Speech EncoderTS

6.1 Impact on Speech Quality

The following performance requirements are stated under the assumption that the noise suppresser is tested as an integral part of the AMR speech codec with the speech codec operating at the rates defined within the test plan (Annex C) .The performance requirements must be met for all these stated speech codec rates.

6.1.1 Initial Convergence Time

The initial convergence time shall be a maximum of T seconds with T equal to 2s. The definition of this time interval shall be understood strictly in accordance with its means of use in subjective listening experiments. Its use shall be defined by a process whereby the first T seconds of each sample processed through the AMR speech codec with and without noise suppression active, is deleted before presentation to listeners. It is assumed that this process does not reduce intelligibility, or introduce clipping or similar effects into the resultant speech plus noise material.

6.1.2 No Degradation in Clean Speech

The noise suppression function must not have a statistically significant distorting effect on clean speech, in comparison with the performance of the AMR codec without noise suppression applied. This requirement also applies when VAD/DTX is active.

The requirement is checked with the use of a paired comparison test where the requirement is met if AMR/NS is preferred or equal to AMR within the 95 % confidence interval.

6.1.3 No degradation of Speech and no Undesirable Effects in Residual Noise in Conditions with Background Noise (residual noise = background noise after AMR/NS)

The noise suppression function must not introduce any degradation of speech and no undesirable effects in the residual noise, when there is (acoustic) background noise in the speech signal. This requirement also applies when VAD/DTX is active.

The requirement is checked with the use of a modified ACR test with specific instructions where the requirement is met if AMR/NS is better than or equal to AMR within the 95 % confidence interval in all conditions.

6.1.4 Quality Impact compared to AMR

The AMR speech codec with noise suppression activated must produce an output in noisy speech which is preferred amongst test listeners with statistical significance, compared to the case where noise suppression is not used. This requirement also applies when VAD/DTX is active.

The requirement is checked with the use of a CCR test where the requirement is met if AMR/NS is preferred to AMR within the 95 % confidence interval in at least 4 of the 6 conditions tested. Preference or equality within the 95 % confidence interval is required for the remaining conditions.

Additionally, it is required that the subjective SNR improvement as measured by the methodology [Annex B] (where the measure is conducted on the associated CCR tests [Annex C] meets the following requirements:

(a) In at least 2 of the 6 conditions tested the SNR improvement shall not be less than 6dB within the 95% confidence interval.

(b) In at least 2 of the remaining 4 conditions the SNR improvement shall not be less than 4dB within the 95% confidence interval.