4. Audio Level

It is desirable to standardize the "on the wire" audio level for speech transmission to avoid users having to manually adjust the playback and to facilitate mixing in conferencing applications. It is also desirable to be consistent with ITU-T Recommendations G.169 and G.115, which recommend an active audio level of -19 dBm0.

Assuming linear 16-bit PCM with a value of +/-32767, -19 dBm0 corresponds to a root mean square (RMS) level of 2600. Only active speech should be considered in the RMS calculation.

If the endpoint has control over the entire audio-capture path, as is typically the case for a regular phone, then it is RECOMMENDED that the gain be adjusted in such a way that an average speaker would have a level of 2600 (-19 dBm0) for active speech. If the endpoint does not have control over the entire audio capture, as is typically the case for a software endpoint, then the endpoint SHOULD use automatic gain control (AGC) to dynamically adjust the level to 2600 (-19 dBm0) +/- 6 dB.

The RECOMMENDED filter for normalizing the signal energy is a second-order Butterworth filter with a 300 Hz cutoff frequency.

---