A sidetone loss is appropriate for terminals using handsets and headsets in duplex calls. There are separate requirements for listener sidetone (LSTR) and talker sidetone (STMR). The listener sidetone performance is considered as the major parameter affecting the user perception of the system. Talker sidetone is important to give the user some comfort in using the terminal.
The value of the Listener sidetone (LSTR) shall not be less than 15dB. This shall be met at the nominal volume setting.
LSTR >= 15dB
Listener Sidetone
The sound field is calibrated in the absence of any local obstacles. The averaged field shall be uniform to within +4dB/-2dB within a radius of 0.15M of the Mouth Reference Point when measured in one-third octave bands from 100Hz to 8KHz (bands 1 to 20).
A calibrated half-inch microphone is mounted at the Mouth Reference Point and the sound field measured in one-third octave bands. The spectrum shall be pink noise [P.64, Annex C] to within +/-1dB and the ambient level shall be adjusted to 70dBA (-24dBPa(A)), tolerance +/-1dB.
The Artificial Mouth and Ear are placed in position and the handset mounted.
Measurements are made in one-third octave bands from 100Hz to 8KHz (bands 1 to 20). For each band the Sound Pressure Level at the Ear Reference Point shall be measured.
The listener sidetone path loss is expressed in dB and the LSTR shall be computed according to [P.79, formula 2.1]with m=0.225 and the weighting factors of [P.79, table 3].
The sidetone masking rating (STMR) shall be 13dB +/-5dB. This shall be met at the nominal volume setting and should ideally be independent of the volume control setting.
STMR = 13dB +/-5dB
Sidetone Masking Rating
This is measured using a pure tone signal with a level of -4.7dBPa at the Mouth Reference Point. The sound pressure level at the Ear Reference Point is then measured at each frequency given in [P.79, table 3, bands 1 to 20].
The sidetone path loss LmeST (dB) and the STMR (dB) is calculated using [P.79, formula 2.1] with m=0.225 and the weighting factors of [P.79, table 3, column 3].
The third harmonic distortion shall be less than 10%.
"Third Harmonic Distortion" < 10%
Sidetone Distortion
This is tested using a pure tone signal of -4.7dBPa at the Mouth Reference Point, The distortion is measured at the Ear Reference Point using frequencies of 315Hz, 500Hz and 1KHz.
This requirement is not expected to be difficult to comply with.
Copyright ©2002 Andy Fletcher andy@x31.com