Send Loudness Rating with ITU Real Speech Test Sequence

The purpose of this sequence is to measure the Send Loudness Rating (SLR) following the ITU-T P.79 standard

Introduction

The purpose of this sequence is to measure the Send Loudness Rating (SLR) following the ITU-T P.79 standard. This sequence can be used with handsets, headsets, and conference call devices. First, real speech from the ITU-T P.501 standard is played out of a mouth simulator. The Device Under Test (DUT) microphone then captures the signal and transmits this back to SoundCheck. SoundCheck calculates the frequency response function in 1/3 octaves and calculates SLR based on that frequency response.

Loudness Rating is a fundamental metric used in telephony. Loudness ensures that the levels passed between the DUT and the network match. If the levels do not match the potential for voice signals to become clipped, distorted, or under ranged becomes a major issue and will lead to poor voice quality reproduction. All communication devices (e.g. mobile phones, landline phones, wired headsets, Bluetooth speakerphones, conference call phones) must use Loudness Rating to set the level of the DUT correctly.

Required Software

• 2007 – Telephone Loudness Rating
• 2013 – WAV File Equalization Module
• 2033 – Active Speech Level

Required Hardware

• Head and torso simulator with mouth – B&K HATS or similar
  Or
• ITU-T P.51 compliant mouth simulator – GRAS 44AB or similar
• Audio Interface – Listen AmpConnect or similar
• Digital Interface – Virtual Audio Cable plus Reference Softphone (e.g. Linphone, Bria 5) for VoIP phones, Portland Tool & Die BTC-4148 for Bluetooth devices, USB port on PC for USB devices, etc.

Setup Diagram

Hardware Setup and Calibration

1. . In the Hardware > Audio tab be sure to use a Vp that follows your codec overload point and A/D = Analog. The below screenshot uses a Vp of 1.572, which is the overload point of an “A Law” codec. Other codecs will use different Vp values, as an example:
   a. “u Law” = 1.579 Vp
   b. ITU-T G.722 = 2.183 Vp
   c. CVSD = 1.094 Vp (Bluetooth codec)
2. The Input Signal Path is “Send (DUT to SC)”. This is the electrical signal from the DUT‟s microphone that is analyzed by SoundCheck. This Signal Path must use the Send (DUT to SC) calibrated device file which applies 1V/V sensitivity and dB reference 775 mV.
3. The Output Signal Path used is Mouth Sim. Make sure to calibrate the mouth using the Speaker Equalization calibration sequence run from the Calibration Editor. Instructions can be found in the SoundCheck User Manual.
4. Note: the stimulus used in this sequence is set to 89.7 dBSPL Active Speech Level (i.e. -4.7 dBPa) at the point which the mouth simulator was equalized. This feature requires module 2033 Active Speech Level to work. Most mouth simulators are equalized at the Mouth Reference Point (MRP), or 25mm from the Lip Reference Point (LRP).
5. Note: the stimulus step applies the mouth simulator equalization to the WAV file before it is played out of the mouth simulator. This feature requires module 2013 WAV File Equalization to work. It is important that you apply the mouth equalization otherwise the frequency response, and corresponding SLR, will be incorrect.

Get Sequence

This sequence is included in the Sequences folder of your SoundCheck installation.