HEAD acoustics "Quality Standards"

Hands-free Telephones
Voice over Internet Protocol
Network Echo Cancellers

Widespread use of digital signal processors in telephony results in numerous non-linear characteristics, such as switching, echo cancellation, VAD, AGC, and others.

Extensive subjective evaluations performed by HEAD acoustics for the ITU-T have identified the parameters judged to be most important for the voice quality of time-variant devices. Based on these assessments, objective tests have been devised to measure those parameters for the following situations:

Hands-free Telephones

HQS-HFT HEAD Quality Standard for Hands-free Terminals

Hands-free telephones require a specific methodology to evaluate their transfer characteristics. Standard parameters such as loudness ratings, frequency responses, terminal coupling loss etc. can not be used to determine the subjectively relevant quality parameters.

The quality of a hands-free telephone is mainly determined by parameters such as duplex capability, quality of background noise transmission, switching and echocancelling characteristics. These parameters were found to be the most subjectively relevant ones based on several years of testing and research by HEAD acoustics. To evaluate these parameters, new test signals and test procedures were developed and included in the HEAD Quality Standard for Hands-free Terminals. Tests cover the following aspects of hands-free performance:

• Delay measurements in sending and receiving directions
• Level switching characteristics in single talk
• Frequency response and loudness ratings in sending and receiving directions
• Distortion
• Simulated double talk
• Attenuation of level switching device
• Switching times
• Sensitivity to speech-like signals
• Background noise transmission
• Echo measurements in single talk and double talk
• Recordings with real speech

Voice over Internet Protocol

HQS-IP HEAD Quality Standard for IP Terminals, Gateways and Networks

Speech quality assessment in the IP network is a challenging topic due to the various types of signal processing involved. Conversational speech quality ins significantly influenced by echo cancellers working together with non-linear processors, various kinds of speech coders and voice activity detection used to reduce bandwidth, jitter buffers and their control along with packet loss concealment to handle typical transmission degradations introduced by the IP network, and the transmission delay itself.

HQS-IP is developed, optimized, tested and successfully used for this purpose. Tests implemented in HQS-IP cover all aspects of conversational speech quality, such as:

• Delay measurements in sending and receiving direction
• Listening speech quality tests under single talk conditions in sending and receiving direction
• Echo performance tests
• Detailed evaluation of double talk performance
• Quality of background noise transmission

Network Echo Cancellers

HQS-EC HEAD Quality Standard for Echo Cancellers

Speech quality assessment of echo cancellers is quite a challenge due to the various kinds of signal processing involved (e.g. adaptive filters and non-linear processors in telephones networks, or various speech coders, voice activity detection, and other signal processing in VoIP scenarios).

Additionally, echo cancellers typically also introduce undesired artifacts such as temporal clipping, limited double-talk performance or modulation of background noise. All these aspects have a significant influence on conversational speech quality. However, current national and international standards are not suited to assess the corresponding parameters.

To solve this problem, HQS-EC has been developed by HEAD acoustics, providing advanced tests for the analysis of:

• Speech transmission quality
• Echo
• Double talk performance
• Quality of background noise transmission

For manufacturers, HQS-EC offers objective guidelines to optimize their echo cancellers. For administrators, it provides selection criteria to ensure a high quality for their systems and services.