ITU-T P.30 PDF

Introduction

Group Audio Terminals (GATs) are terminals which have been specifically designed to be used by several users.

GATs cover a wide range of products ranging from the hands-free telephone when it is used by several users, to the sophisticated teleconference studio.

The CCITT recommends that GATs satisfy the specifications¹ in this Recommendation.

GATs must also comply with Recommendation P.34 as far as loudness is concerned, when they are connected to the telephone network. If they use voice-activated circuits, Recommendation P.34 may also be applied. Such terminals are sensitive to the acoustics of the location where they are utilized and they may resort to sophisticated acoustical echo processing devices.

The first generation of GATs will operate mainly on 4-wire digital networks and will make use of the wideband (WB) speech coding algorithm specified in Recommendation G.722. Such terminals urgently need specifications that can be based on the present Recommendation.

A typical GAT configuration is represented in Figure 1/P.30.

Such a terminal includes one or several microphones, one or several loudspeakers, sending and receiving amplification. Optionally, it includes a sound-managing and mixing device to the loudspeakers and from the microphones, a coder-decoder for digital networks, a voice-activated gain processing device and an echo processing device.

The location where the GAT is to be used is very important. Several measurements defined in this Recommendation have to be made at the location where the GAT is to be used. These are referred to as "in situ" measurements. They are to be made with the full complement of equipment in the conference room, but with no conferees present.

The present Recommendation is divided into three parts:

- interconnection specifications,

- transmit specifications,

- near-end specifications.

Two test signals are used in this Recommendation:

- an acoustic test signal as defined in Recommendation P.50 (see Note): i.e. an artificial voice as defined in Recommendation P.50 produced by a sound source (an artificial mouth) as described in § 2 of Recommendation P.51 and,

- an electric test signal whose long-term spectrum is identical to the acoustic signal; when applied by a source with a matched internal resistive impedance, it provides a level of -22 dBV.

Both test signals are filtered in the transmission system bandwidth.

Note - The preferred acoustic signal to be used in the measurements for the audio alignment is defined in Recommendation P.50. However, other signals such as speech-shaped noise or pink noise may be used in some applications.

¹ The specifications in this Recommendation are subject to future enhancement and therefore should be regarded as provisional.