Quality of Service Monitor for Voice Communications System

This system was used by a major satellite communications organisation for assessing the quality of service of a new mobile communications system that used low-rate voice coding techniques. Due to the low data rate the speech quality was below telephone standards, and the impairments were dependent on channel characteristics. The only way to assess the performance was to transmit a statistically significant number of pre-recorded speech phrases across the system (in each direction) and record the received phrases. These recordings were then passed to an organisation that specialised in subjective testing and statistical analysis of the results.

The system comprised:

Main Control Rack. This was located in the lab of the organisation conducting the tests. It contains a control PC with modem and audio monitor PSTN links to the earth station, two DAT recorders, and three Giastar units: an audio interface unit, a patch panel with distribution amplifiers, and a PC-controlled audio switch matrix. The PC (not shown) controls the entire system, providing both automatic and manual operation. Click for screenshots. The audio interface unit may be connected directly to the mobile earth station (MES) using either two-wire or four-wire hardwired connection, however this is frequently inconvenient for two reasons:
- The terminal requires line-of-sight to the satellite.
- Some terminals have hardwired handsets.
These problems are solved by the Remote MES Interface.
Remote MES Interface. This is a rugged interface unit, connected to the control rack by an umbilical cable that allows the MES to be sited in a convenient location some distance from the Main Control Rack. The cable interface carries signals for:
- Remote control from the Main Control Rack.
- Monitoring of MES status by the Control Rack.
- Forward and Return audio.
- Intercom
The lower section is a soundproofed anechoic chamber containing a handset test jig. The upper section contains the control and interface electronics, plus off-the-shelf measuring amplifier and graphic equaliser for calibration of the chamber transducers.
Earth station (LES) interface. The two units above were originally used to make calls between the MES and the public telephone network (PSTN). However, when going via some remote earth stations where the quality of the PSTN link back to the UK was dubious it was not obvious whether the satcomms link or the PSTN was the weakest link. This portable equipment, mounted in ruggedised flight cases, can be installed in a land earth station (LES) to allow forward signals to be transmitted and return signals to be recorded directly, with no PSTN leg in the audio link. The LES Interface is controlled and monitored via two PSTN links. The LES Interface comprises (from the top of the rack down):
- RS232 controlled multi-channel mains switch and watchdog. This is a standard Giastar product, the PMU1 .
- Huge idiot-proof installation instruction sheet.
- Sony 7010 DAT recorder.
- Control PC. Uses the Giastar DAA card for a PSTN monitor/intercom channel.
- Giastar Interface box, providing:
  - Modem control link.
  - Audio monitor channel, and intercom, between Main Rack and LES Interface.
  - Gain-controlled LES audio interface.
  - Audio and PSTN patch panels.
  - Various test and monitor functions.

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