STI Audio Quality Test

 

Overview

 

The Survey Technologies, Inc. Audio Quality Test is designed to measure distortion and noise in the audio spectrum of a radio link.  The option includes three hardware elements, a tone generator, A/D converter and a low pass filter amplifier, which are used with Field Test 4 versions 5.2 or later and AQT driver.

 

Diagram of the hardware used in the Audio Quality Test

Tone Generator

 

            The audio generator is based upon a DTMF generator integrated circuit. The tone generator can be set to create either or both tones at frequencies of 940 Hz and 1209 Hz.  The generator includes circuitry to adjust amplitude, equalize the amplitude of the two tones and filter any harmonics of the tones.  It is contained in a handheld box and powered by a small 110Volt AC to 12 Volt DC power supply.  Output voltage of the generator is adjustable from 0 to 1.5 Volts P-P.  Output impedance is in the 1 kOhm range. 

 

Audio Filter Amplifier

 

            The low-pass filter, unity gain amplifier is used as an interface between the customers’ mobile radio audio output and the A/D input.  It filters frequencies above 5 kHz from the mobile.  The input and output impedance of the amplifier is in the 1 kOhm range and it has a dynamic range of 5 Volts P-P.

 

STI-9400, ADT Driver

 

            The laptop PC must have Field Test 4 version 5.2 or later, the Audio Quality Test driver and an 8-channel A/D converter installed in a PCMCIA slot.  All application specific software for this option is included with the AQT option.  Up to 4 mobile radios may be interfaced to conduct Audio Distortion tests and RSSI measurements with this option if the laptop has adequate processing power and PCMCIA bus bandwidth. 

Audio Quality Test

 

            The Audio Quality Test option combines the ability to measure RSSI voltage, converted to signal strength through a calibration table, with the ability to digitize a sample of audio signal and compute SINAD from the sample.

           

            Either one or both of the two tones are broadcast from the site under test using the audio generator connected to the site audio input.  The amplitude must be adjusted to obtain a fully modulated, low distortion signal from the mobile.  Audio from the mobile is passed through the low-pass filter then fed to one of 4 A/D inputs.  The audio signal is sampled at 10,000 Hz rate for about four tenths of a second producing 4096 samples.  The resulting amplitude array is passed through an FFT algorithm to convert the time samples to frequency domain samples.  This conversion produces a Nyquist frequency of 5,000 Hz and a frequency resolution of 2.5 Hz.  The low-pass filter reduces harmonics above the Nyquist frequency of the sampled data.

 

            The AQT algorithm then adds the total spectral energy from 100 Hz to 6 kHz and compares it with the energy in the band containing the tone(s).  The ratio of this energy in dB is SINAD, the Audio Quality Measurement.

 

            The choice between one or two tones is affected by the number and energy of harmonics generated by system distortion.  For example, under the effects of harmonic distortion a single 1 kHz tone test will create 4 evenly spaced harmonics at reduced amplitudes below the Nyquist frequency.  By comparison, two tones under the effects of harmonic distortion will cause 8 harmonics as well many more sums and differences to these tones and their harmonics.  While multi-path distortion is not well described by harmonic distortion, experiments have shown that two-tone tests increase the sensitivity to this form of distortion as well.  Since normal audio is composed of many frequencies, the two-tone test is a closer simulation and increases the sensitivity of the Audio Quality Test.

 

RSSI Measurement

 

Signal strength measurements may also be sampled from an RSSI Voltage obtained from the mobile.  RSSI can be calibrated in dBm through a calibration table, which the system uses to convert measured volts to dBm.  During the automated measurement process, RSSI measurements are interleaved with Audio Quality measurements. 

 

Additional Features

           

            For diagnostic and setup purposes time and frequency domain displays are available for each of the 4 channels.  In addition, raw audio samples can be saved and analyzed in depth after the drive test is completed.

         

 

   Frequency Domain                                             Time Display