Real-Time Vocoder

Last updated: September 2, 2009

Overview

The real-time vocoder allows you to perform the audio test in two aspects:

Perform the functional test for the analog voice capability of the mobile station.
Calculate the audio input gain, audio output gain and internal codec loopback gain of the Test Set, which help you to perform the acoustic measurement in 3GPP TS 26.131 and TS26.132.

Real-Time Encoding and Decoding

The real-time vocoder functionality allows you to perform the functional test for the analog voice capability of the mobile station when the Operating Mode of the real-time vocoder is set to Normal :

the test set can real-timely encode the analog audio signal applied at the test set's AUDIO IN connector using the appropriate vocoder algorithm (depending upon the selected voice service option) and then sends the encoded audio frames to the mobile station. See A Simplified Signal Flow for Real-Time Encoding .
Also, the test set can simultaneously decode the encoded audio frames that is received on the traffic channel using the appropriate vocoder algorithm (depending upon the selected voice service option) and then converts to an analog signal output at the test set's AUDIO OUT connector. See A Simplified Signal Flow for Real-Time Decoding .

A Simplified Signal Flow for Real-Time Encoding

A Simplified Signal Flow for Real-Time Decoding

Vocoder Calibration

The real-time vocoder functionality allows you to calibrate the audio output gain, audio input gain and internal codec loopback gain of the test set when the Operating Mode of the real-time vocoder is set to Calibration .

Audio Output Gain Calibration
The audio output gain calibration is performed when the Calibration Type is set to Audio out Path . The test set sends a full scale sinewave to the AUDIO OUT port. The audio output gain can be determined by measuring the output analog signal.

The audio output gain calibration result helps to perform the acoustic measurements of the mobile station, for example, the sending sensitivity frequency response.

Audio Output Gain Calibration
Audio Input Gain Calibration:
You can get the audio input gain of the test set in two ways:
- Option 1: Set the Calibration Type to Audio In Path . The test set samples the input signal from the AUDIO IN port and loops them back to the AUDIO OUT port via the DAC. If an audio output path calibration has already been done, the audio input gain can be derived from the loopback gain and the audio output gain.
- Option 2: Set the Calibration Type to Audio Out Path , looping the full scale sinewave from the AUDIO OUT port to the AUDIO IN port. As the audio output gain can be obtained by audio output path calibration, the audio input gain can be determined by reading the Encoder Headroom
The audio input gain calibration result helps to perform the acoustic measurements of the mobile station, for example, the receiving sensitivity frequency response.

Audio Input Gain Calibration
Internal Codec Loopback gain Calibration
The internal codec loopback gain calibration is performed when the Calibration Type is set to Internal Codec. The test set samples the input signal from the AUDIO IN port, encodes the samples with the voice encoder and loops them back to the voice decoder, then sends the signals to the AUDIO OUT port via the DAC.

The internal codec loopback gain calibration result helps to perform the acoustic measurements of the mobile station, for example, the echo loss.

Internal Codec Loopback gain Calibration

Setting Real-Time Vocoder Parameters

The following parameters can be set from the Real-Time Vocoder Parameters menu. To access the Real-Time Vocoder Parameters menu, see How Do I Use Real-Time Vocoder? .

Expected Audio In Peak Voltage
When the test set is in the real-timer vocoder mode (see Voice SO Mode ), Expected Audio In Peak Voltage sets the peak voltage of the analog audio input signal that can produce a full scale ADC output. It is used to adjust the audio input path gain before the ADC to produce a full scale ADC output. For example, if the input signal is 1V peak, then setting the Expected Audio In Peak Voltage to 1V will get the ADC output to be full scale. Then,
- If the input signal is less than 1V peak, then there will be an encoder headroom to show how close to the full scale on the ADC the input audio signal is (see Encoder Headroom ).
- If the input signal is more than 1V peak, then it is likely to clip the ADC. In this case, you can set the Expected Audio In Peak Voltage to a value more than 1V (e.g. 2V) to avoid the clip.
Whenever you change this setting, it is not applied until the test set is set to the real-timer vocoder mode.

GPIB command: CALL:RTVocoder:INPut:VOLTage[:EXPected][:SELected]
Maximum Audio Out Peak Voltage
When the test set is in the real-timer vocoder mode (see Voice SO Mode ), Maximum Audio Out Peak Voltage sets voltage of the peak analog audio output relative to the DAC full scale output. It is used to adjust the analog audio input path gain after the DAC to get a desired peak voltage output at the test set's AUDIO OUT port. For example, if the Maximum Audio Out Peak Voltage is set to 1V, then the peak output of a full scale sine wave will be 1 V.

Setting this value makes the test set adjust the audio output path gain to achieve the desired audio output level at the test set's AUDIO OUT port. However, the actual output signal also depends on the voice signal decoded from the received R-FCH/R-Traffic voice data. The decoded signal may not be normalized to DAC (Digital to Analog Converter) full scale range (The Decoder Headroom gives information on how close to full scale the current audio signal coming out of the vocoder is). So it is possible that the final signal level is lower than the configured level.

Whenever you change this setting, it is not applied until the test set is set to the real-time vocoder mode.

GPIB command: CALL:RTVocoder:OUTPut:VOLTage[:MAXimum][:SELected]
Encoder Data Rate Mode
Sets how to select the encoding data rate to produce the encoded data frames.
- When Auto is selected, the test set determines the encoding data rate based on the received signal level.
- When Fixed is selected, the test set uses the encoding data rate set by Encoder Data Rate .
- When Limited is selected, the test set uses the encoding data rate no greater than the setting of Encoder Data Rate .
GPIB Command: CALL:RTVocoder:ENCoder:DRATe:MODE[:SELected]
Encoder Data Rate
Sets the encoding data rate used when the Encoder Data Rate Mode is set toFixed or the maximum encoding data rate used when the Encoder Data Rate Mode is set toLimited .

GPIB Command: CALL:RTVocoder:ENCoder:DRATe[:SELected]

NOTE
This setting is only applied to the encoder. The decoder uses the data rate specified in the voice traffic data from the mobile station.
Operating Mode
Sets whether the real-time vocoder works at the normal or calibration mode when a voice call (such as SO1, SO3, SO17, SO68, SO70 or SO32768 , see Service Option ) is established and the Voice SO Mode is set to Real-Time Vocoder .
- When Normal is selected, you can perform the normal functional test for the analog voice capability of the mobile station.
- When Calibration is selected, you can calibrate the audio input gain, audio output gain and the internal codec loopback gain of the test set.
GPIB Command: CALL:RTVocoder:OPERating:MODE
Calibration Type
When the Operating Mode is set to Calibration , Calibration Type determines whether the audio output path, the audio input path or the internal codec loopback gain of the test set is calibrated.

GPIB Command: CALL:RTVocoder:CALibration:TYPE
Audio Out Frequency
When the Operating Mode is set to Calibration and the Calibration Type is set to Audio Out Path, Audio Out Frequency sets the frequency of the sinewave sent out by the test set.

GPIB Command: CALL:RTVocoder:OUTPut:FREQuency