A quick test to ensure the sound quality of mobile phones

Although the main application of mobile phones is data communication, voice is undoubtedly still the most attractive killer application of mobile phones. Therefore, the sound quality of mobile phones is one of its most important features. Sound quality is affected by three factors: the choice of voice coder (Vocoder), the mechanical design of the mobile phone, and the choice of sound sensor (microphone and speaker).

The choice of voice coder is determined when the standard is formulated and cannot be changed by any application design. The mechanical design includes sound isolation, coupling path to the ear, and noise cancellation. The choice of speaker and microphone is also an important factor, but it is also determined during the development process. The verification of design quality is a long and difficult process. The difficulty lies in the simulation of human voice and ear, and how to take human physiological function factors into consideration when conducting sound testing. Although there are ready-made equipment on the market for such testing, they are generally quite expensive, and the process of verifying the sound quality of mobile phones is very time-consuming.

Manufacturing considerations

Once a cell phone is in production, all of these factors should have been verified and, if there are any, resolved, but is that true? Will the testing process catch a defective speaker or microphone? Agilent Technologies' Agilent 8960 wireless communications test instrument already has an audio test function that can be used for GSM or cdma2000 testing.

The idea is to excite the microphone with noise and the speaker with an electronic sound that can be analyzed. We don't need to worry about the quality of the fixture. The fixture needs to be calibrated with a known good design. The response of the good design is learned and then the test results of each cell phone are compared with it.

Sources

One possible test sound signal is an artificial sound called P-50, which is the name given by the ITU standard to describe the process of sound generation. The statistics of this sound show a very good match with real speech, including amplitude variation, frequency response, and the probability that the signal matches the amplitude and frequency of real speech over a period of time. However, comparing these probability of match takes a long time, which is not suitable for manufacturing processes. A

better alternative is to use discrete multi-carrier modulated signals (discrete tones), which can then be analyzed using Fourier techniques and cover the frequency range from 300 to 3000 Hz. This signal is not ideal for modern speech coders, but because it is compared to a known good device, it can detect a bad sound transducer. In addition, the signal used to test the speaker in the test instrument can be further improved by turning off the noise cancellation function of the speech coder, which can produce a better sine wave sound.

Figure 1 shows the speaker test method. The speaker is a type of speaker that is used to test the speaker. The speaker is a type of speaker that is used to test the speaker. The speaker is a type of speaker that is used to test the speaker. ? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie? The magpie?

Figure 1: Mobile phone speaker test

The test steps for microphones are basically the same, but in the opposite direction, as shown in Figure 2. The sound is also generated in the test instrument and then sent to the sound output port to drive the speaker fixed near the microphone. Then, the output signal of the mobile phone voice encoder is transmitted to the test instrument through the RF link to decode the sound and calculate the frequency response.

Figure 2: Microphone test of mobile phone

By comparing the shape and amplitude of the analyzed signal with the reference test signal, it can be determined whether the components of the mobile phone under test are reliable and qualified designs. Figure 3 shows the speaker test results of a GSM mobile phone.

Figure 3: Spectrum of the speaker of a GSM mobile phone

Conclusion

These tests are very fast, typically taking less than a second for each test. During manufacturing, the phone is already tested with the tester in a real connection, so adding these tests does not require additional setup time. However, these tests will require special fixtures, but they are not difficult to design. Using the audio test capabilities provided by the 8960 Wireless Communications Tester can ensure that the phone is manufactured with higher quality.