Ensure Acoustical Quality of Cellular Phones in Just Seconds
By Dave Whipple, Agilent Technologies
Despite the focus on data applications in cell phones, there is no doubt that audio is still the killer application. The audio quality of a mobile phone is one of its most important features. Three elements control the audio quality: The choice of the Voice Coder (Vocoder), the mechanical design of the phone, and the choice of the audio transducers, the microphone and the speaker.
The choice of Vocoder is made during the standards process, and is not something that can be changed in a particular design. The mechanical design includes audio isolation, coupling to the ear, and noise cancellation. The choice of speaker and microphone are important elements, also made as part of the development process. Qualification of a design is lengthy and difficult. The hard part is simulation of the human voice and ear, and how the human physiology must be taken into account when performing audio testing. Equipment for this is commercially available, but is expensive. The acoustic qualification of a phone is very time consuming.
Once a phone enters production, all of these elements have been qualified and are fixed. But are they? Will the test process catch a defective speaker or microphone? Agilent Technologies has introduced audio test capability to the Agilent 8960 wireless communications test set for use with either GSM or cdma2000 testing.
The idea is to excite the microphone with noise, and excite the speaker with electrical sounds that may be analyzed. Rather than duplicate the expensive, difficult testing used for qualification, a simple fixture may be used. The fixture is calibrated by using a known good copy of the design. The response is learned for this MS, and the testing of each device is compared to this.
One choice of acoustic test signal is an artificial voice called P-50, which is the name of the ITU standard that describes the generation process. The statistics of this matches real speech quite well. Areas that match include amplitude variations, frequency response, as well as probabilities over time of signals matching the amplitude and frequency of real speech. Matching these probabilities, though, takes a long time. That is not compatible with the manufacturing process.
A better alternative is to use a collection of discrete tones, which can later be analyzed using Fourier techniques. These tones cover the range of 300 to 3000 Hz. This signal is not ideal for modern Vocoders, but will detect defective transducers because of the comparison to a known good device. In addition, the signal used to test the speaker can be improved in the test set by turning off one element of the vocoder, the noise cancellation. This allows much better acoustic generation of sine waves.
The speaker is tested as shown in figure 1 and using the following steps: The acoustic file is generated in the test set, and sent to an internal vocoder, one that has noise cancellation turned off. The resulting audio is transmitted to the MS in a real radio link. A test fixture must have good acoustic coupling to the speaker of the MS. Typically this is a ring of foam rubber that compresses against the body of the MS, with a microphone suspended above the speaker. The volume of the speaker is actually quite low; an air seal is necessary to hear its output. The signal from the microphone is connected to the audio input of the test set, where the frequency response is computed.
Figure 1. MS Speaker Test.
The test of the microphone does the same steps, but in the opposite direction, as shown in figure 2. The audio is generated in the test set, and sent out the audio out port. This drives a speaker that is fixed near the microphone. The output of the vocoder in the MS is then transmitted to the test set via the RF link, where the audio decode is performed, and the frequency response computed.
Figure 2. MS Microphone Test
By comparing both the shape and the amplitude of the analyzed signals against the reference test, it can be assured that the components in the MS being tested are a faithful copy of the qualified design. Shown in figure 3 is the result of a speaker test on a GSM phone.
Figure 3. Spectrogram of the speaker of a GSM phone
These tests are very fast, typically less than one second each. During the manufacturing process phones are already tested in a real link with the test set, so addition of these tests does not require any additional setup time. These tests do require a specialized fixture, but this is not a difficult design. The use of audio testing as allowed in the 8960 wireless communications test set will ensure better quality in production phones.