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Beep is the noise produced by an audio transient signal in a headset or speaker during the turn-on or turn-off of an amplifier that drives the converter . Until now, the industry has still subjectively evaluated this buzz. The descriptions of “lower humming†and “no humming work†represent subjective judgments on quantitative analysis of snoring. To eliminate subjective judgments that measure the performance of the audio amplifier, Maxim identified objective indicators that describe the hum. This paper describes the K CP of this indicator and the testing process of this parameter.
introduction
The special requirements of portable audio equipment are the key to product design. Product A is superior to its competitive product B, and what are the more ideal reasons for use? From the performance point of view, the frequency response flatness between competing products and THD+N indicators The difference is not big, it is difficult to distinguish which product performs better. The main differences in the product can be judged from the user interface, but this depends to a large extent on subjective evaluation. We can use objective audio performance metrics to compare products to illustrate why one product is significantly better than others.
An important metric for evaluating audio performance is the “click†or other strange transient noise that occurs when the device is turned on or off. As people's expectations for product performance increase, no transient noise becomes an important indicator of people's choice of products, and thus a key selling point for portable audio equipment. Until now, the industry has still subjectively evaluated this buzz, and the descriptions of “lower buzzing†and “no humming work†represent subjective judgments on quantitative analysis of snoring. However, the user's expectations are changing, and the designer needs to get an objective indicator of the buzz.
This article describes a method for quantitatively representing click parameters that allows for repeated comparisons of products in different products.
Humming characteristics
A click is an audio transient that occurs in an earphone or speaker when the amplifier drives the converter on or off. In portable applications, reducing power consumption is key to extending battery life, and is typically disabled when certain functional modules are not required to operate. This feature may further highlight the disadvantage of humming. When the device is turned on or off, the ideal component should not have an audio output, and in practice, all audio amplifiers will beep. Depending on the sensitivity of the converter (speaker or earphone) used, the distance of the converter from the human ear, the ability of the amplifier to process transient signals, and the sensitivity of the hearing, no clicks can be heard. Although determining the audio threshold involves many factors, the amplifier output metric (independent of the audio transfer function) can be used to quantitatively compare the performance of the product.
Table 1 lists the factors that may cause amplifier signal transients.
Figure 1 and Figure 2 (in the time domain) for the transient process of two different headphone amplifiers exiting the off state, comparing the first AC-coupled headphone amplifier with the second DC-coupled headphone amplifier, AC-coupled headphones A large transient occurs when the amplifier exits shutdown (Figure 1). This transient produces a noticeable low-frequency sound due to its slower turn-on process. (Note that the time scale is 100ms/div.)
The second transient process, the DC-coupled headphone amplifier (Figure 2), appears to be submerged in the noise floor of the oscilloscope before A-weighted filtering. For this type of amplifier, most of the audio comes from the dc offset voltage that is generated from turn-off to full operation. Since the offset is only a few millivolts, the unfiltered signal does not accurately determine the size of the click. After A-weighted filtering, the click noise generated by the DC-coupled headphone amplifier is extracted from the noise floor to obtain more objective measurement results. (Note that the filtered signal scale V/div is not displayed.)
There are two aspects to consider when analyzing this issue. First, how do you measure transients objectively? Second, what criteria are used to measure test results if needed?
Beep test method
Maxim uses Audio Precision's System 1 and System 2 (recommended) audio analyzers to measure clicks (Figure 3), as well as similar test equipment from other manufacturers. The recommended indicator K CP can objectively measure the click of an audio amplifier.
When starting the measurement, connect the device under test (DUT) output to the load or analog load (dummy load). Load the required SHDN and power on the DUT and AC-couple all DUT inputs to ground. No input signal is required; the input stimulus includes a control signal that the DUT switches between various operational or stop modes of operation. Connect the DUT output to the analog analysis section of the audio analyzer.
Next, select the analyzer's A-weighted filtering (recommended) or unweighted 22Hz to 22kHz filter to limit the measurement bandwidth to the audio range. Note that the oscilloscope's fast high-level transient does not indicate how much energy is present in the audio band. The frequency response of the human ear to the speaker or earphone transient signal is limited. Therefore, increasing the A-weighted filtering (Fig. 4) is more advantageous for analysis because it enhances the frequency components of the human ear. Some audio analyzers cannot use A-weighting, in which case the bandwidth of the human ear's frequency response should be limited. The limiting bandwidth commonly used in audio test equipment is 22 Hz to 22 kHz, and the bandwidth limiting filter can achieve a flat response of 20 kHz (usually the upper limit of the human ear).
Set the detector to a peak reading (instead of the RMS value) and set the detector to sample 32 times per second. For signals such as transients that we want to acquire, RMS detection has no effect. System 2 analyzers support higher sample rates, while 32 sample rates per second provide the same measurement options from System 1 audio analyzers. (32 sample rates per second is the fastest acquisition setting in the System 1 model.) Disable the audio analyzer's range auto-tuning circuit, and manually select to accurately track the expected peak signal amplitude. System 1 and System 2 analyzers range from 1x to 1024x (0 to 60.21dB) in 4x steps (12.04dB). For accurate measurements, it is recommended that the starting point of the audio amplifier beep measurement be in the 1X/Y range.
The SHDN pin is driven with a low frequency square wave for repeated measurements. The SHDN cycle frequency is lower than the audio band and the period should be long enough to ensure that all open and close events are collected (some models have longer turn-on delays). Maxim usually chooses a 0.5Hz period.
The analyzer's histogram option makes it easy to monitor DUT transients when transients occur between work and shutdown. The peak voltage can be easily determined and the histogram can be quickly reset during the measurement. The peak voltage is recorded in dBV (in dB relative to 1V). This indicator is K CP .
The importance of test equipment
The above test methods can support comparisons of similar devices, producing repeatable objective results. The test equipment is preferably capable of maintaining a linear response to any size input. For example, the peak reading when testing a 1 mV impulse response should be 40 dB lower than the 100 mV impulse of the same pulse width. (See Appendix for Test Transient Calibration).
An oscilloscope with external filtering can be used in this click measurement solution. However, experience has shown that high-quality headphone amplifiers typically have a click level in the millivolt range, which is difficult for most oscilloscopes to make accurate measurements. An oscilloscope can be used to test higher voltage devices such as high power amplifiers.
Average repeat test
Different devices of the same model may produce different test results. Therefore, multiple devices should be tested to determine this model performance to balance this difference. For well-designed DC-coupled headphone amplifiers, most of the clicks are proportional to the input offset voltage, and unless equalized (or otherwise eliminated), there are some differences in the input offset voltages of the different devices. When testing a model thoroughly, to ensure consistent results, multiple transients should be measured for each mode of operation. Then, calculate the average. Multiple measurements are recommended if the device is ready for use. Test all channels of stereo or multi-channel products.
Establish absolute voltage level
The absolute voltage level of the click should be specified according to the actual application of the amplifier. For example, suppose a device generates a -50dBV transient when it is turned off. If the DUT is a 50W/8 power amplifier, the full scale is +29dBV. Thus, the ratio of the perceived click of the amplifier to the maximum peak voltage is:
-(+29 - (-50)) = -79dB
However, if the DUT is a 20mW/16 headphone amplifier with a full-scale range of -1.9dBV, it will be small relative to the peak voltage ratio: -48.1dB.
Set indicator level
Although we have shown how to obtain an objective measure of the hum index, there is still a question: What is the accuracy?
Consider the following problem. After measuring the two headphone amplifiers using the above method, you get a repeatable Class B click suppression result. The first amplifier has a K CP of -59dBV and the second is -61dBV. Is the noise of the second amplifier really much smaller than the first one? Or are both results acceptable? The measurement is objective, but the understanding of “acceptability†is still subjective.
An acceptable and detectable level of click suppression depends on several factors: the efficiency of the headphone/speaker to be tested, the typical distance between the human ear and the converter, the SHDN cycle frequency, and the background noise level during listening. .
In many applications, we can specify a credible benchmark for benchmarks, although many factors can affect the establishment of acceptable hum levels. Note that the Maxim headphone amplifier B-type beep test results (Table 2), all tested with a 32-load resistor, each K CP value represents the average of four samples per port.
The above data is Maxim's test results for K CP performance. To finally eliminate the subjective factors in amplifier performance testing, Maxim recommends that other semiconductor vendors adopt this approach, as well as the defined K CP parameters.
Appendix calibration equivalent equipment
The objective test protocol for obtaining click performance indicators in this application note uses Audio Precision's System 1 and System 2 Audio Analyzers. If you do not have a System 1 or System 2 Analyzer, the following methods can be used.
K CP performance measurements can be made using equivalent test equipment from other manufacturers. Figure A shows the general test setup for the audio analyzer and DUT.
The test setup should be calibrated before recording the test results and directly comparing the results. In addition, the total energy recorded by the equivalent analyzer needs to be verified. In fact, this record is linear with the input amplitude. Only in this way can the energy of the click be accurately recorded, especially when there is a fast rise transient in the audio band. Simple calibration requires a function generator and an equivalent analyzer. (Refer to Figure B for an example.) Calibrate as follows: 1 Load a 0.5 Hz square wave of known amplitude at the input of the equivalent audio analyzer. 2 Set the equivalent analyzer to detect the A-weighted peak voltage. 3 Record peak voltage readings for various input signal amplitudes.
Table A below shows the calibration results for the System 2 Audio Precision Audio Analyzer set to A-weighted, 32-times/second sampling. The 1X/Y auto range is set to a 1mV PP to 40mV PP input signal that produces a 6dB weighting factor. This 6dB weighting factor is related to the A-weighted restricted transfer function of the Audio Precision analyzer. When the input signal is greater than 40mV PP , the calibration result is non-linear for this particular setting. This range is suitable for most amplifiers.
This calibration can be applied to the same analyzer to ensure accurate measurement of the click performance. In addition, after determining the same calibration value and the appropriate input signal range, an equivalent audio analyzer can be used to accurately compare the click performance of the two amplifiers.
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