Quantitative Analysis of Portable Audio Devices

introduction

The special requirements of portable audio devices are key to product design. Why is product A better than its competitor product B and more ideal to use? From a performance perspective, the frequency response flatness and THD+N indicators between competing products are not much different, so it is difficult to distinguish which product has better performance. The user interface can judge the main differences between products, but this depends largely on subjective evaluation. We can use objective audio performance indicators to compare products and explain why one product is clearly better than others.

An important metric for evaluating audio performance is the presence of "clicks" or other strange transient noises in headphones (or speakers) when the device is turned on or off. As people's expectations for product performance increase, the absence of transient noise has become an important criterion for people to choose products, and therefore a key selling point for portable audio devices. Until now, the industry has still evaluated this click from a subjective perspective, and descriptions such as "low click" and "clickless operation" represent subjective judgments on the quantitative analysis of clicks. However, user expectations are changing, and designers need objective metrics for judging clicks.

This article describes a method for quantitatively expressing click-and-pop parameters that allows for repeated comparisons of products across different products.

Characteristics of the Clicking Sound

Click-and-pop refers to the audible transients that occur in headphones or speakers when an amplifier drives a transducer to turn on or off. In portable applications, where reducing power consumption is key to extending battery life, certain functional blocks are often disabled when they are not needed. This functionality can further accentuate the problem of click-and-pop. While an ideal component should not have any audible output when the device is turned on or off, in practice, all audio amplifiers will produce click-and-pop. Depending on the sensitivity of the transducer used (speaker or headphone), the distance between the transducer and the ear, the amplifier's ability to handle transients, and the sensitivity of the hearing sense, the click-and-pop may or may not be audible. Although there are many factors involved in determining the audio threshold, amplifier output specifications (independent of the audio transfer function) can be used to quantitatively compare product performance.

Table 1 lists the factors that may cause amplifier signal transients.

Maxim divides audio tests into two categories to properly measure K _CP measurements. Referring to Table 1 above, Items 1 (power-up) and 2 (power-down) are Category A. It is generally assumed that Maxim products with shutdown (SHDN) have a transient mode controlled by the shutdown pin (or register bit) when powered up under normal operating conditions. Category A does not represent normal use and is only relevant when measuring devices that cannot be shut down by software control. Items 3 and 4 (Category B measurements) are closer to normal use.

Figures 1 and 2 show (in the time domain) the transients of two different headphone amplifiers exiting shutdown. Comparing the first AC-coupled headphone amplifier with the second DC-coupled headphone amplifier, the AC-coupled headphone amplifier produces a larger transient when exiting 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, that of a DC-coupled headphone amplifier (Figure 2), appears to be buried in the oscilloscope's noise floor before A-weighting filtering. For this amplifier, most of the audio comes from the DC offset voltage generated when it goes from shutdown to full operation. Since the offset is only a few millivolts, the magnitude of the click and pop cannot be accurately determined with the unfiltered signal. With A-weighting filtering, the click and pop generated by the DC-coupled headphone amplifier offset is extracted from the noise floor, resulting in a more objective measurement. (Note that the V/div scale of the filtered signal is not shown.)

There are two aspects to consider when analyzing this problem. First, how can transients be measured objectively? Second, if necessary, what criteria should be used to measure the test results?

Click Test Method

Maxim uses Audio Precision's System 1 and System 2 (recommended) audio analyzers to measure click-and-pop (Figure 3), but similar test equipment from other vendors can also be used. The recommended metric, K _CP, is an objective measure of click-and-pop in audio amplifiers.

To begin the measurement, connect the device under test (DUT) output to a load or simulated load (dummy load). Load the required SHDN and power supplies on the DUT and AC-couple all DUT inputs to ground. No input signals are required; input stimulus consists of control signals that switch the DUT between various operating or inoperative modes. Connect the DUT output to the analog analysis section of the audio analyzer.

Next, select the analyzer's A-weighted filtering (recommended) or an unweighted 22Hz to 22kHz filter to limit the measurement bandwidth to the audio range. Note that the fast, high-level transients on the oscilloscope do not indicate how much energy is present in the audio band. The human ear has a very limited frequency response to transients from a speaker or headphone. Therefore, adding A-weighted filtering (Figure 4) can help the analysis by enhancing the frequency components to which the human ear is sensitive. Some audio analyzers do not have the option of A-weighting, in which case the bandwidth should be limited to the frequency response of the human ear. A common limit in audio test equipment is 22Hz to 22kHz, with a bandwidth-limiting filter that achieves a flat response around 20kHz (usually the upper limit of the human ear).

Set the detector to read peak values ​​(rather than RMS values) and set the detector to sample at 32 times per second. For signals such as transients that we want to acquire, RMS detection is not useful. The System 2 analyzer supports higher sample rates, but 32 samples per second allows for equivalent measurement options from the System 1 audio analyzer. (32 samples per second is the fastest acquisition setting in the System 1 model.) Disable the audio analyzer's auto-range adjustment circuitry and manually select the range that accurately tracks the expected peak signal amplitude. The ranges for both the System 1 and System 2 analyzers are 1x to 1024x (0 to 60.21dB) with a step size of 4x (12.04dB). For accurate measurements, a 1X/Y range is the recommended starting point for audio amplifier click-and-pop measurements.

Drive the SHDN pin with a low-frequency square wave to make repetitive measurements. The SHDN cycle frequency should be below the audio band and long enough to capture all turn-on and turn-off events (some models have longer turn-on delays). Maxim typically chooses a 0.5Hz period.

The analyzer's histogram option makes it easy to monitor DUT transients as they occur between operation and shutdown. The peak voltage can be easily determined and the histogram can be quickly reset during measurements. The peak voltage is recorded in dBV (dB relative to 1V). This specification is K _CP .

The Importance of Test Equipment

The above test method can support the comparison of similar devices and produce repeatable and objective results. It is best for the test equipment to maintain a linear response to any input size. For example, the peak reading when testing a 1mV impulse response should be 40dB lower than the peak reading when testing a 100mV impulse response with the same pulse width. (See the test transient calibration in the appendix).

An oscilloscope with external filtering can be used for this click-and-pop measurement solution. However, experience has shown that the typical click-and-pop levels of high-quality headphone amplifiers are in the millivolt range, which is difficult for most oscilloscopes to measure accurately. An oscilloscope can be used to test higher voltage devices such as high-power amplifiers.

Average value of repeated tests

Different parts of the same model may produce different test results. Therefore, multiple parts should be tested to equalize such differences before judging the performance of a particular part. For a properly designed DC-coupled headphone amplifier, most clicks and pops are proportional to the input offset voltage, which will vary from part to part unless it is equalized (or otherwise eliminated). When fully testing a particular part, measure transients in each operating mode multiple times to ensure consistent results. Then, calculate the average. Multiple measurements are recommended if the part is to be put into use. Test all channels of a stereo or multichannel product.

Establishing Absolute Voltage Levels

The absolute voltage level of the click-and-pop should be specified based on the actual application of the amplifier. For example, assume a device generates a -50dBV transient when it turns off. If the DUT is a 50W/8 power amplifier, the full scale is +29dBV. The ratio of the perceptible click-and-pop to the maximum peak voltage of the amplifier is:

-(+29 - (-50)) = -79dB

However, if the DUT is a 20mW/16 headphone amplifier, the full scale will be approximately -1.9dBV, which will be smaller relative to the peak voltage ratio: -48.1dB.

Setting indicator levels

Now that we have shown how to obtain an objective measure of click-and-pop, one question remains: How accurately?

Consider the following question. After measuring two headphone amplifiers using the above method, you get repeatable Class B click-and-pop suppression results, with the first amplifier having a K _CP of -59dBV and the second having a K CP of -61dBV. Is the second amplifier really much quieter than the first? Or are both results acceptable? Measurements are objective, but the understanding of "acceptable" is still subjective.

An acceptable, detectable level of click-and-pop suppression depends on several factors: the efficiency of the headphone/speaker under test, the typical distance from the human ear to the transducer, the SHDN cycling frequency, and the background noise level during listening.

Although many factors affect the establishment of acceptable click-and-pop levels in many applications, we can still define a reliable benchmark. Note the test results for Class B click-and-pop in Maxim headphone amplifiers (Table 2). All tests were performed with a 32 Ω load resistor, and each K _CP value represents the average of four samples per port.

The above data is the result of Maxim's test of K _CP performance. To ultimately eliminate the subjective factor in amplifier performance testing, Maxim recommends that other semiconductor suppliers adopt this method and the defined K _CP parameter.

Appendix Calibration Equivalent Equipment

The objective test solution for obtaining click-and-pop performance metrics in this application note uses Audio Precision's System 1 and System 2 audio analyzers. If a System 1 or System 2 analyzer is not available, the following approach can be used.

K _CP performance measurements can be made using equivalent test equipment from other manufacturers. Figure A shows the general test setup of an audio analyzer and the DUT.

Before recording test results and making direct comparisons, the test setup should be calibrated. In addition, it is necessary to verify that the total energy recorded by the equivalent analyzer is, in fact, linear with the input amplitude. Only in this way can the energy of the click and pop be accurately recorded, especially when fast rising transients occur in the audio band. A simple calibration requires a function generator and an equivalent analyzer. (See Figure B for an example.) Perform the calibration as follows: 1 Load a 0.5Hz square wave of known amplitude to the input of the equivalent audio analyzer. 2 Set the equivalent analyzer to detect peak voltage after A-weighting. 3 Record the peak voltage readings for various input signal amplitudes.

Table A below shows the calibration results for System 2 Audio Precision audio analyzer set to A-weighted, sampling 32 times/sec. The 1X/Y auto range setting for input signals from 1mV _PP to 40mV _PP produces a 6dB weighting factor. This 6dB weighting factor is related to the A-weighted limited transfer function of the Audio Precision analyzer. For input signals greater than 40mV _PP , the calibration results become nonlinear for this particular setting. This range is suitable for most amplifiers.

This calibration can be applied to an equivalent analyzer to ensure accurate click-and-pop performance measurements. In addition, by determining the same calibration values ​​and the appropriate input signal range, the click-and-pop performance specifications of the two amplifiers can be accurately compared using an equivalent audio analyzer.