“Today, almost every personal Electronic device has integrated audio capabilities. Whether on busy city streets or in rural areas, people of all ages can be seen listening to music while doing their daily tasks. The sheer number of audio devices has jumped to astronomical numbers in the last five years. The average consumer has access to a far wider range of media sources than ever before, as well as a wider variety of devices to watch and play content.
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Today, almost every personal electronic device has integrated audio capabilities. Whether on busy city streets or in rural areas, people of all ages can be seen listening to music while doing their daily tasks. The sheer number of audio devices has jumped to astronomical numbers in the last five years. The average consumer has access to a far wider range of media sources than ever before, as well as a wider variety of devices to watch and play content. This digital media explosion is the result of the growing use of mobile phones, tablets, laptops and Bluetooth speakers worldwide. Many people have these devices in their homes, or at least a few of them.
Today’s consumers have become accustomed to high-fidelity audio transmission. Gone are the days of static and distracting distortion in sound systems. At the same time, there is a tendency for audio playback to move to the cloud. There are many streaming services now competing for listeners, all offering high-fidelity music for a fraction of the price of a CD collection. This means that each of us listens to a variety of online music, audiobooks and podcasts through multiple playback mediums. The sheer number of audio modules has also led to an increase in the need for audio test benches; audio test benches must not only be flexible enough to provide multiple functions, but also have high fidelity performance.
Going forward, the general trend in technology development is that the underlying technology does not change much, but the final product continues to improve to achieve better performance. The same trend applies to audio. Larger storage space, higher processing power, precision, clarity, and smaller size will all become common demands. The growing popularity of higher-quality audio with ordinary listeners is a manifestation of this trend. In the application area, one example that is expected to drive the demand for high-quality audio systems is speech recognition and voice commands. This technology is used in everything from voice search on smart devices to home automation systems. In order for smart devices to distinguish user commands from noisy environments, high-quality hardware is required. The key metrics for such systems are the achievable dynamic range and the ability to filter out noise and interference. Applications such as video conferencing, virtual presence and virtual reality all require the suppression of background noise to a lower level to improve the user experience.
design challenge
The increase in the absolute number of audio modules produced has also created a “pain point” for manufacturers: how to test each device quickly and efficiently, while ensuring that its performance is not compromised? From a device manufacturer’s perspective, increasing the speed at which tests can be performed clearly reduces costs. However, the quality of the final product cannot be compromised. With these requirements in mind, the key to designing an audio test bench for modern audio equipment lies in the desire to increase performance on the one hand, and the desire to perform tests at uncommon speeds on the other. Audio test benches need to achieve lower levels of noise and distortion in order to quickly and accurately measure the device under test.
The gap between the needs of the average consumer and the needs of professional studio equipment is narrowing. For example, consumers generally prefer higher quality and more sophisticated headphones. It would be very useful to have an all-in-one audio test bench. Typical CD sound quality requires a dynamic range of 92 dB to 96 dB, while analog microphones and professional-grade speakers require more than 120 dB. To be future-proof, audio test benches need to cover a wider dynamic range, with faster test times and higher test throughput. This audio test platform is ideally positioned to address both current consumer-grade needs and future trends toward higher fidelity systems.
find a solution
Finding a one-size-fits-all solution can lead to many design challenges. To incorporate these requirements into a modern audio test platform, test system designers need to overcome many constraints. Some of these constraints include system cost, design size, and power consumption. In many cases, system performance is determined by device performance constraints related to capital or thermal budgets. ADI’s multi-channel 24-bit Σ-Δ ADC, the AD7768, can help overcome these common limitations. Available in 4- and 8-channel versions, the device has excellent signal-to-noise ratio (SINAD) performance and many other features, making it ideal for audio testing.
Suitability of AD7768/AD7768-4 24-bit ADC
24-bit (and above) ADCs are typically used for high-fidelity audio equipment in the audio test field because their 24-bit precision enables a wider dynamic range. Low distortion and excellent noise immunity are necessary to ensure signal integrity. ADI’s new 24-bit Σ-Δ ADC, the AD7768, offers excellent noise and THD performance on all eight channels, and the output data rate can be adjusted according to the audio bandwidth. This makes it an ideal device for the audio field. Typical THD performance is C120 dB.
The wideband, low ripple digital filters of the AD7768/AD7768-4 are also suitable for audio applications. It supports six different decimation rates, and users can choose flexibly according to the target bandwidth. The AD7768 has a stopband attenuation of 105 dB, which provides a steep brick-wall frequency response while keeping the signal of interest unchanged while keeping noise low.
The ability of the AD7768/AD7768-4 to fit into an audio testbench makes it a prominent audio testbench backbone. The development of instrumentation towards modularization and miniaturization has become the current trend. The AD7768/AD7768-4 integrate multiple channels, allowing the system to shrink in size and increase channel density. While testing multiple devices simultaneously, the AD7768/AD7768-4’s ability to keep channel-to-channel crosstalk low is a key differentiating advantage. As the cornerstone of a configurable test platform, it solves many key design challenges and is ideal for modular systems where power consumption is also an important consideration.
An example of its performance is shown in the AD7768/AD7768-4 IMD results in Figure 1. This graph shows that the second-order IMD results are C135.2 dB, and the third-order IMD is C129.3 dB, which is excellent. The AD7768/AD7768-4 IMD testing follows the CCIF standard, which applies two input frequencies of the same magnitude to the device. The resulting FFT shows whether the two frequencies are intermodulated; intermodulation occurs at the corresponding sum and difference frequencies. In this example, the center frequency is 10kHz and the frequency offset is 600 Hz. IMD testing is often used to test whether an audio device will produce an unwanted beat frequency signal when two or more tones are mixed. The presence of such tones can cause disturbing distortions when playing music streams. This distortion produces unpleasant noise compared to the original high-fidelity audio file.
Figure 1. AD7768/AD7768-4 IMD with 9.7 kHz and 10.3 kHz input signals
Test Case
To explore the applicability of the AD7768/AD7768-4 for audio testing, we conducted typical audio test experiments to demonstrate the performance of different consumer audio devices and how the AD7768/AD7768-4 can be used to test various devices. This test uses the AD7768 evaluation board EVAL-AD7768FMCZ and the SDP-H1 platform.
This test case measures multiple audio sources of varying quality and compares their performance. The test case considers many possible test tones, such as IMD tone, log chirp, level test, etc.
The two test tones selected are as follows:
1 kHz sine wave at C60 dBFS. This test is suitable for dynamic range testing, i.e. preventing the device from muting (manually muting the output).
IMD SMPTE test, 60 Hz/7 kHz, 4:1 (12 dB ratio), C20 dBFS. This IMD test shows non-linear distortion products, which are the result of mixing multiple signal tones. In this example, a 7 kHz signal is modulated by a 60 Hz tone (7 kHz ± 60 Hz).
configure
To tune the AD7768 to the desired bandwidth, some calculations must first be made to find the desired master clock (MCLK). Using this MCLK in combination with a specific decimation rate, we can adjust the output data rate of the AD7768/AD7768-4. The MCLK used in this example is 12.288 MHz and the decimation rate is ×64, resulting in an ODR of 48 kSPS. Other combinations may also be used, given the power consumption versus bandwidth trade-off. See the Clocks, Sample Trees, and Power Scaling section on page 41 of the data sheet for more information.
A typical setup is shown in Figure 2. This setup uses the EVAL-AD7768FMCZ board with its onboard SMB connector AC coupled to the audio device. Eight channels can test up to four stereo outputs at a time, with each stereo device having two left and right channels. This circuit can be further optimized to improve system performance. For example, add a high-pass filter to remove noise below 20 Hz.
Figure 2. Connection Diagram
result
Table 1. Test tone results
As can be seen from Table 1, for different devices, the range of results for the low-amplitude input signal and the IMD test varies greatly. Of particular concern is the fact that the inexpensive MP3 player exhibits good dynamic range, but in the IMD tests it is clear that the distortion introduced is considerable. The frequency output of this device indicates poor quality and the maximum IMD level that can be tested is limited by the output drive capability of this device. Therefore, for equal comparison, the test tone for all devices is limited to C20 dBFS.
Laptop audio output has many different driver and processing options. They are developed based on the response of the human ear to produce a more pleasing sound, but cause certain frequencies to be altered. So the laptop’s dynamic range performance is actually the worst when these effects are turned off, but is as good as any other source when turned on.
Figure 3 shows the range of IMDs for various devices from known good sources (orange) to poor MP3 players (green). For MP3 and cell phones, IMD artifacts are clearly visible at 7 kHz and ±60 Hz.
Figure 3. IMD SMPTE test sample, 7 kHz input
Differentiating Factors for AD7768/AD7768-4 Solutions
The maximum output data rate (ODR) of the AD7768/AD7768-4 is 256 kSPS. By adjusting the master clock (MCLK) and/or decimation rate, this ODR can be tuned to a typical audio bandwidth of 48 kSPS, 96 kSPS, or 192 kSPS, depending on the application needs. This is especially useful in power-sensitive applications because the AD7768/AD7768-4 consume relatively low power per channel compared to other audio ADCs. It is also invaluable for other applications that require high dynamic range and large bandwidth, such as sonar.
With many modern test benches moving to modular systems, thermal requirements are starting to become an issue.
The AD7768/AD7768-4 allow the user to trade off signal bandwidth (or dynamic range) and power consumption, resulting in lower power consumption and a wider range of uses. The dynamic range versus ODR curve in Figure 4 shows this flexibility. In addition to flexibility, the AD7768/AD7768-4 have several device-level and system-level power saving methods. For more information, please refer to the data sheet.
Figure 4. Dynamic Range vs. ODR (Per Channel)
The high channel count is also an advantage of the AD7768 for the following reasons:
quick test
Eight channels or four stereo devices can be tested in parallel at the same time. As a result, test time or test cost is now reduced by a factor of four, which is very important for test systems. The number of audio modules around us is constantly increasing, and future audio test benches will be more and more concerned with the cost of test. An example of an application where channel density is important is a home theater system with 7.1 surround sound.
Superior performance
By combining multiple channels, the achievable performance of the end system can be further improved. When four channels are combined into one, the dynamic range figures shown in Figure 4 can be improved by up to 6 dB.
smaller size
Multi-channel audio platforms may have size constraints due to increased channel density, system constraints, plant space constraints, or a move to modular benchtop instrumentation. By integrating eight ADCs into one package, the system follows the trend toward instrument miniaturization.
Epilogue
The number of audio modules is increasing, and the quality requirements of these devices are also increasing. As a result, there is an increasing demand for modern audio test benches in the industry. To exceed these demands and adapt to the growing trend of future smart devices, modern audio test benches must be high-performance, adjustable, configurable, and fast. Home automation, speech recognition and speech-to-text applications are no longer a concept of the future, but a reality. As voice control and similar technologies develop and expand, the burden on the testbed will increase. The AD7768/AD7768-4 can provide a helpful solution for this. Test results show that the AD7768 can be used to test a wide variety of audio equipment on the market today, from low-end equipment to high-fidelity systems.
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