Measuring HDMI Audio

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From its origins in 2002 as a new, smaller connector for HDTV, High-Definition Multimedia Interface (HDMI) evolved continuously over the years to become one of the most widely adopted interfaces. This article provides practical answers on how to perform HDMI audio measurements, dealing with the interface’s multiple options and configurations.

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Measuring HDMI Audio

Measuring HDMI Audio - image of ax articleHow do you measure HDMI audio? This is a question we often hear as HDMI soundbars become more commonplace and the rapid adoption of surround sound and spatial audio systems with televisions continues. While some manufacturers offer expensive custom interfaces to test HDMI devices, there is a much more cost-effective method. Some software-based audio measurement systems can be used with a $100 off-the-shelf interface that enables test signals to be sent via HDMI to a soundbar or surround sound system for measurement. In this article, we discuss how the HDMI protocol handles audio and demonstrate acoustic measurements on a soundbar using Listen SoundCheck software with a commercially available HDMI audio extractor. This method can be extended to a wide range of HDMI connected devices including multichannel surround sound systems.

HDMI Audio
Modern HDMI systems offer Audio Return Channel (ARC) functionality. This allows a single HDMI connection to be used as both an audio input and an output, providing a two-way flow of audio, in addition to video, between devices. This eliminates the need for a dedicated audio cable to connect a soundbar or AV receiver to the television, simplifying configuration and reducing cost and cabling. ARC supports up to six-channel compressed audio and has a maximum bandwidth of 1Mb/second. It supports various audio formats including PCM (2 channel), Dolby Digital (up to 5.1 channel), and DTS Digital Surround (up to 5.1 channel).

A newer version, Enhanced Audio Return Channel (eARC) improves upon ARC with a significant boost in bandwidth and speed, enabling transmission of higher quality audio from a TV to a soundbar or AV receiver. It carries up to 32-channel audio, including eight channels of 24-bit/192kHz uncompressed audio, and supports modern advanced audio formats such as DTS-HD Master Audio, DTS:X, Dolby TrueHD, and Dolby Atmos.

Many audio devices that are used with audiovisual systems, such as soundbars, speakers, gaming headsets, and so forth, are unable to accept an HDMI input with video directly. A small device called an HDMI extractor is commonly used to split the audio and visual HDMI signals so that the audio signal can be played directly on a soundbar or surround sound system. Audio extractors typically cost around $100 and can be purchased online or from electronics or AV stores.

These widely available devices also enable HDMI audio devices to be measured using a software-based audio analyzer. To measure audio, a test stimulus must be sent from the computer to the device under test (DUT). Although soundbars usually feature an HDMI port, as do most computers, typically the device’s audio channels will not be accessible if they are directly connected via an HDMI cable.

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Measuring MEMS Microspeakers

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Measuring MEMS microspeakers can be challenging due to their small form-factor, but it is increasingly important as they are now widely available and seeing fast evolution and adoption in true wireless stereo (TWS) earbuds and in-ear monitors (IEM). Accurate measurement is essential to understanding design considerations, as well as ensure product quality. In this article, published in the June 2024 issue of AudioXpress, Steve Temme and Michael Ricci (xMEMS) discuss the challenges of measuring fully piezoelectric MEMS microspeakers and demonstrate how SoundCheck reveals valuable distortion data.

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Introduction to Measuring MEMS Microspeakers Article

Measuring MEMS microspeakers - image of AX articleMEMS microspeakers are driving one of the fastest technological transitions in the miniature speaker market. MEMS transducers first gained widespread attention in the late 2000s with the first commercially available MEMS microphone, and over the course of a decade, they replaced electret microphones in the majority of mobile devices. The MEMS speaker revolution is shaping up for an even faster transition.

Early speakers incorporating MEMS technology were launched in 2014, but it was not until 2021 that the first fully piezoelectric MEMS microspeakers became commercially available. Manufacturers are already rapidly adopting these into TWS and pro-audio IEMs, earning praise from consumers and product reviewers alike, and propelling this technology into the mainstream.

MEMS Speaker Overview
MEMS microspeakers are constructed in an entirely different way from conventional miniature speakers. Rather than using inductive coils and magnets, they rely on a voltage-driven capacitive actuator, and they are entirely manufactured using a monolithic solid-state fabrication process in a semiconductor wafer foundry. This fabrication process delivers much tighter speaker-to-speaker consistency than is possible with a conventional driver made up of an assembly of many moving parts. They offer full-range performance, eliminating crossover networks that introduce complexity and sometimes even phase differences. This is particularly important considering this year’s other big audio trend—spatial audio, where the phase response is critical to accurate reproduction. Piezo-MEMS speakers are also compact in size and low power, making them economical on both space and power consumption. Finally, they are surface-mount technology (SMT) reflowable, offering yield and quality advantages in device manufacturing.

MEMS microspeakers are now commercially available and seeing fast evolution and adoption in true wireless stereo (TWS) earbuds and in-ear monitors (IEM). With the emergence of these small form-factor transducers comes the need to address how to measure its performance and ensure product quality.

While these advantages are leading to rapid adoption in devices such as hearing aids, earbuds, professional in-ear monitors, smart glasses, and more, they are not without their challenges. The
maximum sound pressure level (SPL) in the low frequencies is typically less than is required for a leak-tolerant active noise cancelling (ANC) system compared with conventional headphone drivers, limiting their use for ANC earbuds to a mid/high frequency tweeter in a two-way configuration. However, in recent months, manufacturers have made rapid progress in overcoming these
limitations, thus broadening their applications. For example, xMEMS’ new Cypress Piezo-MEMS speaker, showcased at the recent Consumer Electronics Show (CES), delivers up to 140dB SPL in the low-frequency band—more than sufficient for ANC applications.

The cost of MEMS-driven products is also decreasing as this technology is more widely adopted. While early MEMS earbuds were priced well over $1000, recently launched models such as the Creative Labs Aurvana bring this technology’s high-quality sound to TWS products below a $200 price point. As this technology moves toward becoming the de facto standard, the outlook for this market is positive.

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Transitioning Audio Tests from R&D to the Production Line

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Steve Temme discusses the transition from R&D to production testing in this March 2023 issue of AudioXpress. Production line audio testing poses many challenges such as noisy environments, harsh operating conditions, high throughput, relative limits, and more. In this article, Steve Temme shares his observations and outlines the main considerations to ensure a successful operation.

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Introduction to Production Line Audio Testing Article

Your product design is finalized, your prototypes are close to perfect, and your audio product is ready to move to production. But how do you ensure that the product hitting the retail shelves sounds exactly as you intended?
Your production line is probably thousands of miles away, with time zone and language complications. Pandemic-related restrictions make it hard to visit to configure, supervise, and train factory personnel. Furthermore, production is often outsourced to a contract manufacturer, adding another layer of complexity to both culture and information transfer.
Every production facility has one priority: to build products that meet the specifications with the greatest efficiency and highest yield possible for optimum profitability. Think about what that means for end-of-line tests. Their goal is to have as many units pass as possible. Tests must be fast to avoid slowing down the production line, and the test environment is generally noisy, often with variable temperature and humidity. The quest for high yields and manufacturing efficiency, along with challenging operating conditions and sometimes poor training, can result in unreliable measurements. When this happens, the only loser is you and your brand’s reputation.
I’ll never forget being told about a production line operator who increased pass-rate by opening the test box during measurements so that the factory background noise would mask the high-order harmonics to fool the perceptual Rub & Buzz algorithm. This might improve his yield, but it’s not so good when these products ship to customers! While this is one of the more outrageous violations of end-of-line test protocol, I have also seen many other questionable measurement practices over the years. In this article, I’ll try to de-mystify production tests and look at some simple measures that you can take to ensure a smooth transition.

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End of Line Distortion Measurements

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End of Line Distortion Measurements - LIS 2022Steve Temme discusses the importance of detecting manufacturing-induced defects such as Rub & Buzz, Loose Particles, and other distortion measurements during end-of-line testing and explains the various algorithms that are used. He compares conventional and perceptual metrics for the measurement of Rub & Buzz, including Listen’s new enhanced Perceptual Rub & Buzz algorithm, and discusses why it can be beneficial to use both conventional and perceptual measurements in tandem.

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INDUSTRY FEATURES
End-of-Line Distortion Measurements

Steve Temme focuses on various test methods, and introduces Listen’s newest perceptual distortion algorithm, comparing it to other conventional and perceptual techniques in this article about distortion measurements.

It is important to catch speakers with Rub & Buzz and transient distortion defects at the end of the production line, as such defects sound harsh and quickly give the impression of poor quality. These manufacturing-induced distortion defects are easy to screen for, but what exactly should you be measuring? In this short guide to end-of-line distortion measurement, we discuss various test methods, and introduce Listen’s newest perceptual distortion algorithm, comparing it to other conventional and perceptual techniques.

What Causes Rub & Buzz and Loose Particles

Rub & Buzz and Loose Particle faults are pretty much exclusively introduced during the manufacturing process. Rub & Buzz is usually caused by small aberrations in the manufacturing process that make some parts rub or rattle when the speaker is operated, such as off-center voice coils, poorly glued surrounds, or incorrectly trimmed lead wires. This can be a random manufacturing issue with one product, or a drift in production line characteristics over time. Loose particles are caused when foreign particles, such as glue or magnet fragments, become trapped in the gap behind the diaphragm or dust cap during the manufacturing process.

Conventional Detection Methods
Although Rub & Buzz, and Loose Particles sound different to the trained ear than they do to the average consumer, both manifest themselves as an annoying vibration sound that may be more prominent at certain frequencies and worse at higher output levels. Despite this similarity, the detection methods are very different. Rub & Buzz is a periodic distortion effect. Conventionally, it is measured using Higher Order Harmonic Distortion (HOHD) analysis, examining typically the 10th order harmonic and above. These harmonics contribute most significantly to Rub & Buzz as their separation from the fundamental frequency makes them less masked and more easily detected by the ear. There are several widely used measurement methods. A high-pass filter algorithm is common, but has its limitations as it combines all the harmonics into an overall metric, and does not distinguish between periodic and transient distortion. Other methods sequentially measure individual harmonics using a tracking filter that moves from one harmonic to the next. While this technique provides accurate and detailed information on each harmonic, it is not fast enough for use on automated production lines.

 

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The Evolution of Production Line Rub & Buzz Measurements

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Steve Temme discusses the evolution of production line Rub & Buzz measurements in this April 2022 issue of AudioXpress. Starting with  simultaneous analysis of higher order harmonics, he explains the progression of improvements including the greater accuracy offered by normalized distortion measurements, and progressing to the introduction of perceptual metrics. He covers the introduction of the first perceptual distortion algorithm introduced in 2011, and the newest enhancements to this which offer the repeatability necessary for successful end-of-line perceptual distortion measurement, where the reduction in false rejects and resulting higher yields add significant value to speaker and headphone manufacturers.

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The Evolution of Production Line
Rub & Buzz Measurements

Rub & Buzz defects in loudspeakers, headphone drivers, and microspeakers generate an immediate perception of poor quality. This article follows the development of production line Rub & Buzz detection, starting with the conventional methods that, although developed 25 years ago, are still valuable and well-used today, and ending with the latest perceptual algorithms.

 

Production line detection of loudspeaker Rub & Buzz defects is important because the harsh sounds created by such faults instantly result in customer perception of poor quality. Such defects are almost always due to manufacturing errors, for example, the loudspeaker spider or surround may not be properly glued, the voice coil lead wires could be the wrong length, or the voice coil may be rubbing. If not immediately identified and corrected, these faults cause high volumes of waste and low yield, or worse still, customer returns.

Not all Rub & Buzz faults detected by conventional mechanisms are audible—the masking effects of the human ear and background noises can render some of them inaudible even though they are clearly present when the signal is analyzed. In the quest for higher yields, it’s valid to question whether a speaker with inaudible faults needs to be rejected. This is particularly true for smaller and lower-margin products (e.g., microspeakers used in mobile phones, laptops, and some smart devices). Over the past decade, this desire for lower costs and higher yields has driven the development of accurate perceptual distortion algorithms that reliably identify whether a fault is audible, rather than simply whether it exists.

This article follows the development of production line Rub & Buzz detection, starting with the conventional methods that, although developed 25 years ago, are still valuable and well-used today, and ending with the latest in perceptual analyses. The trade-offs between each are discussed, and the reasons why both conventional (higher-order harmonic) and perceptual distortion measurements provide valuable data points for end-of-line tests are explained. It should be noted that these days, production line Rub & Buzz measurements are equally likely to be used on headphone drivers or microspeakers as they are conventional loudspeakers, and perceptual measurements are rapidly gaining traction in the quest for higher yields of such lower cost products.

 

What Causes Rub & Buzz
Rub & Buzz defects are almost exclusively introduced in the manufacturing process. The term “Rub & Buzz” refers to the presence of higher-order harmonics, which create a harsh buzzing sound that often becomes particularly obvious at higher playback levels. Such sounds are created when something abruptly impedes the free movement of the speaker mechanism, for example, loose glue joints, vibrating voice coil lead wires hitting another loudspeaker part (e.g., the cone), or an incorrectly centered and rubbing voice coil.

How to Measure Free Field Speaker Response without an Anechoic Chamber

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Did you knowSimulated Free field measurement you can make free field speaker response measurements without an anechoic chamber? In the March 2021 issue of VoiceCoil, Steve Temme explains his unique method for achieving this. The article explains how the ‘splice’ method results in a full range frequency response from a combination of near-field and windowed far-field measurement, and compares the results with anechoic chamber measurements and the manufacturer’s published response curve.

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Simulated Free Field Speaker Response Measurement

Source: “How to Measure Free-Field Speaker Response without an Anechoic Chamber” by Steve Temme, Voice Coil, March 2021.

Main Theme: The article presents a practical and cost-effective method to obtain simulated free-field speaker response measurements without needing an anechoic chamber, particularly useful for those working with limited resources.

Key Ideas/Facts:

  • Challenge: The COVID-19 pandemic highlighted the difficulty of conducting free-field speaker measurements outside a lab setting, as anechoic chambers are not easily transportable.
  • Traditional Solutions (and their limitations):
    • Anechoic Chambers: Expensive and not readily accessible. Suffer from low-frequency cut-off limitations depending on chamber size.
    • Large Open Spaces: Impractical due to logistical challenges and uncontrollable environmental factors.
  • Proposed Solution: The article advocates for the “splice measurement” technique, a long-established but often overlooked method.
  • How it Works: Combines a near-field measurement (accurate at lower frequencies) with a time-windowed far-field measurement (captures higher frequencies).
  • Software Simplification: SoundCheck software (specifically version 18 or newer with the LogTSR module) greatly simplifies the process with a dedicated test sequence.
  • Practical Implementation:
    • Requires: SoundCheck software, audio interface, microphone, amplifier (optional), and a standard room.
    • Procedure: The software guides the user through near-field and far-field measurements, suggests a time-window for the far-field data, and then automatically splices the measurements together.
  • Case Study: Indy Acoustic Research successfully used this method to measure a Tannoy System 600 speaker, achieving results comparable to measurements taken in an anechoic chamber.
    • “These results demonstrate how simulated free-field measurement using a splice sequence to combine near-field measurements and time-windowed far-field measurements yield results that align well with those made in an anechoic chamber.”
  • Advantages:
    • Cost-effective alternative to anechoic chambers.
    • Achievable in a typical room setting.
    • Relatively simple setup and execution using readily available software.

Significance: This technique provides a valuable tool for audio engineers and acoustic professionals to obtain accurate free-field speaker measurements without significant investment or reliance on specialized facilities. This is particularly beneficial for smaller companies, independent researchers, or anyone needing to conduct these measurements remotely.

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ABOUT LISTEN INC

Listen is a global leader in audio measurement and analysis, with over 30 years’ experience developing innovative test methods and algorithms. SoundCheck, our powerful software-based audio analyzer, pairs with our test interfaces, precision microphones, and accessories to offer a complete solution for testing loudspeakers, headphones, microphones, smart devices, communication systems, hearing aids, automotive audio, and more. Trusted by engineers worldwide, Listen delivers the accuracy and flexibility essential for modern audio testing, from the R&D lab to the production line.

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