“Now, that’s changing thanks to technology we call smart amplifiers or smart amplifiers, because unlike traditional amplifiers, they can safely and temporarily push speakers to their limits. By sensing the speaker’s actions and applying advanced algorithms while music is playing, the smart amplifier can get a lot of sound from your phone’s tiny speakers without hurting your ears.
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By Russell Crane and Matthew Kucic
Smart Amplifiers Can Improve Your Audio Solution Compared to Standard Class D Amplifiers
Remember the days when you strung together two tin cans to make a phone call? Unfortunately, due to the miniaturization of devices (including speakers), cell phone audio sounds a lot like those tin cans. Unless you’ve upgraded your phone to a top-of-the-line phone in the past few years, you’re going to be a pain in the ass to use the speaker for voice or audio. That’s because phone makers have been slow to make audio a differentiator among high-end phones, and there’s little to no low-frequency audio (often called bass).
Now, that’s changing thanks to technology we call smart amplifiers or smart amplifiers, because unlike traditional amplifiers, they can safely and temporarily push speakers to their limits. By sensing the speaker’s actions and applying advanced algorithms while music is playing, the smart amplifier can get a lot of sound from your phone’s tiny speakers without hurting your ears.
What’s in the speakers today?
Before discussing how smart amplifiers work, it’s important to understand another critical part of the audio signal chain: the speakers. No matter what the amplifier is, if the speakers are not designed properly, it will not overcome any audio processing or amplification issues. It’s like throwing rocket fuel into a lawnmower engine — all that power goes unusable. But if you’re starting with a reasonable engine, adding a smart amp is like adding a turbo to push it to the limit in a controlled way.
A loudspeaker consists of a frame, magnets, voice coil and diaphragm (Figure 1). Current flows through the voice coil, which reacts with the speaker’s stationary magnet to magnetize it. The motor moves a membrane attached to the coil up and down and emits sound waves that are actually audible. We call the movement of the vibrating diaphragm the limit, and this movement is limited. When the offset limit is exceeded, audible distortion occurs. In extreme cases, damaged speakers can cause malfunctions. Traditional amplifiers use simple equalization (EQ) to limit excursion. However, to protect against all speaker changes, operating conditions and audio signals, these filters are generally conservative – giving up the ability to push the speaker to its true limits.
Anatomy of a Speaker
The second problem with speakers is that when electricity is passed through the voice coil, some of the energy is converted into heat instead of sound. Push the speaker hard, and this heating can melt the varnish on the magnet wire, damaging the voice coil. As the voice coil heats up from the energy delivered by the amplifier, the voice coil must be cooled by the magnets into the surrounding structure. In conventional amplifiers, the maximum power is limited to a value that, if supplied continuously, will not damage the speaker. This maximum power value must cover all loudspeaker variations, operating conditions and signals. Therefore, this value is usually lower than what the speaker can handle and is therefore conservative.
What makes an amplifier smart?
How can we extract the maximum sound pressure level (SPL) from a given speaker while still ensuring safe operation? We can use smart amplifiers. Audio has a peak-to-average ratio (PAR) that allows us to push instant peaks while maintaining an average or safe level. Smart amplifiers fall into two categories. The first is feedforward, where speaker models are created and audio is fed through these models to predict speaker behavior. Feedforward tends to work with larger speakers, with less variation and more linear operation. Even with larger speakers, we have to account for headroom speaker changes, but the dynamic system can temporarily push the speakers to the limit for loud audio.
The tiny speakers commonly used in smartphones require more advanced smart amplifiers. The second category is feedback smart amplifiers, which add current and voltage (IV) sensing to digital-to-analog converters (DACs) and class-D feedforward solutions. This IV sensing allows us to directly measure the voice coil temperature of the speaker and detect changes in the speaker due to changes between units, ambient temperature and speaker loading (such as placing a hand on the speaker port). This information allows the algorithm to extract other SPLs from the speakers that would otherwise be lost by limiting the output to cover these changes.
To utilize voltage and current sensing information, smart amplifiers require a processor (preferably a digital signal processor or DSP) to parse this data and apply sophisticated algorithms to extract optimal performance and sound while maintaining safe operating conditions for the speaker . Smart amplifiers with or without an integrated DSP can meet the designer’s cost, time-to-market and performance requirements.
Meet your speakers
With a basic understanding of how speakers and smart amplifiers work together to deliver better sound at louder volumes, we can discuss how to use this technology to bring your product to market. The first step is to create a speaker signature that measures multiple aspects of the speaker to identify its limitations. These limitations must be fully understood in order to get the maximum sound and the highest quality sound from the speaker without damaging it. Take detailed measurements to develop accurate loudspeaker models. One way is to use TI’s PurePath Console 3 (PPC3) and accompanying learning board. The combination can perform these measurements using an easy-to-follow procedure.
These measurements include, but are not limited to, system checks, offset characterization, thermal characterization, and SPL measurements. Although the measurement of the offset can be done using the parameters from the loudspeaker datasheet, a more precise method is to use a laser displacement sensor to measure the offset and extract the desired parameters. TI’s Smart Amplifier Learning Board provides all the data acquisition needed by using a laser along with a microphone for SPL measurements, allowing engineers to easily characterize loudspeakers. Once complete, users can quickly view different graphs of measurement data, including offset versus frequency and safe operating area limits.
TI’s PPC3 can simplify the adjustment process. Its advanced tool suite automatically combines low-end impedance measurements with high-frequency microphone measurements. This will create a clean full-range SPL measurement to start tuning. By easily selecting various alignment filters and allowing the software to automatically generate the required compensation filters, the low frequency bass region can be quickly adjusted to push the bass. Smart amps dynamically adjust this filter to push maximum bass without exceeding excursion limits. Next, you can use SmartEQ for easy speaker sounding. The user simply specifies the target EQ curve and PPC3 will calculate the necessary filters to adjust the speaker’s measured SPL response to the target EQ. The tool does all the math,
all together
Once you’ve characterized and fine-tuned your speakers, it’s important to make sure that your selection will work safely and reliably on a larger sample of speakers. Reliability testing is an important step before bringing your product to the assembly line. Regardless of the speaker manufacturer, there are always differences between speakers. While the tuning sounds good and appears to be within a safe operating range, other speakers may not be as powerful as what you’ve been working on in the previous steps. It is recommended that you obtain a larger sample of speakers through the life test. The sample should have at least 20 speakers, and you should test it for longer periods of time and extreme temperatures to simulate expected customer use cases.
If you are using the TI TAS2555 smart amplifier, there is no need to integrate the sequencing and setup of the smart amplifier into the host processor as the DSP is fully integrated into the smart amplifier. This greatly reduces software development time. Also, if your application processor has been upgraded or changed, there is no need to reintegrate the sorting and setup.
When you move to the production line, fast and robust testing procedures can be implemented to ensure that the final product conforms to the parameters set during development. Production line software can help screen speakers to ensure they are within preset ranges and have not been damaged during assembly. Additionally, impedance changes between speakers can be measured and stored. This step ensures that the full cooling headroom of each speaker can be used.
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