In today's audio industry, when discussing high-end products, it might seem like dynamic range compression—often associated with distortion—is now being praised. However, unless you're a strict audiophile, sound quality is rarely compromised. Most consumers weigh factors such as area, device cost, and design expenses before deciding to implement dynamic range compression. The introduction of Texas Instruments' first stereo Class D amplifier with dynamic range compression seems almost inevitable.
Dynamic range typically refers to the difference between the loudest and softest sounds, measured in decibels (dB), calculated as 20Log10(P/Po). Speech generally has a dynamic range of 20-40 dB, while music ranges from 40-60 dB, and symphonies can reach up to 70-120 dB. An audio amplifier must have a dynamic range that exceeds these values to ensure clarity and richness.
The phrase "sing loudly and clearly" has become popular across regions, sparking interest in the portable market. Portable devices also require amplifiers that can produce loud sound, but they are limited by speaker size and power supply voltage. Higher power often leads to more distortion and lower sound quality, while smaller size and lower power consumption are key priorities for portable products.
During the design phase, maintaining a consistent output volume throughout the battery life is crucial. For example, with lithium-ion batteries, the voltage fluctuates between 3.0V and 4.2V. If power is supplied directly, the sound will be louder when the battery is new and gradually decrease as it drains. Limiting maximum power consumption is another challenge, as different audio sources (like MP3s or FM radio) have varying input levels. This requires flexible control over output peripherals such as headphones and speakers, as their sensitivity may differ. At the same time, the goal is to maximize sound level while maintaining a good signal-to-noise ratio.
To address these challenges, one common solution is to use a boost circuit to maintain sufficient sound output as battery voltage changes. Texas Instruments previously offered products like TPA2014D1 and TPA2013D1, which provide constant voltage and maximum output power. Another approach is dynamic range compression, a technique widely used for over two decades.
What exactly is dynamic range compression? As the name suggests, it involves artificially reducing the dynamic range by amplifying small signals while limiting large ones. A typical CD has a dynamic range of 96 dB, but the human ear can only perceive about 60 dB in a small portable device, since speech itself only spans 20-40 dB. The extra range is often wasted in terms of power and accuracy, making it difficult for users to hear subtle details. With dynamic compression, small signals can be amplified tenfold, reducing the dynamic range from 1:1000 dB to 1:100 dB. While this reduces the overall range, it ensures that small signals remain clear. According to Zhang Hong, a business development manager at TI China, if an amplifier has a dynamic range of 100 dB, and the string section of a piece is much smaller than that, there's no distortion. When playing rock music, the loud drum hits can be compressed into softer tones, which the listener may not even notice, ensuring a better listening experience.
This technology is particularly useful for music playback and conference calls. For instance, when using a mobile phone or MP4 player to learn English, the sound might be too quiet to catch all the details. Increasing the volume helps, but it could disturb others. Dynamic range compression solves this problem by balancing volume and clarity.
Texas Instruments is a pioneer in stereo amplifiers and currently holds around 40% of the Class D amplifier market share. According to IMS Research, the Class D amplifier market is expected to grow steadily in the coming years. Zhang Hongwei noted that since most mainstream portable consumer electronics already use Class D amplifiers, there won't be major shifts in market share. With the rise of digital TV and broadcasting, more digital audio sources will emerge, and some pure digital amplifiers may enter the market. However, he predicts no significant changes in the next two to three years.
Class A, B, AB, and D amplifiers each have distinct characteristics. What sets TI’s first stereo Class D amplifier with dynamic range compression apart from traditional A and B amplifiers? And what is the logic behind these naming conventions?
Generally, transistor power amplifiers are built using triodes, which consist of multiple NP junctions. When no voltage is applied, the junction is off. A bias voltage above the threshold (around 0.6V for silicon transistors) turns the junction on, allowing current to flow and enabling the transistor to function.
Class A amplifiers operate with a fixed bias set to half the maximum output power, meaning the amplifier is always active, even without a signal. This ensures the transistor works in the linear region, resulting in minimal distortion and rich sound quality, especially for small signals. However, Class A amplifiers are inefficient, typically operating at less than 25%, and require higher voltages, which limits their ability to handle large dynamic ranges.
Class B amplifiers lack a fixed bias and rely on the signal to turn the transistor on. When the signal is below the threshold voltage, the transistor remains off, leading to distortion for small signals. This results in loss of detail and an incoherent listening experience. Class B amplifiers also suffer from crossover distortion due to the push-pull configuration, where one transistor turns off while the other turns on.
To reduce this distortion, designers introduced a small fixed bias slightly below the threshold voltage, allowing the transistor to conduct slightly even without a signal. This creates a Class AB amplifier, which eliminates crossover distortion and offers higher efficiency than Class A while maintaining better sound quality than Class B.
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