Audio Applications

 Audio Amplifiers are designed to increase the amplitude of a small input signal while maintaining the linearity of the original signal. In other words, the detail and characteristics of the original signal will be maintained. The audio amplifier market has seen many changes, resulting in a wide selection of amplifier ICs available to design engineers.

Audio amplifier ICs are now available to address specific applications with a certain amplifier class and number of channels. Audio amplifiers are available in single channel (mono), two-channel (stereo), or multi-channel amplifiers that may support 6 or more channels of high quality audio. Various classes are used to differentiate between amplifier types for audio applications:

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When transmitting audio or video signals over any distance, the connectors are potentially the most fragile part of the signal path. Paying attention to these seemingly mundane devices can pay major dividends in terms of signal integrity and product reliability. Many analog audio and video connections use standard RCA connectors, sometimes referred to as "phono connectors". RCA connectors are commonly used in consumer audio applications such as connections for component hi-fi systems, but are also frequently used for IEC 60958 Type II (S/PDIF) digital audio signals.

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If you want to do any digital processing with analog audio you'll need to convert your analog signals into the digital domain using Audio Data Converters. As their names imply, Audio A/D Converters convert signals from the analog world into the digital domain and after processing Audio D/A converters will convert the digital signals back to the analog domain. There are different types of data converters, but the key specs to compare in audio applications are their resolution (number of bits) and the sampling rate. These specification will affect the dynamic range which should be pre-determined as part of your design. Compact discs provide high quality music (20 Hz to 20 kHz) are sampled at 44.1 kHz with 16-bit precision. High resolution audio more commonly uses 24-bit resolution with samples rates of 192 kHz or more. On the other end of the spectrum, telephone quality speech (200 Hz to 3.2 kHz) can be sampled at 8 kHz with 12 bit precision.

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Evaluation boards let you quickly determine just how well the audio processor that looked good on the datasheet will perform in your pending design. Full development kits go even further and include a wide range of on-board peripherals, application programming interfaces (APIs), reference designs, firmware, software libraries, sample source code, user interfaces, and a serial or USB connection to connect the development kit hardware to a PC host. With a development board you can quickly prototype, test, and debug your design, eliminating the time and cost involved in developing a series of prototype boards.

Development boards typically come with either an onboard debugger or a separate debugger and an onboard debug interface. Check the software library for functions and filters that can be useful in audio applications- they can save a good deal of programming time and effort. Development kits pay for themselves (and more) in terms of shortened development time and avoidance of errors in the design process.

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When an event occurs that requires attention there are two ways to generate an alert: sight and sound. Usually an LED lights up and an audible alarm sounds. The audio alert can be anything from the buzzer on a cell phone to a resonant voice that announces, "I'm sorry Dave, I'm afraid I can't do that."

Audio alert devices are simple electromechanical transducers that span a tremendous range of technologies (buzzers, horns, strobes, sirens, tone generators), tones (chirps, chimes, beeps, whoops, and warbles), frequencies (6.3-9100 Hz), volumes (6-127 dB), and voltage ratings (0.1-220V). With so many audio indicators, there is bound to be an appropriate device available for any given application.

Product selection is very much application driven. How loud does it need to be? How immediate is the need for attention? A buzzer may be sufficient to alert the person in the next room that the dryer load is finished; but a smoke detector will need to issue a piercing alarm. When an event occurs that requires attention and it's not safe to assume that someone will quickly see a visual alert, an audio alert or alarm should be a required part of the system design.

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Amplifying the output from a microphone enough to fill a large auditorium requires a great deal of gain. In order to obtain the best possible signal-to-noise ratio (SNR) it's imperative to amplify the microphone's output as quickly and cleanly as possible. If you start with a noisy signal, no amount of downstream filtering will give you a satisfactory result.

Microphone preamplifier ICs are specialized amplifiers that provide high gain, low noise, low distortion, and- if embedded in the microphone- low power; their characteristics are dictated by the type of microphone.

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Potentiometers (Pots) are variable resistance devices commonly used as the interface to control various features in audio products. They are simple but important devices that allow you to dial in the desired amount of resistance which may affect volume, tone or other aspects of the audio signal. Typically pots are analog devices but digital pots that mimic the behavior of their analog counterparts are also available. Potentiometers come in rotary or slide panel packages with various sizes, shaft configurations and tapers, and can be found in products designed for the pro audio, consumer, portable and automotive markets.

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Audio signals almost invariably require processing, whether to filter out noise, remove echoes, reduce distortion, optimize frequency response, or compress them for transmission or storage. Audio signal processors offer a variety of tools to assist in optimizing sound for audibility, intelligibility, and fidelity. In commercial audio applications, audio signal processors are typically employed within or just after the mixing stage, but before amplification.

Audio signal processors generally fall into one of two categories: Audio SoCs- integrating multiple digital audio management features into a single, complex package- and audio DSPs, which range from hardware implementations of specific codecs to fully programmable DSPs. Audio DSPs are available with a wide range of features, so you can select the feature set to match design requirements.

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Speakers are electromechanical transducers that convert an audio signal to audible sound, the opposite of microphones, with which they share many of the same characteristics. Like microphones, there different types of speakers.

The smallest speakers, if you can call them that, are the electret transducers used in hearing aids. Marvels of miniaturization, they include an electret microphone, programmable DSP processor, audio amplifier, and electret audio output device, all of which must work for days off a tiny zinc-air battery. The electret transducer makes that possible. Earbuds are the next step up in size, but they don't face the same power constraints as hearing aids. They're typically dynamic speakers, with a very small coil attached to a moving diaphragm. Larger headphones use the same design, though high-end ones may use electrostatic drivers that are essentially condenser microphones in reverse, with an electrically charged plate suspended between two perforated metal electrodes to which the audio signal is applied. Electrostatic speakers are more linear than moving coil designs and produce very crisp sound. The downside is they require a relatively high voltage power supply and they can't move enough air to reproduce low frequencies well.

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While digital designers aren't used to seeing transformers, they're very useful in audio applications for isolation and impedance matching, both of which can be accomplished at the same time. Audio power amplifiers connect to 8-ohm loads, which is far lower than the output impedance of the amplifier. An output transformer matches the impedances and isolates the speaker from the amplifier's collector or plate voltage.

When evaluating audio transformers it's important to consider the impedance, power rating, isolation voltage, and frequency range. If the transformer will be operated near an RF or other EMI source, shielding may be necessary. Shielding shouldn't be an issue with a low-impedance audio output transformer, but it definitely could be with a transformer feeding a high-impedance, high-gain input stage; in that case the transformer should be placed as close to the input jack as possible and far away from noise-emitting sources, for which its windings could act like an antenna.

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Analog audio can move readily from a microphone to an amplifier as long as the cable and connectors are shielded and in good repair. But audio that needs to travel to a remote site with some distance needs to be digitized, encoded, transmitted, decoded, and converted back to analog at the far end. The audio transmitter accepts audio, channel status, and user data, which is then multiplexed, encoded, and driven onto a cable; at the receiving end the audio receiver does the reverse. This is clearly a more complicated task than moving analog sound a few feet over a microphone cord.

Digital audio is rarely transmitted raw- it's always compressed and later decompressed by a codec. The choice of codec depends on how much signal loss can be tolerated due to compression and how noisy the signal path is liable to be. Audio transmitters usually contain ADCs and hardware-based codecs; receivers are just the reverse; and transceivers contain the essential elements of both.

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Several types of test equipment can be used to capture and measure audio signals. The simplest test involves putting a single audio tone (usually 1 kHz) through the audio system and measuring the output with an oscilloscope and distortion analyzer, determining the acceptable input level before achieving an unacceptable level of total harmonic distortion (THD), which usually occurs just before clipping. Sweeping the input frequency will indicate the system's bandwidth (+/- 3 dB).

Specialized audio signal generators can generate signals ranging from a short impulse, a burst, or a continuous signal. The signal can be a standard shape (pulse, ramp, sine wave, etc.), a sweep of frequencies, a controlled amount of noise, or an arbitrary waveform.

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