オーディオ・サブシステム: PC スピーカ

オーディオ・サブシステム: テキサス・インスツルメンツ製 PC スピーカ・ソリューション

設計上の考慮事項

Audio Subsystem: PC Speaker

Notes:

  • *PCM1808: requires external mux for = 2 inputs
  • **Audio DSP with low tier I2S amplifier could be used in place of TAS57xx part for a lower cost option
The core subsystems include:

Digital-Input Class-D Speaker Amplifier

Output Power per Channel

  • After determining the number of speakers in a system, specify the output power for each channel.
  • Maximum power is decided primarily by power supply (output voltage and current) and speaker impedance.
  • Efficiency of Class-D amplifiers is typically between 80% and 90%, which reduces demands on power-supply designs when compared to Class-AB amplifier requirements.
  • The maximum input signal level dictates the required power amplifier gain to achieve the desired output power.
  • For best noise performance, the gain should be as low as possible.

Output Filter Design

  • Most of TI’s Class-D amplifiers operate without a filter when speaker wires are less than 10 cm.
  • EMI from high-frequency switching is a major design challenge.
  • When speaker wires are long, place a second-order low-pass (LC) filter as close as possible to the amplifier’s output pins.
  • The filter must be designed specifically for the speaker impedance because the load resistance affects the filter’s quality factor, or Q.

Audio Converters

Reducing Noise on Microphone Inputs

  • Microphone signals are susceptible to noise injection because of the low peak-to-peak range of 10 mV.
  • Placing the codec or ADC close to the microphone often conflicts with user preference, industrial design or mechanical design requirements.
  • Look for devices that can work with digital microphones or have differential inputs, both of which provide significantly better noise immunity.

Processing Allocation and Software Re-usability

  • Host processors in handheld consumer electronics are being given more tasks, pushing processor MIPS allocations and design schedules.
  • One solution is to offload a number of audio functions to a DAC or codec.
  • Audio functions include 3-D effects, equalization, notch filters or noise cancellation.
  • Look for devices with broad, easy software re-usability and the ability to allocate the processing to either input or output functions.

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