人参与 | 时间:2021-06-20 02:02:10

As the volume is lowered digitally, the difference between the noise level of the DAC and the analog output becomes smaller because the level of noise from the DAC is the same, but fewer bits are used to represent the audio. Therefore, a 24-bit DAC starts behaving like a 16-bit DAC or worse.

By changing the volume (attenuation) of the output in the analog domain, the noise floor of the DAC is essentially lowered along with the audio itself. Therefore, maintaining the actual signal-to-noise ratio while attenuating the output.


For those in the consumer market (CD, MP3 etc.), simply using a higher-performance DAC (e.g., 105 dB) than the source (e.g., CD at 96 dB) will allow the dynamic range to be preserved, while attenuating (that is, multiplying the sample by less than one) the signal.

For those who need full control over the audio and do not enjoy the difference in sound when the audio is attenuated, a digitally-controlled volume – typically using a PGA – can be implemented, which will change the gain/attenuation in the analog domain with the benefit of digital control.

About the Author Dafydd Roche is the Analog Professional Audio Marketing Manager at Texas Instruments. A graduate from the University of York (UK), Dafydd pours his passion and knowledge of audio and music making into his work, doing his part to help enable audio design engineers to make products that end users can't wait to use. In between a hectic life of customer visits, internal meetings and tradeshows, Dafydd still manages to find time to make and record music with fellow musicians in the Dallas area.


Related links: Accelerate audio algorithms with enhanced DMADesign of High-Performance Balanced Audio InterfacesDesign of High-Performance Balanced Audio Interfaces – Part 2Design of High-Performance Balanced Audio Interfaces – Part 3TI launches 216-kHz stereo A/D converters with 124-dB dynamic range

[Part 4 examines jitter, delay, and other errors in ADCs.]


DAC Dynamic Performance The ac specifications that are most likely to be important with DACs are settling time , glitch , distortion, and spurious-free dynamic range (SFDR) .

The settling time of a DAC is the time from a change of digital code to when the output comes within and remains within some error band as shown in Figure 2-38. With amplifiers, it is hard to make comparisons of settling time, since their error bands may differ from amplifier to amplifier, but with DACs the error band will almost invariably be ± 1 or ± 1/2 LSB.

Quality-of-service support: The WiMAX MAC layer has a connection-oriented architecture that is designed to support a variety of applications, including voice and multimedia services. The system offers support for constant bit rate, variable bit rate, real-time, and non-real-time traffic flows, in addition to best-effort data traffic. WiMAX MAC is designed to support a large number of users, with multiple connections per terminal, each with its own QoS requirement.

Robust security: WiMAX supports strong encryption, using Advanced Encryption Standard (AES), and has a robust privacy and key-management protocol. The system also offers a very flexible authentication architecture based on Extensible Authentication Protocol (EAP), which allows for a variety of user credentials, including username/password, digital certificates, and smart cards.

Support for mobility: The mobile WiMAX variant of the system has mechanisms to support secure seamless handovers for delay-tolerant full-mobility applications, such as VoIP. The system also has built-in support for power-saving mechanisms that extend the battery life of handheld subscriber devices. Physical-layer enhancements, such as more frequent channel estimation, uplink subchannelization, and power control, are also specified in support of mobile applications.

IP-based architecture: The WiMAX Forum has defined a reference network architecture that is based on an all-IP platform. All end-to-end services are delivered over an IP architecture relying on IP-based protocols for end-to-end transport, QoS, session management, security, and mobility. Reliance on IP allows WiMAX to ride the declining costcurves of IP processing, facilitate easy convergence with other networks, and exploit the rich ecosystem for application development that exists for IP.

Next: WiMAX Physical Layer

顶: 634踩: 189