Design of audio signal chain for non-audio experts
In this article, we will begin to decode audio product specifications in order to find the best product suitable for audio applications-not the more advanced the better, but the key to meeting product requirements, just like gloves to fit your hands.
converter
Audio converters are divided into three major categories: analog-to-digital converters (ADC), digital-to-analog converters (DAC), and ADCs and DACs integrated in the same device (CODEC). In the software environment, the codec is similar to the software used to encode and decode the MP3 format. But in the hardware environment, it is an interface to the analog domain.
Control interface
Various control interfaces. Some simple converters generally use a hardware control interface. Control pins are usually connected to VDD, GND or GPIO processor pins. If your system does not change the configuration, or your processor space has a lot of GPIO, this is the easiest way to start. Permanent hardware settings (configured when you design the printed circuit board) will uninstall an extra software driver that you may have to write using the software-controlled converter. Some examples of hardware-driven ADCs and DACs include PCM1803 / PCM1789.
The software control interface can generally be driven by I2C or SPI serial ports, which can be seen on some microprocessors and DSPs. Some devices driven in software mode usually have greater flexibility than their hardware-controlled versions. Software controlled converters usually have some internal registers that can be written to from an external source. From the perspective of the overall system solution, the complexity of mixing is increased to some extent.
Nonetheless, there are some tricks to make it simpler: you can write to your "drive" to change the settings during runtime; you can also dump all the configuration or register the code in flash memory . In this way, during startup, all configurations are sent out of the powered-up serial port.
Dynamic range, SNR and THD + N
The audio standard used to measure product performance (not only the converter, but also the entire signal chain) is defined by the Audio Engineering Association (AES): "AES17-1998 (r2004): Digital Audio Engineering AES Standard Method-Digital Audio Equipment Measurement Standard (AES17-1991 revision) ".
These tests are based on the difference between full-scale (maximum input / output) and background noise levels. For example, use an input of -60dB below the full scale of 1 kHz, and then measure the background noise to achieve the signal-to-noise ratio (SNR) or dynamic range (the same in the converter) test. For THD + N measurements, engineers run the test equipment (DUT) at -1 dB below full scale and then make similar measurements. I suggest you download the AES document and read it carefully.
Since most output signal chains require an audio converter and audio amplifier, and audio amplifiers generally bring their own noise, you need to specify a converter that is higher than your requirements. CD sound quality is often referred to as the 96-dB dynamic range (the actual number may be slightly higher, but the basic calculation method is to multiply the number of bits by 6, ie 16 bits x6 = 96 dB).
ADCs are very similar. Ideally, the best quality input conversion can be achieved by using an input amplifier that converts the highest level input signal just below the ADC full-scale input. This results in the best SNR performance of the converter.
For information on how to specify the audio converter suitable for your system, you can visit the TI E2E community forum to discuss the audio community, or see "Why use a better DAC?" And "Understanding excellent professional audio design: Block-by-block method "(both articles are in" Audio DesignLine ").
About the Author
Dafydd Roche is currently the Marketing Manager of Home Entertainment and Professional Audio Products at TI's High Performance Analog Products Division. Dafydd graduated from the University of York (UK), he put all his knowledge in audio and music manufacturing into his work, which helps designers and consumers to achieve clearer input and higher Output.
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