In 2015, there has been little controversy about the application of audio storage, audio transmission and audio streaming, all of which are digital audio. In addition to selling rare tapes in boutiques or enthusiasts, the rest are digital audio. When used properly, digital audio is very flexible and has high audio quality. PCM (Pulse Code Modulation) recording, lossless surround format, and even lossy compression can play acoustic music for our daily records. Of course, the sound that travels in the air is not digital. The pressure wave generated by the human voice or the instrument is recorded by some kind of sensor. These sensors respond to the pressure wave analog with a corresponding voltage. Similarly, at the end of the digital inductive chain, the digital audio signal will eventually vibrate the air, causing the original acoustic wave analog voltage to drive the sensor to generate a pressure wave.
Near the beginning of the digital chain, an analog-to-digital converter (ADC) is used to convert the analog electrical signal to the corresponding digital audio signal. Near the end of the digital chain, a digital-to-analog converter (DAC) is used to convert the digital audio signal back to an analog electrical signal. The quality of the link between the analog-to-digital converter and the digital-to-analog converter and its internal digital chain is the key to determining the overall sound quality presented to the listener.
Audio converter testTraditional methods of audio testing can also be used for evaluation of analog-to-digital and digital-to-analog converters including: frequency response, signal-to-noise ratio, phase-to-channel phase, crosstalk, distortion, group delay, polarity, and more. However, the conversion between successive sampling domains introduces a series of new nonlinear mechanisms (especially for low-level signals). This article is aimed at the problems that arise in analog-to-digital/digital-to-analog audio conversion and some solutions to these problems.
Of course, analog-to-digital and digital-to-analog converters are also widely used in non-audio applications, with higher sampling rates than audio converters. The high-performance oscilloscope has a bandwidth of 33 GHz and a sampling rate of 100 GS/s, which is similar to Lamborghini. Although the sampling rate of the audio converter is not close to this value, they are required to cover a larger dynamic range, and the high-performance 24-bit analog-to-digital converter has a signal-to-noise ratio of more than 120 dB. Even high-end oscilloscopes typically use only 8-bit digitizers. The 24-bit converter advances the measurement of noise and other small signal performance characteristics to the forefront. Therefore, the measurement of this converter requires an outstanding analog performance analyzer.
Test deviceCommonly used test devices are simple and easy to understand.
Analog-to-digital converter testing requires that the analyzer be able to provide a pure stimulus signal at the driver stage for the converter input. For converter integrated circuits, the analyzer must be required to have a digital input format and protocol that matches the output of the integrated circuit, such as an I2S, DSP, or custom format. The digital format of commercial converter devices is usually AES3-S/PDIF - compatible with mainstream devices. For devices that can synchronize with an external clock, the analyzer is required to provide a clocked synchronous output.
Figure 1. Block diagram of the analog-to-digital converter test block
For digital-to-analog converter testing, the analyzer must have a digital output in a suitable format and an extremely high-performance analog input.
Figure 2. Digital-to-analog converter test block diagram
This chart uses the AES3 digital transmission format to test commercial converters. The audio analyzer model is Audio Precision APx555.
As mentioned above, the analog to digital and digital to analog converters are shown as special features of the converter. The Audio Engineering Institute has recommended methods for measuring the performance of multiple converters according to the ASE17 standard. The following examples demonstrate and compare a number of distinctive converter problems.
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