In 2015, there’s not much question about audio storage, transmission or streaming: It’s digital. Apart from rare sightings of vinyl or open-reel tape in boutique sales or creative enclaves, audio is digital. Done right, digital audio is flexible, robust and of very high quality. PCM recording, lossless surround formats and even lossy compression (at least at high data rates) provide the soundtrack for our lives.
But, of course, sound in air is not digital. The pressure waves created by a human voice or a musical instrument are recorded after exciting a transducer of some sort, and the transducer responds with an electrical voltage that is an analog of the pressure wave. Likewise, at the end of the chain, the digitized audio signal must eventually move air, using a voltage that is the analog of the original sound wave to drive a transducer that creates a pressure wave.
Near the beginning of a digital chain, then, we must use an ADC (analog- to-digital converter) to transform the analog electrical signal to a digital representation of that signal. Near the end of the chain, we must use a DAC (digital-to-analog converter) to transform the digital audio signal back into an analog electrical signal. Along with the transducers, these two links in the chain—the ADC and the DAC—are key in determining the overall quality of the sound presented to the listener.
Testing and Converters
The conventional measurements used in audio test can also be used to evaluate ADCs and DACs. These measurements include frequency response, signal-to-noise ratio, inter-channel phase, crosstalk, distortion, group delay, polarity and others. But conversion between the continuous and sampled domains brings a number of new mechanisms for non-linearity, particularly for low-level signals. This article looks at problems seen in audio A-to-D and D-to-A conversion, and some methods that have evolved to address these issues.
The typical test setups are straightforward.
For ADC testing, the analyzer must provide extremely pure stimulus signals at the drive levels appropriate for the converter input. For converter ICs, the analyzer must have a digital input in a format and protocol to match the IC output, such as I2S, DSP, or a custom format. For a commercial converter device, the digital format is typically an AES3-S/PDIF-compatible stream. For devices that can sync to an external clock, the analyzer should provide a clock sync output.
As mentioned previously, ADCs and DACs exhibit behaviors unique to converters. The Audio Engineering Society has recommended methods to measure many converter behaviors in the AES17 standard. The following examples examine and compare a number of characteristic converter issues.
Common audio converter architectures, such as delta-sigma devices, are prone to have an idling behavior that produces low-level tones. These “idle tones” can be modulated in frequency by the applied signal and by DC offset, which means they are difficult to identify if a signal is present. An FFT of the idle channel test output can be used to identify these tones.
Signal-to-Noise Ratio (Dynamic Range)
For analog audio devices, a signal-to-noise ratio measurement involves finding the device maximum output and the bandwidth-limited rms noise floor, and reporting the difference between the two in decibels.
With audio converters, the maximum level is usually defined as that level where the peaks of a sine wave just touch the maximum and minimum sample values. This is called “full scale” (1 FS), which can be expressed logarithmically as 0 dBFS. The rms noise floor is a little tricky to measure because of low-level idle tones and, in some converters, muting that is applied when the signal input is zero. AES17 recommends that a –60 dB tone be applied to defeat any muting and to allow the converter to operate linearly. The distortion products of this tone are so low they fall below the noise floor, and the tone itself is notched out during the noise measurement. IEC61606 recommends a similar method, but calls the measurement dynamic range.
For ADCs, clock jitter can occur within the converter, and synchronization jitter can be contributed through an external clock sync input. For DACs receiving a signal with an embedded clock (such as AES3 or S/PDIF), interface jitter on the incoming signal must be attenuated.
Sinusoidal jitter primarily affects the audio signal by creating modulation sidebands—frequencies above and below the original audio signal. More complex or broadband jitter will raise the converter noise floor. A common measurement that reveals jitter susceptibility is to use a high-frequency sinusoidal stimulus and examine an FFT of the converter output for jitter sidebands, which are symmetrical around the stimulus tone.
Jitter Tolerance Template
AES3 describes a jitter tolerance test, where the capability of a receiver to tolerate defined levels of interface jitter on its input is examined. A digital audio signal is applied to the input. The signal is jittered with sinusoidal jitter, swept from 100 Hz to 100 kHz. As the jitter is swept, its level is varied according to the AES3 jitter tolerance template. Jitter is set at a high level up to 200 Hz, then reduced to a lower level by 8 kHz, where it is maintained until the end of the sweep.
An interface data receiver should correctly decode an incoming data stream with any sinusoidal jitter defined by the jitter tolerance template. As jitter level rises, poor tolerance will cause a receiver to decode the signal incorrectly, and then fail to decode the signal, occasionally muting or sometimes losing lock altogether.
Anti-aliasing filters are used in ADCs to eliminate signal content above the Nyquist frequency—half the sampling frequency; signal content higher in frequency than the Nyquist frequency reflect unwanted audio band artifacts into the audio band. Anti-aliasing filter effectiveness is tested by introducing swept out-of-band (above Nyquist) input stimulus. The resultant converter output is then analyzed for out-of-band induced content.
Tests for the high-level nonlinear behavior of an ADC are similar to those for non-linearities in analog electronics, using standardized tests for harmonic distortion and intermodulation distortion. But audio converters bring new mechanisms for non-linearity, particularly for low-level signals. The AES17 Standard and Audio Precision’s Technote 124 describe effective testing methods for audio converter measurements.
David Matthew is technical publications manager Audio Precision