Acousticbuoy DAC2488 multi-format Digital to Analog Converter
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Signal Images

The first hurdle we aimed to overcome has to do with the interference arise from the process of signal sampling. Sampling theory has indicated that when we take sample of a signal there are “images” of the original signal around multiples of the sampling frequency. For example, a 1 kHz signal that was originally sampled at 44.1 kHz will have image components at 43.1 kHz, 45.1 kHz, 87.2 kHz, 89.2 kHz, etc., the phenomenon is illustrated in the diagram below,

In our daily life, Radio Frequency (RF) signals from mobile phone, television, radio, and many other electrical appliances will also generate “images” that lead to further degradation in the performance of a digital system and also the Signal-Noise-Ratio in an analogue system. The diagram below shows the sampling images found in a typical audio system.

As we will find out in any audio systems, it really does not matter where the images are located, even if they are in the ultrasonic range and inaudible, crystal clear sound field will not be possible as long as there is unwanted frequency existed in the system. In order to get a near-perfect reconstruction of the original signal, it is necessary to annihilate any unwanted frequency and stop them from spreading out to the rest of the audio system.

It should be obvious the purpose of removing the unwanted frequency in a typical amplifier is to ensure that the analog circuitry will not become non-linear due to the inaudible frequency from both the D/A conversion (Internal interference) and other RF sources (External interference). It is equally important to realize the significance of minimizing the needs for data correction will also help to reproduce the original recording, as we all know “corrected” data not necessary equal to the original data.

The following figure shows one of the methods which audio systems designer used to remove the unwanted image signals, by filtering the input signal with a digital filter and use a simple analog filter at the DAC output.

In the old days, before the emergence of the digital audio era, the reconstruction filter was typically a complex, analog, active or passive filter. This filter had to have very flat frequency response over the audio band and then drop precipitously just above 20 kHz, with an ultimate attenuation of at least 80dB. These high order filters are often known as the brick-wall filters, and they required exceptionally precision components to satisfy the high audio standards and quality requirements. The brick-wall filters also introduced large phase shifts near the edge of the audio band, but whether this phase shift introduced audible artifacts has been a controversial subject for many years, with the early detractors of digital audio often blamed the so-called “gritty” sound of digital recordings on the high phase shift introduced by these filters.

In recent years, due to the high cost, high power consumption and large phase shifts characteristics of the brick-wall filters, the digital over-sampling filters have been developed and effectively replaced the brick-wall analog filters in many applications nowadays.

In the design of DAC2488, a 1:1 digital interface transceiver (WM8805) with PLL advance jitter attenuating (Less then 50 ps RMS) is used for filter out the undesired “images” or noise in the digital domain before the digital signal is fed to the DAC (ADI-AD1852) for digital to analog conversion. Although there must still be an analog output filter to complete the D/A conversion, it can be a simple third order design with minimal phase shift without degrading the sound quality.

Equally important, when designing the DAC2488, a hermetic aluminum (6mm thick) casing is used to eliminate any undesirable interference induced by RF source or electro-magnetic field.

In the design of DAC2488, a 1:1 digital interface transceiver (WM8805) with PLL advance jitter attenuating (Less then 50 ps RMS) is used for filter out the undesired “images” or noise in the digital domain before the digital signal is fed to the DAC (ADI-AD1852) for digital to analog conversion. Although there must still be an analog output filter to complete the D/A conversion, it can be a simple third order design with minimal phase shift without degrading the sound quality.

Equally important, when designing the DAC2488, a hermetic aluminum (6mm thick) casing is used to eliminate any undesirable interference induced by RF source or electro-magnetic field.


Block diagram of AD1852