Measurement and Evaluation of Analog-to-Digital Converters Used in the Long-Term Preservation of Audio Recordings

The analog-to-digital (A/D) converter lies at the heart of the encoding side of a digital audio system, and is perhaps the most critical component in the entire signal chain. The A/D converter must discretely sample the analog signal, quantify the amplitude of the sample, and represent the measurement as a binary word. Whereas conversions made with the A/D converter’s counterpart—the digital-to-analog (D/A) converter—can subsequently be improved for higher-fidelity playback, errors introduced by the A/D converter will accompany the audio signal throughout digital processing and storage and, ultimately, back into its analog state. Thus, the choice of the A/D converter irrevocably affects the fidelity of the resulting signal. For critical applications such as the long-term preservation of historic audio signals, to the greatest extent possible, the A/D converter must exhibit audio transparency—that is, it should neither add to nor subtract from the sound. To assess the degree of transparency, the converter’s electrical measurements and subjective aural performance, as well as the converter’s operating parameters such as sampling frequency and word length, must be considered. Finally, the signal-level input to the converter, converter-component design, and external conditions such as grounding and shielding can greatly affect the fidelity of the resulting file.