|Audio Tape Digitisation Workflow
An A/D converter is a relevant component in the digitisation pathway and hence should not colour the audio or add any extra noise. It is vital to choose a high quality A/D converter corresponding to the minimum specifications defined by IASA (IASA TC-04, pp. 6—8), providing balanced line inputs with level control potentiometers on the front panel. In practice, the A/D converter incorporated in a computer’s sound card does not meet the specifications required due to low cost circuitry and the inherent electrical noise of the computer.
To obtain the most accurate, highest-quality transfer of the original possible, it is necessary to keep the signal path in the transfer chain as direct and clean as possible. Additionally, the ability to monitor the signal from both the playback machine and post-A/D converter is necessary to enable verification of the A/D conversion and to analyse potential problems heard during transfer.
Use an external (stand alone) A/D converter that will convert from analogue to digital in accordance with the IASA specification. Connect the A/D converter to a good quality sound card which has a reliable digital input and passes a digital audio data stream without any change or alteration. Use high quality cables and connectors and route the signal from the playback machine to the A/D converter using the cleanest, most direct signal path possible. There may be no unused devices in the signal path. It is recommended to avoid inserting a mixing console to the signal path, as such a device may introduce additional noise and distortion. Whenever possible use balanced signal lines rather than an unbalanced. The archival transfer should be performed in an appropriately designed, critical listening environment. Choose a room that is quiet and is removed from other work areas and traffic, and design the monitoring chain to allow instant comparison of the signal pre and post A/D conversion.
Make sure that the soundcard is properly synchronised with the A/D converter (set soundcard as word clock "slave", respectively in "external sync mode"). Some A-D converters are able to re-calibrate themselves periodically or have to be recalibrated (e.g. to compensate for offset and gain drifts due to temperature and component changes). It is recommended to consult the manual about this individual procedure.
IASA TC-04, pp. 6—8.
Mike Casey & Bruce Gordon: Sound Directions - Best Practices for Audio Preservation, pp. 16-17.
Ken Pohlmann: Measurement and Evaluation of Analog-to-Digital Converters Used in the Long-Term Preservation of Audio Recordings
Wikipedia: Analog to Digital Convertors
Understanding analog to digital converter specifications
Ankita Agarwal: Analog sampling basics