HF-Bias Signal Pick-Up and Pre-processing for Wow & Flutter Correction of Analogue Magnetic Tape – Analyses and Limitations in Practical Application
Nadja Wallaszkovits, Phonogrammarchiv, Austrian Academy of Sciences, Vienna
Heinrich Pichler, Audio Consultant, Vienna
In conventional transfer processes of analogue magnetic audio tape, the main focus is set on the reproduction of the signal band carrying the primary audio content. Due to various limitations of standard playback systems, some additional technical information is lost, like the information possibly provided by the HF bias signal recorded on the original tape.
Since the audio, signal as well as the HF bias signal, are similarly affected by wow and flutter, these deviations from the standardised tape speed are reflected in the HF bias signal, provided the bias frequency is constant. The correction of wow and flutter has already been discussed in theory, mostly referring to the signal processing part. Practical implementations of signal processing have already been developed, using various automated and semi-automated detection routines in combination with non-uniform re-sampling methods.
The paper describes the problems and limitations of the practical implementation of HF bias signal pick-up from analogue magnetic tape at original replay speeds, to be implemented in a standard archival workflow, using slightly modified standard playback facilities. The signal pre-processing in analogue as well as digital domain are compared, and basing on analyses of bias signals from professional as well as semi-professional recordings, the various practical problems are discussed: level instability and unknown frequency of the recorded HF bias signal, frequency variations mainly with semi-professional devices of older generations due to the instability of the bias oscillator, as well as effects of signal distortions, interferences and ultrasonic artefacts.
Nadja Wallaszkovits received her master’s degree in comparative musicology from the University of Vienna and graduated as an audio engineer from the School of Audio Engineering (SAE) Vienna. Since 1988 she has been working as recording and balancing engineer in the studios of the Vienna Concert House and for private national and international recording companies, focusing on classical, jazz and pop music, including film and video sound and synchronisation. In 1998 she joined the Phonogrammarchiv of the Austrian Academy of Sciences, where she has been managing the audio department as a chief engineer since 2004. She is specifically responsible for audio restoration, digital archiving and rerecording of historical tape collections. She works as a consultant for archival technology and systems implementation for National and International Project Partners. She lectures at the University of Vienna and has held several training seminars, recently in the context of the Project TAPE. Nadja Wallaszkovits is Member of Technical Committee of the Audio Engineering Society (AES), Chair of the Audio Engineering Society Austria (AES Austria) and Member of Technical Committee of the International Association of Sound Archives (IASA).
Heinrich Pichler received his masters degree in information technology from the University of Technology in Vienna in 1968 and graduated as PHD in 1972. Until 2002 he was assistant professor at the University of Technology Vienna. He is working in the field of audio, video and circuit design. He is expert on the court of commerce in Vienna and consulting engineer in this fields. He lectures at the University of Technology in Vienna and on the University of Applied Sciences in Vienna. He is author of more than 100 papers in the field of audio and electronic circuit design and holds more than 40 patents. Heinrich Pichler is member of AES, IEEE, ITG and other national and international scientific organisations.