METHODS FOR DETECTION AND REMOVAL OF PARASITIC FREQUENCY MODULATION IN AUDIO RECORDINGS

1. METHODS FORDETECTION AND REMOVAL OF PARASITIC FREQUENCY MODULATION IN AUDIO RECORDINGS CZYZEWSKI A., MAZIEWSKI P., DZIUBINSKI M., KACZMAREK A., KULESZA M., CIARKOWSKI A. & KOSTEK B. Multimedia Systems DepartmentGdańsk University of Technology (GUT)Poland

2. Schedule • Wow & flutter – basic facts and definitions • Methods of estimating parasitic frequency modulation in audio • Our algorithms and software • Sound examples • Conclusions

3. Wow & flutter –parasiteeffects • The wow & flutter defects can be found in: • Recordings, especiallyarchival on magnetic and optical sound tapes • Those defects can occur during: • sound recording • copying procedure • The main sources of those defects are: • irregular velocity of the sound carrier movement • mechanical tape damages • Parasite wow (flutter) distortions can be characterised as an undesirable changes of all sound frequency components • Eliminating them should help to understandthe content of some still unresolved archival speech recordings

4. Wow & Flutter problem Driftfmod < 0.5Hz Wow fmod < 0.5 – 6Hz > Flutter fmod < 6Hz – 100Hz > Frequency-modulation noise fmod > 100Hz

5. Standards related with wow & flutter Examples of well known standards : • Magnetic Tape Recording and Reproducing (Reel-to-Reel) NAB Standard 1965 (National Association of Broadcasters) • Messgerate fur Frequenzschwankungen bei Schallspeichergeraten (Measuring Equipment for Frequency Variations in Sound Recording Equipment) DIN 1966 • Measurement of Wow and Flutter in Recording Equipment and in Sound Reproduction CCIR Recommendation 1970 • Method for Measurement of Weighted Peak Flutter of Sound Recording and Reproducing Equipment IEEE 1953 (1971) AES 1982 (2003) It is generally accepted that flutter should be less than 0.15% DIN weighted to be inaudible. Measurement methods (2) are described in IEC 386, and a suitable test film is available from the SMPTE (No. P35-FL).

6. Wow Restoration algorithms • The defect is difficult to estimate and to restore • We developed several algorithms for wow restoration

7. Preservation towards storage and access. Standardised Practices for Audiovisual Contents in Europe FP6-IST-707336 PrestoSpace partnership • Coordinator : INA • Project Steering Board : BBC, RAI, INA, ORF, Joanneum Research (Austria), B&G (NL), Sheffield University (GB) • Partners : CTM Debrie, ACS, Media-Matters, CRCDG, Centrimage, Vectracom, SWR, Front Porch Digital, Hi-STor, Houpert Digital Audio, LIMSI-CNRS, Snell&Wilcox, TI Partners, Universities… • Three partners to be identified in second phase of the project • External User Group : 30+ considered archives.

8. Methods of the restoration of pitch variation defectsbased on comparison of the multiple copies of degraded audio • time warping algorithms applied in time or spectral domain (appropriate algorithm should generate the correct time warping function or pitch variation function). • adaptive filtering (estimation of time delay (offset) between selected copies). • correlation methods (finding similarities based on cross-correlation function). • statistical methods (maximum likelihood estimation or true Bayesian estimation – depending on knowing the priori information).

9. Methods of the restoration of pitch variation defectsbased on one copy of the archive recording • speech applications – lowpass filtering followed by pattern matching. • speech - cepstral analysis which reveals information about the fundamental frequency. • music or speech - application of methods based on a knowledge of the shape of the nonlinear function of audio distortion. PVC = Pitch Variation Curve

10. Methods of the restoration of pitch variation defectsbased on one copy of the archive recording • multiple pitch tracking based on : • McAulay and Quatieri representation (pitch tracking through the use of multiple concurrent pitch “tracks”) • Analyzing high-frequency bias • Analyzing recorded power-line hum (AC power supply) • pitch tracking algorithms should generate the correct pitch variation function (PVC) • based on pitch variation the restoration of the signal can be done for example by resampling it with a truncated sinc function.

11. Our algorithms… presto8.exe (7)

12. Carrier feature analysis • Tracking power-line hum frequency Hum is commonly found in archival recordings. Tracking its frequency allows estimating the depth of wow effect. Due to limitations of DFT methods in case of low-frequency signals, the AR pseudospectrum estimation method is used. • Tracking high-frequency bias Assuming that a high frequency bias was recorded on the magnetic tape it could serve as an ideal target for parasite wow characteristic determining. This idea was also exploited in the prepared software.

13. Wow Restoration Plugin No Wow O R I G I N A L R E S T O R E D Wow

14. Conclusions • MQ analysis can be effective in case of parasite frequency modulation detection • Formant structure analysis can significantly improve efficiency of the detection process • Precise wow tracking can be achieved based on a cepstrally smoothed spectrum representation • Recorded power line hum provides a good basis for wow tracking (similarly to high-frequency bias in magnetic recordings) • Interactive procedures (parameters adjustment) leads to more reliable results • It seems that an interactive approach employing an algorithmic „toolbox” for wow&flutter restoration is the best solution of this problem

15. Thank you for your kind attention