Dolby Laboratories 585 - Manual (Page 36)

Model 585 Time Scaling Processor Principles of Operation
6-2
6.2 Development of Time Scaling
Time scaling of audio was first achieved in the 1950s with the invention by
Fairbanks, Everitt, and Jaeger
1
of a modified tape recorder. Their modified machine
discarded and repeated sections of audio signal by using a rotating playback head, and
became known as the sampling or splicing technique.
6.2.1 Time-Domain Solutions
Based on a time-domain representation of the signal, time-domain techniques are
efficient and have been attractive for use in real-time implementations. The Fairbanks
splicing technique was a very low-fidelity solution, producing a choppy product that
sometimes obscured the content.
Lee
2
translated the splicing technique into a digital system in 1972, replacing the
magnetic tape with a circular memory buffer and using read/write memory pointers in
place of the rotating head. Further advances were made on this solution throughout
the ’70s, ’80s, and ’90s. These advances generally applied signal analysis to the
splicing technique, making use of increased computer processing power during that
time period. The solutions developed tended to focus on either speech or musical
instruments, but not combinations of both.
The more recent solutions are significant improvements on the original splicing
technique, but all time-domain based processors exhibit a characteristic drawback that
at minimum makes the listener aware that the audio has been altered. Characteristic
artifacts of these methods often include audible echoes, stutters and an overall loss of
definition in the audio.
6.2.2 Frequency-Domain Solutions
In 1967, an alternative to time-domain techniques was introduced by Schroeder
3
. This
method used either a Fourier transform or a filter bank to produce a frequency
representation of the signal. A direct manipulation of the frequency components of
the signal was followed by a transformation back into the time domain to achieve
time scaling.
The technique introduced by Schroeder was developed for speech. Later
modifications applied this approach to music, and variations were produced that could
handle specific signal types, either music- or speech-oriented, but still not in
combination.
1 Fairbanks, G., W. Everitt, and R. P. Jaeger. “Method for Time or Frequency Compression-Expansion of Speech.”
Transactions of the Institute of Radio Engineers, Professional Group on Audio AU-2 (1954): 7–12.
2 Lee, Francis F. “Time Compression and Expansion of Speech by the Sampling Method.” Journal of the Audio
Engineering Society 20, no. 9 (1972): 738–742.
3
Schroeder, M., J. Flanagan and E. Lundry. “Band-Width Compression of Speech by Analytic-Signal Rooting.”
Proceedings of the IEEE 55 (1967): 396–401.