dbx
dbx is a noise suppression method for audio recordings that was developed in 1971 by the company dbx , which is now part of the Harman Group .
Like most analog noise reduction systems, DBX uses the compander method . With dbx, the entire frequency spectrum of the audio signal is compressed in a ratio of 2: 1 before recording and expanded in the same ratio during playback. Due to the high compression factor, dbx offers a noise and background noise reduction of up to 40 dB. With high-quality tape recorders, a dynamic range can be achieved that even exceeds that of a 16-bit digital recording. In contrast to some other compander systems, such as Dolby , compression and expansion are based on a linear characteristic. This means that the system works independently of the level - an exact calibration of the playback level is not necessary.
Originally, dbx was developed exclusively for the professional market in order to increase the dynamics of high-quality tape recorders in studios. In the early days there was only one variant of dbx, which therefore only bore the name dbx . A few years later, the company realized that the system could also be used successfully in tape recorders and cassette recorders for home use with small modifications. For this was dbx II developed. The original dbx, which was intended for professional use, was designated as dbx I from this point onwards for clear differentiation .
Differences between dbx I and II
dbx I compresses the entire audio spectrum in the range from 20 Hz to 20 kHz in a ratio of 2: 1. In contrast to Dolby A , for example , the signal is not divided into separate frequency bands, but the entire signal is processed with a single compressor or expander. However, this basically results in the problem that so-called "noise breathing" or "noise pumping" occurs, since the expander always opens the entire frequency spectrum for each signal, regardless of its frequency. With broadband signals, the noise is masked by the audio signal and therefore not perceived by the human ear. However, if a signal with predominantly low to medium frequency components occurs, this means that the noise in the upper frequency range is audible and modulated by the audio signal, so the noise resonates with the beat of the music. To counteract this disruptive effect, the treble is increased significantly before recording (so-called pre- emphasis ) and decreased by the same factor during playback.
Due to the functional principle, it is necessary for the system to function properly that the compressed signal is recorded and played back as precisely as possible by the tape recorder used. Since the entire frequency spectrum is always processed in one unit, even the smallest deviations in a single frequency range lead to a malfunction of the entire system and affect not only the frequency range concerned, but the entire signal, as the ratio of compression and expansion in such a case no longer coincides. This also means that even minimal drop-outs , which would otherwise be barely audible, are significantly amplified by using dbx. For these reasons, very high demands are placed on the tape recorder used for dbx I. Specifically, a frequency range of 20 Hz to 20 kHz with a maximum deviation of ± 1 dB is required, and due to the strong pre-emphasis, there must also be very good treble control. According to the manufacturer's information, only tape recorders with a tape speed of at least 38 cm / s are suitable for proper operation with dbx I, whereby good results can still be achieved with precisely calibrated tape recorders using very high-quality tapes even at 19 cm / s.
dbx II basically works according to the same procedure. However, this has been changed slightly in order to enable use with simpler tape recorders and cassette recorders, which are not able to meet the requirements for proper functioning of dbx I. With dbx II, the entire audible audio spectrum is recorded and played back, but only the frequency range of 30 Hz-10 kHz is taken into account for controlling the compressor and expander, which can also be reliably covered by simpler hi-fi devices. This means that errors that are outside this range do not affect the functioning of the system. Since there are usually no isolated tones below 30 Hz and above 10 kHz in common audio signals, such as speech or music, this restriction generally has no significant effects on the sound quality. Nevertheless, the reduced precision of the treble reproduction is perceived by some people in certain pieces of music and perceived as annoying. Since the pre-emphasis has also been reduced compared to dbx I, dbx II is more susceptible to the already described effect of noisy breathing.
Due to the differences between dbx I and II, the two systems are not compatible. Playback is only possible with the same dbx system that was used for the recording.
dbx encoded records
Between 1971 and 1982 around 1100 records were published that were compressed using the dbx process. The dbx-II system was used for this, but the sensitivity for low frequencies below 50 Hz was also reduced in order to avoid impairment of the system due to low-frequency interference, which occurs more frequently when a record is mechanically scanned. Most dbx-II compander systems have their own "dbx-Disc" button for playing these records. When used on LPs, the dbx Type II system reduced the audibility of dust and scratches to tiny pops and clicks (if they were audible at all) and also eliminated the noise completely. dbx-encoded LPs theoretically had a dynamic range of up to 90 dB.
Further development
In the home sector, dbx could not prevail against Dolby. One of the reasons for this was the fact that recordings compressed with dbx cannot be played back in acceptable quality on devices that are not equipped with a dbx decoder. Dolby B offers much less noise reduction, but playback is also possible without Dolby. In this case, the recordings only sound slightly accentuated in height, but otherwise flawless. In addition, Dolby B was already established on the market at the time of the introduction of dbx II and was later supplemented by the improved Dolby C and S versions. In addition, dbx had to compete with a large number of other providers of noise suppression systems in the following years ( e.g. adres from Toshiba , Super D from Sanyo and HighCom from Telefunken / Nakamichi ), none of which could prevail against Dolby.
The long-term success of dbx was also lacking in the professional sector, since Dolby A had already established itself as the standard here and in the following years compander systems were also presented for the professional market, which offered considerable advantages over dbx (in particular Telcom c4 and Dolby SR ).
Dbx was able to achieve a certain spread in the semi-professional sector, where Dolby B was not considered effective enough due to its low impact and systems such as Dolby SR or Telcom c4 were not affordable due to the high acquisition costs.
The company dbx today sells high-quality studio signal processors based on its chip technology and has completely withdrawn from the home device market.
literature
- Hubert Henle: The recording studio manual. Practical introduction to professional recording technology. 5th, completely revised edition. Carstensen, Munich 2001, ISBN 3-910098-19-3 .
- Gustav Büscher, Alfred Wiegelmann: Little ABC of electroacoustics (= Radio-Praktiker-Bücherei. Vol. 29 / 30a). 6th, completely revised and expanded edition. Franzis, Munich 1972, ISBN 3-7723-0296-3 .
Web links
- dbx SysInfo (English)
- dbx Professional Products (English)
Individual evidence
- ↑ dbx Professional Products: Model 150X Type I Noise Reduction Service Manual. In: dbx Professional Audio. Retrieved February 17, 2019 .
- ↑ Talbot-Smith, Michael .: Audio engineer's reference book . 2nd ed. Focal Press, Oxford [England] 1999, ISBN 0-240-51528-5 , pp. 3-39-3-40 .
- ↑ https://web.archive.org/web/20170302203101/https://www.discogs.com/label/322174-DBX-Series
- ↑ http://www.audioinvest.no/dbx/dbxsysin.htm