Audio Engineering

Noise Reduction

The Music Telegraph | Text 2019/04/13 [12:18]
Audio Engineering

Noise Reduction

The Music Telegraph| 입력 : 2019/04/13 [12:18]

▲ Noise Reduction and Tape Hiss

: The fidelity of a tape recording is limited by "tape hiss", high frequency random noise resulting from the fact that the magnetizations of the oxide particles of the magnetic emulsion do not add to exactly zero even in the absence of a recorded signal. Noise reduction systems like the Dolby and dbx systems help to overcome this by pre-emphasis of high frequency, low-intensity music signals upon recording so that they, along with the tape hiss, can be de-emphasized upon playback.   

 

▲ Dolby noise reduction

: The Dolby systems for noise reduction employ circuitry which pre-emphasizes high frequencies before they are recorded onto tape in order to make them larger than the tape hiss noise with which they compete.

: The circuitry is amplitude-sensitive: only soft high-frequency sounds are emphasized. If loud high-frequency sounds were emphasized, it might drive the tape into its distortion levels. Upon playback, a matched de-emphasis circuit is employed to restore the high frequencies to their proper balance with the other part of the recorded signal.   

 

▲ dbx Noise Reduction    

: The dbx system is an compression-expansion scheme for increasing dynamic range and for decreasing tape hiss. 

: Two inherent problems with recording onto tape cassettes are the limitation on dynamic range for distortion-free recording and the presence of high-frequency tape hiss. The live music to be recorded might have a dynamic range of 70 decibels, but you will do well to get 40 dB of dynamic range recorded on the tape. If you have carefully matched electronics to squeeze or compress the signal to 40 dB before recording and expand it back to 70 dB upon playback you can not only retain more of the dynamic range of the original music but also accomplish some tape noise reduction in the process. Note that soft music signals are made larger than the tape hiss level for recording, and then they are made smaller again on playback. This playback expansion also reduces tape hiss dramatically. Note also that the compression avoids distortion from tape saturation.

© Georgia State University



 

 

Noise Reduction

 

Analog Noise Reduction

- Analog taping process causes noise (tape hiss), especially noticeable with 16 or 24 track recordings

- Two ways of noise reduction on analog tape:

i) Single Ended: Filters used during the Mix (filters to reduce hiss)

ii) Double Ended: a compander (Compressor + Expander) or compression system - they would compress while recording (encoding) and expand when playing back (decoding)

 

 

*Double Ended technologies & models:

1. Dolby A

- had to align a dolby tone on every track and print is set on each track

- compressed and expanded in frequency ranges

 

2. dBX

- had a single compansion band (20 ~ 20kHz)

- had artifacts in LF range

 

3. Dolby SR (Spectral Recording)

- No alignment tones

- works in 5 frequency bands

- only compress when necessary and expands as necessary when playing back

- the best Noise Reduction system ever made

 

*Best way to mix

- 1" 2 track with Dolby SR at 15 ips

- Put 1k, 10k, 100Hz, & 30 ~ 60 Hz, & 15 kHz tones

 

 

 

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