Noise Reduction – Analogue and Digital Signals
Analogue and digital formats are representations of how sound waves continuously impede on surrounding air pressure levels. An analogue signal (or recording) is one that has been modulated onto a substrate, such as record groove. The quality of the substrate is indicative of the final quality of the audio.
Digital formats are produced by converting the physical properties of the signal into a sequence of numbers, which are stored. The sampling rate determines the overall quality of a digital signal, and unlike analogue formats, where the original signal must be kept in good condition to retain its information, (A damaged record may not work) digital signals can be copied provided the ‘original’ set of numbers can be recovered from the signal being duplicated.
There are four main techniques for removing unsolicited noise from analogue audio signals;
- Single-ended pre-recording
- Single-ended hiss reduction
- Single-ended surface noise reduction
- Dual-ended systems
Single-ended pre-recording.
Targets the signal at the time of recording by affecting the medium used in the recording process.
Single-ended hiss reduction.
Reduces the noise while it is occurring, both before and after the signal has been recorded. It can also be employed for live to air broadcasts.
Single-ended surface noise reduction.
Is a process applied to assuage any scratching, marring or imperfections on the surface of phonograph records which can affect the playback audio quality.
Dual-ended systems
Employ the use of ‘the emphasis process’ which involves pre-emphasizing, or increasing the magnitude of the higher frequencies with respect to the lower frequencies during the recording process. Conversely, during playback de-emphasizing, or decreasing the magnitude of the higher frequency signals with respect to its lower counterparts helps to improve the signal-to-noise ratio and eliminate undesirable hiss.
Modern digital recordings have reached a level where undesirable audio noise has been all but removed. However there are arguments against the cold, clinical feel of such recordings, particularly within the music industry. Many artists still opt for the tape-to-tape analogue recording methods, which are much warmer sounding and do have a degree of background noise which adds to the ambiance of a song.
In a home theater system, digital signals are much more desirable because the crystal clear sound they produce enhance the experience of watching a film.
