noise reduction

Remembering Ray Dolby pioneer of analogue noise reduction

We have already written about noise reduction this week, but did so without acknowledging the life of Ray Dolby, one of the inventors of video tape recording while working at Ampex and the inventor and founder of Dolby Noise Reduction, who died on 12 September 2013.

An obituary in The Guardian described how:

‘His noise-reduction system worked by applying a pre-emphasis to the audio recording, usually boosting the quieter passages. The reverse process was used on playback. Removing the boost – lowering the level – also removed most of the tape hiss that accompanied all analogue recordings. Of course, people did not care how it worked: they could hear the difference.’

 Dolby managed to solve a clear problem blighting analogue tape recording: the high frequency noise or tape hiss inherent when recording on magnetic tape.

Dolby 365 / 363 Dual Channel / Stereo A-Type Noise Reduction

Like many professional recording studios from the 1960s onwards, the Great Bear Studio uses the Dolby A noise-reduction system that we use to play back Dolby A encoded tape. On the Dolby A the input signal is split into four individual frequency bands and provided 10 dB of broadband noise reduction overall.

Dolby SR (Spectral Recording) modules and board from a BVH 3100 1 inch C format video

We also have a Dolby SR system that was introduced in 1986 to improve upon analogue systems and in some cases surpass rapidly innovating digital sound technologies. Dolby SR maximises the recorded signal at all times using a complex series of filters that change according to the input signal and can account for up to 25dB noise reduction.

Posted by debra in Audio Tape, Video Tape, 0 comments

Audio Noise Reduction and Finn’s World War Two Stories

We get a range of tape and video recordings to digitise at the Great Bear. Our attention is captured daily by things which are often unusual, interesting and historically significant in their own way.

Last week we received a recording of Pilot Officer Edwin Aldridge ‘Finn’ Haddock talking about his experiences in the Second World War. Finn, who has since passed away,  had made the tape in preparation for a talk he was doing at a local school, using the recording in order to rehearse his memories.

Despite the dramatic nature of the story where he is shot down in Northern France, sheltered by the French resistance and captured by the Germans, it is told in a remarkably matter of fact, detached manner. This is probably because the recording was made with no specific audience in mind, but was used to prompt his talk.

Finn’s story gives us a small insight into the bravery and resilience of people in such exceptional circumstances. The recording tells us what happened in vivid terms, from everyday facts such as what he ate during his shelter and capture to mass executions conducted by the Gestapo.

The now digitised tape recording, which was sent to us by his niece, will be shared among family members and a copy deposited with the local history club in Wheatley Hill, where Finn was born.

Finn was also interviewed by the Imperial War Museum about his experiences, which can be accessed if you click on this link.

On a technical note, when we were sent the tape we were asked if we could reduce the noise and otherwise ‘clean up’ the recording. While the question of how far it is reasonable to change the original recording remains an important consideration for those involved in digital archiving work, as was discussed last week on the Great Bear tape blog, there are some things which can be done if there is excessive hiss or other forms of noise on a recording.

screen grab of spectogram from Izotope RX of an audio file

The first step is to remove transient noise which manifest as clicks and pops which can affect the audibility of the recording. Family home recordings that were made with cheap tape recorders and microphones often picked up knocks and bangs, and there were some on Finn’s tape that were most probably the result of him moving around as he recorded his story.

The second step is to deploy broadband noise reduction, which removes noise across the audio spectrum. To do this we use high pass and low pass filters which effectively smooth off unwanted noise at either end of the frequency range. The limited frequency range of the male voice means that it is acceptable to employ filters at 50 Hz (high pass) and 8000 Hz (low pass) and this will not affect the integrity of the recording.

It is important to remember that noise reduction is always a bit of a compromise because you don’t want to clean something up to the extent that it sounds completely artificial. This is why it is important to keep the ‘raw’ transfer as well as an uncompressed edited version because we do not know what noise reduction techniques may be available in five, ten or twenty years from now. Although we have a lot of experience in achieving high quality digital transfers at the Great Bear, any editing we do to a transfer is only one person’s interpretation of what sounds clear or appropriate. We therefore always err on the side of caution and provide customers with copies of uncompressed raw, edited and compressed access copies of digitised files.

Finn’s story noise reduced

The ‘raw’ transfer

A further problem in noise reduction work is that it is possible to push noise reduction technology too much so that you end up creating ‘artefacts’ in the recording. Artefacts are fundamental alterations of the sound quality in ways that are inappropriate for digitisation work.

Another thing to consider is destructive and non-destructive editing. Destructive editing is when a recording has been processed in software and changed irrevocably. Non-destructive editing, not surprisingly, is reversible, and Samplitude, the software we use at the Great Bear, can save all the alterations made to the file so if certain editing steps need to be undone they can be.

Again, while in essence the principles of digital transfer are simple, the intricacies of the work are what makes it challenging and time consuming. 

 

Posted by debra in Audio Tape, 0 comments