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Reducing noise using a plurality of recording copies, This technique is already patented; does it work; is it used?
post Mar 26 2013, 12:55
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I was pondering the usefulness of noise removal based on several copies of a recording, possibly from different physical copies. A Google search reveals that such a technique is already patented:

The present invention provides a method for reducing noise using a plurality of recording copies. The present invention produces a master file with lower noise than the available recording copies, and avoids the problems of losing musical content caused by prior art pop and click removers. The system comprises a recording playback unit, a computer system with a sound input capability, and a high capacity storage system such as a CD recorder. In operation, a plurality of recording copies of a single recording are played on the playback unit. These recordings are digitized by the computer and a separate recording file is formed for each copy of the recording. The recording files are then synchronized. The samples from each of the recording files are then averaged to reduce the noise components. A variety of threshold comparison techniques can be employed to eliminate samples and/or recording files that are outside of a computed range for that sample...


Do any of you know:

1) Does it work in practice?
2) Is software that does this available commercially or open source?
3) Are there available software/algorithms for synchronizing several audio streams (an essential step before comparing copies)?

Any pointers appreciated.
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post Mar 26 2013, 19:24
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I theory this can work. Since the signal is coherent and the noise is random. This DOES work is when you combine the left & right channels of a stereo recording to make a lower-noise mono recording. If my math is right, the signal increases by 6dB (With two signal sources) but the noise increases by only 3dB. That increases your signal-to-noise ratio by 3dB (not much). But in real-world situations with two different copies of a recording, I don't think this works.... The problem is the synchronzation which has to be "exact". That means less than one wavelength at the highest frequency if analog, or sample-perfect if digital.

For example, if you record/digitize two copies of a vinyl record (or record the same record twice) and you time-align it at the beginning and mix the recordings, the time will drift slightly and you'll get nasty phasing/flanging/comb filtering effects. And you won't get "digital alignment" or the same exact digital data both times because of analog variations, and because the samples will "taken" at a different point along the analog waveform every time you digitize. Any audio editor can mix two or more recordings, so it's an easy thing to try for yourself.

If you start-out with two digital recordings, they should be identical. In that case there is no information-increase by combining the two. And if they are not identical, one is probably better than the other and you should use the best one.

You can probably use some DSP "tricks" to continuously time-align by time-streatching or time-squeezing one of the signals. But once you've done that, I'd suggest that you'd get better results by selecting the best (lowest nose) recording moment-to-moment, rather than combining the good with the bad. With this method, the synchronization wouldn't need to be perfect, just perceptually perfect. In fact, Wave Repair has a feature something like that which allows you to copy a short section of one stereo channel into the opposite channel in order to eliminate a vinyl click/pop that occurs in only one channel, or that's time mis-aligned in the left & right channels. (The loss of stereo for a fraction of a second is not noticeable as long as the sound blends "nicely".)
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post Mar 26 2013, 20:43
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Thanks, I also figured that the sychronization is the issue here, if that is solved, various approaches can be used to determine the "best" signal.

The reason I started thinking about this approach is that similar techniques are applied for image processing, e.g. diffing noise exposure from night time exposure and SIFT for stiching etc. In essence, digital photography is also sampling, and identifying and aligning certain (possibly subsample) interest points should be possible for audio, too.

I actually tried manual alignment, and the results were not terrible, but not better than any of the originals, I'd say a bit worse.

After doing a little more research I found the MATCH plugin for Sonic Visualiser, which sort of does what I had in mind, but I haven't figured out how to process the aligned audio to get two aligned files:

This method is described in a 2005 paper by Simon Dixon:

Dynamic time warping finds the optimal alignment of two time series, but it is not suitable for on-line applications because it requires complete knowledge of both series before the alignment can be computed. Further, the quadratic time and space requirements are limiting factors even for off-line systems. We present a novel on-line time warping algorithm which has linear time and space costs, and performs incremental alignment of two series as one is received in real time. This algorithm is applied to the alignment of audio signals in order to follow musical performances of arbitrary length. Each frame of audio is represented by a positive spectral difference vector, emphasising note onsets. The system was tested on various test sets, including recordings of 22 pianists playing music by Chopin, where the average alignment error was 59ms (median 20ms). We demonstrate one application of the system: the analysis and visualisation of musical expression in real time.

I also found another paper by Pablo Sprechmann, Alex Bronstein, Jean-Michel Morel and Guillermo Sapiro that seems to do exactly what I had in mind, but I found no reference to any available software implementation:

A method for removing impulse noise from audio signals by fusing multiple copies of the same recording is introduced in this paper. The proposed algorithm exploits the fact that while in general multiple copies of a given recording are available, all sharing the same master, most degradations in audio signals are record-dependent. Our method first seeks for the optimal non-rigid alignment of the signals that is robust to the presence of sparse outliers with arbitrary magnitude. Unlike previous approaches, we simultaneously find the optimal alignment of the signals and impulsive degradation. This is obtained via continuous dynamic time warping computed solving an Eikonal equation. We propose to use our approach in the derivative domain, reconstructing the signal by solving an inverse problem that resembles the Poisson image editing technique. The proposed framework is here illustrated and tested in the restoration of old gramophone recordings showing promising results; however, it can be used in other applications where different copies of the signal of interest are available and the degradations are copy-dependent.

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Posts in this topic
- verdemar   Reducing noise using a plurality of recording copies   Mar 26 2013, 12:55
- - DVDdoug   I theory this can work. Since the signal is coher...   Mar 26 2013, 19:24
|- - verdemar   Thanks, I also figured that the sychronization is ...   Mar 26 2013, 20:43
|- - Arnold B. Krueger   QUOTE (verdemar @ Mar 26 2013, 15:43) ...   Mar 27 2013, 03:03
|- - saratoga   QUOTE (Arnold B. Krueger @ Mar 26 2013, 21...   Mar 27 2013, 03:05
||- - Arnold B. Krueger   QUOTE (saratoga @ Mar 26 2013, 22:05) QUO...   Mar 27 2013, 12:53
||- - Woodinville   In order to make this work you must have: 1) exac...   Mar 28 2013, 00:21
||- - saratoga   QUOTE (Arnold B. Krueger @ Mar 27 2013, 06...   Mar 28 2013, 01:00
||- - Arnold B. Krueger   QUOTE (saratoga @ Mar 27 2013, 20:00) The...   Mar 29 2013, 14:05
||- - 2Bdecided   QUOTE (Arnold B. Krueger @ Mar 29 2013, 13...   Mar 29 2013, 15:21
|||- - verdemar   Sprechmann et al. use the multi-stencil fast march...   Mar 29 2013, 18:17
||- - Kees de Visser   QUOTE (Arnold B. Krueger @ Mar 29 2013, 14...   Mar 29 2013, 22:42
|- - soulsearchingsun   QUOTE (Arnold B. Krueger @ Mar 27 2013, 03...   Mar 27 2013, 08:36
- - extrabigmehdi   I know it's done for pics. If there are a cons...   Mar 26 2013, 19:33
- - Dynamic   QUOTE (verdemar @ Mar 26 2013, 11:55) Do ...   Mar 26 2013, 19:38
- - bryant   http://www.hydrogenaudio.org/forums/index....showt...   Mar 26 2013, 19:39
- - verdemar   Several replies while I was writing myself... @br...   Mar 26 2013, 20:56
|- - bryant   QUOTE (verdemar @ Mar 26 2013, 11:56) @br...   Mar 28 2013, 03:54
|- - verdemar   QUOTE (bryant @ Mar 28 2013, 03:54) Of co...   Mar 28 2013, 14:46
|- - 2Bdecided   QUOTE (verdemar @ Mar 28 2013, 13:46) I p...   Mar 28 2013, 17:39
|- - verdemar   QUOTE (2Bdecided @ Mar 28 2013, 17:39) It...   Mar 28 2013, 19:53
- - 2Bdecided   Wow! Thank you for sharing those David. The sy...   Mar 28 2013, 10:54
- - 2Bdecided   I don't think already declicked or denoised ve...   Mar 28 2013, 21:16
- - bryant   David and verdemar, thanks for taking the time to ...   Mar 29 2013, 07:56
- - 2Bdecided   Thank you for replying! Another potential use...   Mar 29 2013, 12:08

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