Audio noise reduction techniques
- 1. AUDIO NOISE REDUCTION TECHNIQUES Meenakshi Singhvi Deputy Director All India Radio, Vadodara Meenakshi_singhvi@hotmail.com
- 2. Virtually all audio recordings contain some amount of noise. This noise may join audio signal during recording process or due to long media storage, which is not acceptable by sound engineers.
- 3. To produce best quality audio recordings these unwanted audio noises must be removed to the greatest extent possible.
- 4. Importance of Digitization in Noise Removal
- 5. Sound inherently begins and ends as an analogue signal. Few years ago, removing noise from the audio was a very difficult task because of the high cost involved which was not affordable for a typical user.
- 6. The sound digitization and digital signal processing technologies changed this scenario dramatically. The new generation high-speed PCs equipped with quality sound cards and software applications has made audio noise removal work even more affordable.
- 7. Any vinyl surface is subject to micro fissures, scratches, and soiling. Over a period of time, the result is a constant deterioration in the quality of the audio. This deterioration shows itself in unwanted noise, clicks, and crackles.
- 8. Digitization process itself does not remove any type of the distortion or noise , but it allows us to suppress or eliminate such distortions or noise by applying special soft Noise Removal tools/algorithms to a digital sound.
- 10. Broadband Noise The noise in which the acoustic energy is distributed over a relatively wide range of frequencies called broad band noise or continuous noise. The sounds such as hiss and static fall in this category. It is measured in Signal to Noise Ratio (SNR). The average level of noise is called noise floor
- 12. Narrow Band Noise It is limited to a narrow range of frequencies. Usually this kind of noise has a constant level and frequency and normally caused by incorrect grounding and poorly shielded cables. It includes single frequency such a 50Hz or 60 Hz induced from the mains supply and its harmonics at 100 Hz, 150 Hz and so on.
- 13. Impulse Noise It includes sharp sounds such as clicks and pops. The pops are high amplitude audio impulses compared to the original audio signal and persist for a longer duration that is usually more than 2ms whereas clicks are lower in amplitude, higher in frequency and shorter in duration.
- 14. Digital clicks also exist which are caused by processor overload as we make a digital recording. The recording “stops” for a moment and the resulting skip creates a very short click.
- 15. In audio editing software we can see Clicks and Pops as below
- 16. Irregular Noise It includes sounds such as background conversation, traffic and rain. These types of sounds are very difficult to remove because they are made up of many random sounds that vary in frequency and loudness.
- 18. Minimize Noise Before Recording Before digitizing a LP record, we should clean the record with a soft micro fiber brush and mild cleaning solution to remove dust and then it may be vacuum dried. The stylus and cartridge of Turntable should also be in good condition. While recording from a tape deck we should ensure that heads are clean and demagnetized. Good quality shielded cables can further reduce noise from electrical interference.
- 19. While transferring a LP or a tape, a few seconds of silence may be recorded from the source which will show the amount of existing hum, hiss & static noise as well as it will help to create noise profile.
- 20. Computer Noise The quality of sound card plays a big role on the quality of recordings. When we record through a sound card the analog to digital conversion process adds distortion from quantization errors and electrical noise can be picked up from other components in the computer.
- 21. Lower priced sound cards are poorly shielded which makes them more susceptible to noise. Poor quality sound cards will have low resolution analog to digital converters which will introduce more distortion from quantization errors.
- 22. Moreover sound card should be placed in the slot farthest away from the computer's power supply and processor. Video card should also be placed as far away from the sound card as possible. The best way to avoid picking up electrical noise from inside the computer is to use an external audio interface which connects to a computer via USB or firewire port.
- 24. Set Appropriate Level During recording, it is important to set the recording levels as high as possible to obtain a good Signal to Noise Ratio and maximum Dynamic range. However it should not be set so high to produce clipping and signal distortion.
- 25. Level meters are usually labeled in dB with 0dB equal to the maximum level. Levels below the maximum level are shown in negative dB. Normally recording level peak should average around -6 dB and shouldn't exceed -3 dB. This will avoid clipping, simultaneously maintains a good signal to noise ratio. If recording level is set too low, any noise picked up from the analog circuits in sound card will be more apparent, because it will be proportionally louder.
- 26. Work In Stages It is always good to work on a copy of the original file and to experiment with different settings until satisfying results occur. All noise cannot be removed in one pass, we should work in stages and backup files should always be kept.
- 27. Preview The Result The audio restoration programs and plug-ins normally have `Preview’, ‘Bypass’ and ‘Noise only’ options. The preview option makes us to listen to the result and fine tune the setting before applying the changes. While the noise only option makes us to hear exactly what is going to be removed.
- 28. Record at Higher Resolution For best possible fidelity, we should record at 24 bit resolution and after removing noise we can convert back to 16 bit. Although it takes up more disk space but it is for temporary. It is better to record in an uncompressed format; it gives a chance to clean up the file before it is encoded.
- 29. Select Right Software It is very important to select right kind of software depending on the type of Noise. There are lots and lots of features in all softwares, so after proper comparison of software features and pricing we should go for buying. It is also advisable to first use evaluation copy and then purchase the actual software.
- 31. There are many different methods of audio noise reduction. Method one adopts must be based upon the nature of noise found in the audio signal. Basically Noise reduction is a series of filters which allow us to remove specific frequencies of audio, that is, the frequencies at which the noise occurs.
- 32. For obtaining best results the methods/tools should be used in following sequence which is based on the category of noise: – Reduce Narrow band noise – Reduce impulse noise – Reduce crackle and distortion – Reduce Broad band noise
- 33. Narrow Band Noise It is simply any unwanted signal that remains steady over time (repeated). It includes DC offset, hum and buzz from ground loops and acoustical noise from air handlers or motors.
- 34. Apply a high pass filter which removes rumble and DC offset with a proper slope setting and frequency control to determine the cut off frequency. To eliminate DC offset only set the frequency control at around 10 Hz. The harmonic notch filter set at 50 Hz or 60 Hz will be able to ensure AC hum easily removed. There are Q & gain controls, which set the width and depth of the notch filter, may also be taken care.
- 35. Clicks and Pops – De clicking Clicks and pops removal tool is generally available in all software. It scans the audio signal for spikes, remove them, then recreates the missing sound wave by analyzing pre and post click samples and interpolating the result using high order algorithms.
- 36. The bigger the click, the easier it is to identify and remove. At extreme settings, de clicking also may remove musical transients, like snare, drum attacks, so it is best to make two light passes by adjusting the software for shorter and longer clicks.
- 38. Distortion and Crackle Once the larger pops, clicks and hum are removed, we should try to remove continuous crackle and distortion.
- 39. Crackle is a series of pops and clicks that appears continuously. It can't be identified individually from the signal. It is a form of signal distortion. De crackle tool which is directly available in Audio softwares works very well.
- 40. The other common type of distortion is clipping (overloading) typically caused when a digital recording is made of an analogue source and the input level is too high. The extreme volume overloads the digital converters and the signal is clipped once it exceeds the peak value. This squaring produces additional frequencies in the signal which are noise or distortion.
- 41. Clip removal tool is commonly found to fix this kind of noise. This effect repairs audio by replacing clipped sections of static like noise with new audio data.
- 42. Broadband Noise The noise reduction effect for broadband noise works by capturing a noise profile from file and then using this profile to subtract the noise from the rest of the signal. This noise profile helps to reduce exactly the right kind of noise from audio recordings.
- 43. We should make a 'Noise print' by sampling a piece of 'silence' such as the space between songs. The software's filters are set to match this noise profile. This signal which contains unwanted noise is then subtracted from the audio signal leaving only useful audio signal without noise, this process known as spectral subtraction. It is extremely versatile and can be used for any background noise.
- 44. Noise reduction in Adobe we have to select a segment where there is only noise and no wanted sound. In the beginning normally, there are such a segment. Selecting this segment by dragging the mouse over it and open the Effects- Restoration-Noise Reduction filter. Then click on the capture Noise Profile.
- 45. What simply happens is that the program will "learn" that this is noise and store it in a profile. Now you have to select the whole waveform where you want to reduce the noise. If it is different file then save the Noise profile first, load it after opening the file in which you want to reduce the noise. Noise reduction in Adobe
- 46. Noise reduction in Adobe
- 47. Noise Gating Techniques A noise gate allows a signal to pass through only when it is above a set threshold value. The noise gate is used when the level of signal is above the level of the 'noise'. The threshold is set above the level of the 'noise' and so when there is no 'signal' the gate is closed.
- 48. Noise gating reduces steady noise sources such as rumble from LP records, hiss from audio tape, static from amplifier and hum from the power supply system without greatly affecting the source sound.
- 49. START STOP START STOP START STOP START STOP START STOP START STOP Noise - ex1 original-1 Noise- ex3 dehiss-click Noise – ex2 dehiss Noise - ex5 artifacts START STOP START STOP START STOP START STOP Neeley Parbhato Noisy Neeley Parbhato clean Tut gaye kangna noisy Tut gaye kangna clean KUNTA ABHIMANUE NOISY KUNTA ABHIMANUE CLEAN
- 51. Equalization Optimizing Sound Frequencies Through the process of equalization (EQ) we can amplify or reduce selected frequencies or groups of frequencies. It can make the overall bass, midrange, and treble more uniform from sound file to sound file.
- 52. It is a commonly used method for removing artifacts also while preserving the original audio clip e.g. i.e. analogue recordings might suffer excessive hiss at the high end of spectrum, so corresponding reduction there will help in removing hiss.
- 53. Audio normalization Normalize effect lets you set a peak level for a file or selection. Normalization increases the amplitude of the audio waveform to the maximum level, that does not introduce any new distortion because this is a linear translation.
- 54. Audio normalization Normalization applies a constant amount of gain to the selected region of the recording to bring the highest peak to a target level. Normalization may require two passes, depending upon the software employed. A first pass would determine the highest peak, and the second pass applies the gain to the entire recording.
- 55. Conclusion
- 56. The process of Noise removal should be used carefully as it may cause loss in audio data. Sometimes it is better to leave some noise in the recording than to remove a significant portion of good material.
- 57. THANK YOU
Editor's Notes
- #43: A more sophisticated way of filtering out is subtracting a signal that is exactly equal to the signal that is determined to be the unwanted noise. This is a very effective way and you can reach an amzing good result with it. This method works best if you have a piece of footage where you only have the noise, so that you can sample it and substract.