In this tutorial I am going to attempt to give a brief outline of some of the problems we can strike when doing audio post production or sound mixing work. If you are the one responsible for the project from the outset, including both the recording aspect, and the mixing and post production work, it makes sense to try to address the issues of unwanted noises before committing to tape (ok well more likely hard drive, but I’m sure you know what I mean). If you have received someone else’s recordings, and are responsible for the final mix/master, then it is already likely too late for that, but here are a few pointers that will hopefully help you with fixing the issues already in existence, or helping to avoid future ones that you have control over.
DC Offset Problems
One problem with using cheap soundcards or recorder inputs is they often add a DC voltage offset to the audio when recording. This sound sits at 0 Hz, which we can’t actually hear, but it offsets the wave so that instead of it being centered, it sits above the zero line. This means you that you will have less audio headroom before clipping occurs. Have a look at your tracks, and if tthe wave is not centered around the zero, then you need to apply a DC offset filter to it. If at the time you were recording, the meters in your DAW were reading even when you were not sending them any signal, that could likely be a sign of a DC offset problem. In most recording or editing software you will find that they have a functionlabelled something like “remove DC offset”. Use it if the problem is evident, but preferably do it prior to any compression or EQing. It should get rid of any unwanted offset.
Unwanted Hums and Buzzes
Hums and buzzes in your audio, especially from things like guitar amps can be a problem when recording. Try to elimante the sources of such unwanted sounds before undertaking the recording process. Try to avoid earth loops. Keep all your audio sources on one power circuit (or phase), and any lighting or other appliances on a different circuit. Make sure all your equipment is properly grounded. Check your cables, making sure all wires and shielding are firmly attached at the terminals. It’s amazing how often a loose wire can cause intermittent noises. If the problem is a guitar and amp comination, experiment with different proximities and angles of the guitar in relation to the amp. Facing in one direction you may experience hum, but if you turn your body and the guitar in another direction the hum may disappear. Single coil pickups are more prone to this problem than hum-bucking pickups ( hence the name). Use balanced cables where possible, and avoid long runs of unbalanced cables. Read the tutorial Audio Signals: Basic Guide to levels, Signal Types and Usesand the paragraph titled “Balanced or Unbalanced” for more information on this topic. Also try to avoid having power cables and transformers running near audio cables. Keep them at least a foot (30 cm) apart, if possible, and if they must cross, try to have them do it at right anges, or better still sit the power cable on something above the audio cables so they don’t touch. Hums can be dealt with by some programs and plugins such as The Waves X-Hum, but it is more difficult to remove buzzes from a recording without altering the tone of the recording. The center frequency of the hum is likely to be either around 50 Hz, or 60 Hz, depending on the voltage of the power supplied, and this varies from country to country. Sometimes you can use the “Learn” function of a noise reduction platform to select and remove buzzes and such like from good audio. If you have an area of audio that is essentially silent apart from the offending noise, you can use this section of the audio for the software to “learn the noise,” and then remove it from the rest of the track..
Hiss and white Noise
Hiss is often a result of incorrect gain settings when recording. Perhaps you had a quiet source, or a small a signal, and the gain on the preamp was set high to compensate for this. You may then pick up noise from the preamp, or amplify the self noise of a microphone. Perhaps you just have a noisy piece of equipment. Whatever the cause, use your denoiser, dehisser or noise removal plugin, software or hardware function before you compress, EQ or otherwise tamper with the audio. If you compress before you use noise removal, you will likely be boosting the noise along with the signal, but if you remove the noise first, the compressor then won’t find any white noise to boost! Higher and more aggressive settings on these programs often seriously mess with the signal, adding unwanted artifacts, and change the tone considerably. Try to avoid using overly aggressive settings, if at all possible, try mild settings first. Sometimes making a couple of passes with mild settings does a better job than one pass with a more extreme setting.
Removal of Clicks and crackles
Unwanted clicks and crackles can be a real bane of digital recordings at times. Some suggestions I can offer for minimizing them are as follows:
Defragment your hard drives regularly to ensure the computer runs fast enough to keep up with the high demands of recording, mixing and running video and audio together.
Adjust your DAW program buffers to avoid clicks
Use the latest drivers for your interfaces and other attached hardware
Use crossfades at edit points
Avoid clipping any equipment’s inputs while recording or any outputs while mixing
If you are unfortunate enough to discover clicks in your audio, use your DAW’s or plugin declicker, and try to get as close to the area of the waveform as possible by cutting the region either side of the problem area, and selecting just that small area of the region for processing. Sometimes you can get better results by manually removing clicks using methods such as re-drawing a waveform, with the pencil tool available in the software.
Hopefully you have found some useful answers to unwanted noise problems in this article. I would suggest if you are intending to do your own audio editing, post production, mixing or mastering, that you try to obtain a good software platform and suitable plugins for this task. Learning to use them well is something that does take a bit of experience, and a bit of ear training, but having good tools to work with means half the battle is already won.
Written by Tony Koretz
© copyright February 2012