Unity Audio Pebble - Musikmesse 2013

Q. Why do waveforms sometimes look lop-sided?

When I look at the waveform of a vocal recording, it looks lop-sided, with the waveform going further above the zero-crossing point than it goes below it. It sounds fine, though, so what’s happening here?

This is a naturally occurring asymmetrical waveform, built from the linear sum of a cosine fundamental and its first four harmonics (created in Adobe Audition).

In this image we can see that as the waveform amplitude decays, it settles on the zero line (red). There is no DC offset.

This simple sine wave has a DC offset, which raises the centre of the sine wave well above the zero line. As the waveform amplitude decays, it remains well above the zero line.

This is the typical phase response of a ‘phase rotator’, which can be used to impose symmetry on asymmetrical waveforms by adjusting the phase relationships between fundamentals and harmonics. An increasing negative phase shift is applied progressively as the signal frequency increases, with the highest frequencies being shifted a full 360 degrees relative to the lowest frequencies.

Bruno D’Cunha, via email

SOS Technical Editor Hugh Robjohns replies: This kind of asymmetrical waveform is quite natural and normal, and is particularly common on recordings of speech and vocals, brass instruments, and sometimes also closely miked strings. A lot of percussive sounds are also strikingly asymmetrical, of course.

In the ‘BC’ era (Before Computers) we didn’t look at waveforms, we just listened to them, and this kind of asymmetry was generally inaudible and didn’t bother us, although some people are sensitive to absolute polarity, and can actually tell if an asymmetrical waveform is inverted. Waveform asymmetry has been known about for a very long time, and in the few areas where it can be an issue (such as in broadcast processing), technology has long been in place to deal with it. However, since the prevalence of the DAW and its ubiquitous waveform display, a lot of people have become aware of it and asked the same question.

This asymmetry is due mainly to two things, the first being the relative phase relationships between the fundamental and different harmonic components in a harmonically complex signal. In combining different frequency signals with differing phase relationships, the result is often a distinctly asymmetrical waveform, and that waveform asymmetry often changes and evolves over time, too. That’s just what happens when complex related signals are superimposed.

The other element involved in this is that many acoustic sources inherently have a ‘positive air pressure bias’ because of the way the sound is generated. To talk or sing, we have to breathe out, and to play a trumpet, we have to blow air through the tubing. So, in these examples, there is inherently more energy available for the compression side of the sound wave than there is for the rarefaction side, and that can also contribute to an asymmetrical waveform.

Confusingly — and erroneously — this natural waveform asymmetry is often attributed to a ‘DC offset’, but that’s not the case at all. A DC offset is a specific fault condition where the varying AC audio signal voltage is offset by a constant DC voltage, and the ‘tell-tale’ is that, although the waveform might look asymmetrical, a decaying signal waveform settles away (offset) from the centre zero-line.

DC offsets are virtually unheard-of these days, but can occur in hardware analogue electronics under fault conditions. In the digital world, very early multi-bit A-D converters sometimes suffered a problem in the quantiser that essentially resulted in encoding a fixed-level shift or offset onto the audio sample values — the digital equivalent of an analogue DC offset.

However, a DC offset can be very easily corrected by passing the audio through a high-pass filter tuned to a low frequency (typically 10Hz or lower). It is important to correct DC offsets when they do occur, because editing between an audio clip with a DC offset and one without results in a loud thump or plop at the edit point, which is not good!

In contrast, natural waveform asymmetry cannot be ‘corrected’ with a high-pass filter, and a rather more complicated solution is required called a ‘phase rotator’. Generally, there is no need to ‘correct’ a naturally asymmetrical signal, but occasionally the asymmetry can restrict how much the signal can be amplified because the stronger half of the waveform will reach the clip level before the weaker side. By using a phase rotator process to alter the harmonic phase relationships, a more balanced symmetry can be established, allowing slightly more gain to be applied before both sides reach the clipping level at the same amplitude. Asymmetrical waveforms can also sometimes confuse the side-chain level-detection circuitry (or algorithms) of some compressors, resulting in less effective compression than might be expected.  

Published in SOS May 2013

Solid State Logic Live L500 - Musikmesse 2013

Q. What insurance do I need?

I’ve had a home studio for a few years now that I use mainly for my own projects and some commissioned work. I’m occasionally asked to record people and am usually paid a small fee for doing so, though it doesn’t form a large part of my business. My equipment is insured in the usual way, but do I need to worry about any other factors, such as public liability, when insuring my studio?

If you earn any money at all from your home studio, it’s important to read the small print of your insurance policy, and inform your provider if necessary. Business activities on any scale could invalidate your insurance, whether or not a claim is related to your studio work. It may be that taking out specialist cover is the most sensible option, particularly when you consider the potential costs of public liability claims.

Owen Morrison, via email

SOS contributor Mike Senior replies: 

If your gear is already insured, the main question when recording other people in your home is one of public liability. In other words, if the artist trips over their stompbox and impales themselves messily on your lava lamp, will you be covered when their relatives sue for compensation? If you were recording entirely as a hobby, then you’d probably be covered in such situations under your normal household insurance policy, although (as with anything relating to insurance) this is something that you need to check in the policy particulars to be absolutely sure of. However, the fact that you’re making some money from your recording, albeit small, may disqualify your studio activities from this domestic public liability cover, in which case you might feel you need an additional professional public liability policy. It may be that you can negotiate a certain degree of wiggle room with your domestic insurance provider, though, so it might be worth giving them a ring to confirm exactly what is and isn’t covered in this respect, and how much you are able to earn from your studio work while still qualifying for cover under a domestic policy.

If you’ve insured your gear with a specialist music technology insurer, you may find that you already have professional public liability cover built into this insurance, and that’s all well and good, if so. If not, then you have to make a judgement call as to whether the risk of a compensation claim is high enough to warrant the yearly cost of dedicated public liability cover. It’s not illegal to do studio work without public liability insurance, but unless you know your clients very well, it’s probably a bit of a foolhardy economy given the litigious times in which we live.

Adrian Scott, head of Musicguard, adds: If you use your home for business purposes, however infrequently, you’ll almost certainly need to let your home insurer know to ensure your cover isn’t affected. Most standard home insurance policy wordings specifically exclude business equipment (other than for clerical purposes) from their contents cover, which would include your music equipment. Worryingly, you may find other aspects of your insurance are invalidated simply by running a business from home unless you get their acceptance regardless of how you choose to insure your equipment. An extreme example may be that if the equipment you were using causes a fire due to some form of electrical failure. In this example your insurer may refuse to pay out a claim as the incident occurred due to business activities that they had not been made aware of. Insurers usually state quite clearly that you must inform them if your home is used for business purposes, and may reduce, or even turn down, a claim payout if they feel you haven’t been open with them.

As Mike stated, a specialist music insurance policy will cover your gear whether you’re using it for business or pleasure, giving you the breadth of cover you’d expect, such as theft, accidental damage, loss or mechanical/electrical breakdown, and will almost certainly come with public liability included, or at least be an option. Just make sure that the money you’re already spending on insurance to protect your home and other possessions isn’t wasted because you’ve fallen foul of their small print.    

Published in SOS May 2013

Universal Audio Apollo 16 - Musikmesse 2013

Q. Which should I check first: monitors or FOH?

I’m responsible for live sound at a lot of small shows where there isn’t the budget for a separate monitor desk or engineer. In this situation, I’ve seen engineers handle things in different ways. Some concentrate on getting the sound right on stage first before bringing up the front-of-house speakers. Some make sure the sound out front is right and only then turn up the aux sends on any instruments that the band are struggling to hear. Others go through, instrument by instrument and set levels for both FOH and monitors at the same time. What are the pros and cons of each approach, and which would you recommend?

Sound engineers can differ over whether to set up stage monitoring or the front-of-house sound first. Our contributor likes to get a rough FOH mix done and then move onto the wedges, leaving fine-tuning until the band are on stage.

Lee Entwistle, via email

SOS contributor Jon Burton replies: 

This is a very common situation and one I’ve come across many times. When I’m doing monitors from the same desk as the house sound, I always try to use a Y-split cable on the lead vocals. If there are enough channels this means you can split signal across two channels, one dedicated to the monitors and one to the FOH. This has the advantage that you can set and leave the monitor channel optimised for the stage sound, whilst having an FOH channel that you can equalise and compress during the show, knowing that it is not adversely affecting the sound on stage. Even if I can’t do this, I always create a rough front-of-house sound first. I set the gain for both channels, then set the EQ flat on the desk, but with the high-pass filter in, if there is one. I will then concentrate on checking all the wedges on stage. If there are any equalisers on the monitor sends, I usually flatten these. I then check each monitor in turn, speaking normally through the mic, using the same desk channel and microphone for each monitor. By doing this I can check that each speaker is working correctly. If they are not, which is not unusual, I’ll try to fix them, checking connections and drivers, for example, and, failing that, move the best-sounding ones into the most crucial positions!

If there are graphic EQs, I try not to do too much, as I prefer them to look like smiley faces rather than cross-sections of the Himalayas. If you hack away with a graphic, you’ll usually start causing more problems than you’re solving. If there are no outboard equalisers, I’ll EQ the channel, but only as a last resort.

Having got all the wedges working and sounding OK, I’ll then get all the vocal microphones up in their respective wedges. Once I’m happy that vocals sound good on stage, I’ll start soundchecking the other channels.

I always leave the vocal microphones open but dipped a bit during the soundcheck, as they will be on during the show and will contribute a lot to the overall sound coming from the stage, adding high-end spill to the drums and other instruments.

After I have checked the vocals, I like to continue with drums, getting the drummer to play a simple beat on kick, snare and hi-hat. I prefer to do all three at the same time, as this way the drummer tends to play more naturally, like he or she would in a show, rather than repetitively hitting a drum, which is monotonous for all — and unrepresentative. Checking all the instruments one by one, I then usually leave the FOH master faders at half volume while the band play a song. During this time, I’ll work on the monitors for them, maybe adding keys or kick drum. I always dip the FOH, otherwise the sound of the loud PA in an empty room will drown the stage. If you do leave the PA system at a higher level, you enter into an upward spiral of volume where everybody is competing to hear. I usually know when all the channels sound good and I have a rough balance; the time for fine-tuning will be when the room is full and the first chord is struck!

Before the performance, time is always against you, and I prefer to get the stage sound right as fast as possible, usually before the band arrive. Soundchecking monitors is always easier on a quiet stage without musicians tuning and checking their instruments. Checking FOH is a lot easier, as you can just don a pair of headphones and check your channels, returns and inserts. So I would always prioritise and make sure the monitors are sorted before checking the band.

For more advice see the article ‘Effective Soundcheck’ in SOS July 2012 (www.soundonsound.com/sos/jul12/articles/soundchecking.htm) for advice from some top live engineers!  

Published in SOS April 2013

Focusrite Red 1 500 - Musikmesse 2013