Welcome to No Limit Sound Productions

Company Founded
2005
Overview

Our services include Sound Engineering, Audio Post-Production, System Upgrades and Equipment Consulting.
Mission
Our mission is to provide excellent quality and service to our customers. We do customized service.

Wednesday, August 24, 2016

Q. Is it possible to record in surround on only two tracks?

By Hugh Robjohns
This simplified polar pattern diagram shows how a figure-of-eight mic (red) and a coincident omnidirectional mic (blue) can be combined to produce a directional cardioid pickup pattern (green). Flipping the polarity of the omni reverses the direction of the cardioid pickup.

This simplified polar pattern diagram shows how a figure-of-eight mic (red) and a coincident omnidirectional mic (blue) can be combined to produce a directional cardioid pickup pattern (green). Flipping the polarity of the omni reverses the direction of the cardioid pickup.
Would it be possible to use two figure-of-eight mics to create a surround sound recording on a two-track recorder, which could be decoded and mixed later on? The two mics would be placed at 90 degrees capturing left and right and front and back respectively. I'm after a quick and portable way to make surround recordings in the field, and portable recorders with more than two inputs are expensive! As far as I can see this could work, unless there's something I'm missing about figure-of-eight decoding?

SOS Forum Post
Technical Editor Hugh Robjohns replies:

There is something you are missing — a third mic to resolve the ambiguity inherent in a figure-of-eight mic over which side of the diaphgram the sound strikes. You cannot derive front-to-back directional information with just two crossed figure-of-eight mics.
Imagine I'm in a studio and you are in a control room, without sight lines between the two. There is a single figure-of-eight mic in the studio and I'm talking into it. How could you tell whether I was talking into the front or back of the mic just by listening? The answer is that you couldn't — a figure-of-eight mic provides signals of identical level from front and rear sources. Yes, the polarity of the signal is inverted between front and rear sources, but without a reference to know which polarity was which, you are no better off.

If you think about it, when you combine the two coincident figure-of-eight mics mounted at 90 degrees, you effectively end up with another 'virtual' figure-of-eight pointing midway between the two original mics. You can use that to provide left-right discrimination — which allows the arrangement to be used for stereo — but you will get the same level of output from a source behind and to the right of the array as you would from one to the left front. That doesn't matter in stereo: in fact it is often very useful that rearward sounds are folded back onto the front. However, it is obviously a problem in surround because we need to be able to have rearward sounds coming out of the rear speakers!
Q. Is it possible to record in surround on only two tracks?

So, what we need to be able to do is create virtual polar patterns that have front-back discrimination, and that basically means creating 'virtual' cardioid patterns. These can be derived by combining an omnidirectional mic's polar response with a figure-of-eight.

If you have two coincident figure-of-eights, facing left/right and front/back, and you add to that a coincident omnidirectional mic, you can combine them in various ways to create virtual cardioid patterns pointing in almost any direction you like. Let me explain how...
Consider what happens if you mix together the output of an omnidirectional mic and figure-of-eight, the two capsules being coincident. The omni mic picks up sound from all directions equally. The figure-of-eight picks up sound only from the front and back, rejecting sound sources to the sides, and its rearward pickup is in the opposite polarity to the front.

If we arrange the polarity of the omni to be the same as the front of the figure-of-eight mic, then when you mix the two mic outputs together their contributions will add together for frontal sound sources. So the resulting 'virtual mic' will be very sensitive to frontal sound sources.
Sources to the side are not picked up at all by the figure-of-eight mic, but the omni still hears them. So the 'virtual mic' created by the combination of the two is not as sensitive to sounds from the sides as it was from the front, but it does still hear them.
The four coincident capsules of a B-format Soundfield mic, and a diagram showing the intersecting pickup patterns of the three figure-of-eight capsules (X, Y and Z) and one omni-directional capsule (W), courtesy of Soundfield.The four coincident capsules of a B-format Soundfield mic, and a diagram showing the intersecting pickup patterns of the three figure-of-eight capsules (X, Y and Z) and one omni-directional capsule (W), courtesy of Soundfield.

Sources to the rear are heard by both the figure-of-eight and omni, but the figure-of-eight's output is of the opposite polarity to the omni. Hence, when the two outputs are combined they will cancel each other out. Thus the virtual mic hears nothing at all from the rear, and if you draw this polar pattern out accurately you'll discover that we have just created a cardioid microphone (facing forwards). You can also create other first-order polar patterns (sub-cardioid, hypercardioid and so on) by varying the ratio of the omni and figure-of-eight contibutions (in other words changing the gain of each).

So you see that by introducing an omni mic into the array, you can resolve the inherent front/rear ambiguity of the figure-of-eight pickup patterns, by converting them into cardioid patterns, and by adjusting the ratios and polarities of the signals from the two figure-of-eights, you can make those cardioids face in pretty much any direction you like. You can now generate any number of virtual microphone outputs which 'hear' only the sources in front of them — front left, centre front, front right, rear left and rear right, for example.

This is the basis of horizontal Ambisonic encoding — the 'B-format' — as used by the Soundfield mic. The omni component is called W, the front/back figure-eight is called X and the left-right figure-eight is called Y. The Soundfield mic also adds a third figure-eight element for the up/down axis (called Z), although this isn't really needed in most surround applications. You can read more about the Ambisonic approach in SOS October 2001.

Another (arguably more practical) way of recording horizontal surround sound is the MSM format. This uses the same basic concepts, but is constructed from a pair of matched cardioids facing to the front and rear, plus a sideways-facing figure-of-eight. Again, all three should be coincident. The front cardioid and the sideways figure-of-eight are decoded as a conventional M&S pair for the frontal sound stage, while the rear cardioid and the same figure-of-eight are decoded as another M&S pair for the rear channels.

However, whether you adopt the crossed figure-of-eights plus omni approach (WXY format), or the front/rear cardioids plus figure-of-eight approach (MSM format) you do need to be able to record at least three channels, and there is no getting away from that!
If you really want to encode surround onto a two-track machine, you have to use some form of phase/amplitude matrix system like Dolby Pro Logic or RSP Circle Surround. However, while these formats are acceptable for final mixes delivered to the end user, they are far too restrictive for source recordings because you can't easily manipulate the signals to alter the spatial surround characteristics later on, as you can with either of the three-channel surround recording techniques described above.

Published December 2005




Monday, August 22, 2016

Power & Electrical Safety On Stage

By Mike Crofts
PA Basics

Staying safe on stage is more than a matter of simply making sure that willing hands are available before taking a dive. Knowing how to properly handle the mains power we all need is also crucial to performance health...

Whatever the size, complexity or cost of your live sound rig, one of the first — if not the first — question on your mind when you get to a venue will usually be "where do I plug it in?" Depending on the venue, the answer can vary from a wall-socket behind a plant pot to a dedicated and professionally-installed supply that is reserved for your exclusive use, fully tested and certificated, and for which (with any luck) you'll have brought an appropriate connector. Whatever you encounter, you'll need to know some basic rules. When it comes to portable live-sound systems, this means firstly, using a suitable electrical supply; secondly, using suitable equipment; and, thirdly, connecting and using that equipment safely.

How much power will the average band's gear actually need? The only way to know for sure is to add up the power requirements of each individual item. 

How much power will the average band's gear actually need? The only way to know for sure is to add up the power requirements of each individual item.Photo: Mike Crofts

How Much Mains Power?

What constitutes a suitable supply will depend, of course, on what you need to plug into it: if it's your own equipment you'll presumably know what supply capacity is required, but there may be other factors to consider if additional gear needs to be connected to the same supply. Such gear might include a visiting disco, a lighting rig, or other event equipment — for example, fridges at summer events.

A good first step, then, is working out what current your equipment will draw from the mains. The power rating of each piece of gear should be stated on a panel fixed close to the mains connector, or where a fixed mains lead enters the equipment. The power rating may be expressed as a current (in Amps) or as a power figure in Watts. It's generally best to work out the total current your gear will draw, adding up all the individual figures to find the total load you'll be connecting to the mains. To convert Watts to Amps, divide the Wattage figure by 230 (mains voltage). As an example, a piece of equipment with a mains power rating of 100 Watts (not 100W of audio power) will draw a little under half an Amp. In a small venue that is only offering 13-Amp sockets of the normal domestic type, you can then work out how you need to wire up. If the total connected load of your system — including the backline equipment — is comfortably within the rating of a single or double 13-Amp socket, it's perfectly alright to connect it all from a single point. After all, that's what they're designed for! Try to avoid too many connections between this point and your equipment. It's much better to have a single power lead of the required length than two shorter ones joined together: less to go wrong!

A professionally made distribution box with meters to indicate AC mains voltage and current. 

A professionally made distribution box with meters to indicate AC mains voltage and current.Photo: Mike Crofts

Working It Out

One common mistake is assuming that audio output power is the same as the mains power required to operate the gear. If an amplifier were 100 percent efficient, you could, in theory, use all the mains power as audio output power, but this is not the case in practice, as some of the power used by the amplifier is dissipated as heat. A typical full-range 'active' speaker with built-in amp modules, rated at 240 Watts audio output, would have a mains power rating somewhere around 350 Watts. A useful rule of thumb (if you don't have the manufacturer's stated figures) is to multiply the audio output power by 1.4 to get an idea of how much mains power would be needed, then divide by 230 to find out the current consumption.

The table below gives a rough guide to the supply current likely to be required by a band with three backline amps and a vocal PA (based on UK voltage). Bear in mind that equipment may demand a much bigger supply current when it is first switched on, so don't be tempted to turn everything on from a single switched socket — you wouldn't want to do this anyway, for many other reasons, such as risking a huge pop through your speakers! Also consider that the power that you can safely run your system on may not be enough to realise its full performance capability. Any system capable of delivering good bass power will need to draw a hefty current from the mains, and if, in the above example, we were to replace our typical small speakers with, say, a pair of Mackie SA1521s, the makers recommend that each speaker's mains supply is capable of providing seven Amps at 230 Volts! This is, of course, not a constant current requirement, but it does illustrate how important a good power source is for getting the best from your gear.

Equipment

Mains Power needed

Mains Current needed

Amps
2 x 240W active PA speakers480W x 1.4 = 672W672 / 230 = 2.92 Amps2.92
Mixer100W (stated)100 / 230 = 0.44 Amps0.44
2 x rack processors20W each (stated) = 40W40 / 230 = 0.17 Amps0.17
3 x 100W backline amps300W audio x 1.4 = 420W420 / 230 = 1.83 Amps1.83
TOTAL = 5.36
ROUNDED UP = 5.5 Amps

User Beware

While we're talking in Amperes, it's worth remembering that electrical current is a dangerous animal; a current of only 50 Milliamps (0.005 Amps) can be fatal, and our typical small rig above is using over a thousand times more current than this. Safety is thus a huge consideration, and the use of a suitably rated supply is only the beginning. The best way to stay safe is to use only well-maintained equipment (including cables and connectors) that are properly designed for the task in hand, and to make sure that they are used as the manufacturers intended.

A professionally made distribution box with four 16-Amp outputs, all RCD protected, for limited outdoor use. 
A professionally made distribution box with four 16-Amp outputs, all RCD protected, for limited outdoor use.Photo: Mike Crofts

If the venue in question is unfamiliar to you and you are responsible for providing and operating the PA, always check that the supply you're asked to use is suitable. Just because it's a 13-Amp socket doesn't mean that it's capable of supplying 13 Amps: it may have been DIY-installed as a spur from a domestic ring main, originally to light a garden shed or run a fountain or something! If you're operating in any kind of business or commercial premises, they should have an up-to-date electrical safety certificate. A quick look at the distribution board or consumer unit should show the overall current rating of the circuit you'll be using, and you can also see if it uses old-style wired fuses or the more modern MCBs (Miniature Circuit Breakers), which react more quickly if the rated current is exceeded.

Fuses and MCBs do not protect you from electric shock, so always make sure that your system is fed via a residual current device (RCD). This could be at the main board/box, on the socket itself, or at the point where a separate spur is fed. If you're not sure that this is the case, use your own RCD, either as a plug type or one of the RCD plug-in adaptors readily available for a few quid from any electrical retailer. The RCD should be as far 'upstream' as possible so that it protects as much as possible, and wherever it is, make sure you test it before use, by using the built-in test button. If it doesn't seem to work, find another!

A final word on RCDs: they are there as a backup in case anything goes wrong, not as a substitute for poorly-maintained, faulty or unsuitable equipment.

Going Through A Phase

Most small venues are likely to have a single-phase supply, as will normal domestic premises, and for the purposes of this basic article we won't be looking at the whys and wherefores of 'three-phase' systems, other than to point out that all the sound equipment should be connected to the same phase. Any other electrical equipment, such as lighting, should also share the sound-system phase if it is possible for a person to come into physical contact with both systems — for example, to touch the lights and a guitar at the same time. It's a given that the venue's technical staff should supervise any connection to a three-phase supply.

Distribution Deal

A damaged mains plug recently discovered at the bottom of the cable trunk — to be disposed of straight away! 

A damaged mains plug recently discovered at the bottom of the cable trunk — to be disposed of straight away!Photo: Mike CroftsHaving found a suitable supply point, you now have to feed it to all your equipment. For all gigs where a 'proper' supply is available, I use a professionally made portable distribution box, which has a single 32-Amp inlet and 32-Amp breaker, feeding four 16-Amp outlets, all via separate combined RCD/MCBs. Although, on the face of it, I've got four 16-Amp outlets, giving a total of 64 amps, I can only use 32 Amps overall, with each outlet limited to 16 Amps. I run my front-of-house speakers from two of these feeds, the monitors and desk from the third, and the stage backline from the fourth. This splits up the load and ensures that each feed is fully RCD protected. As mentioned earlier, it is always best to have an RCD as far upstream as possible, and I would ensure that my original 32-Amp source incorporated suitable protection if available.

For smaller indoor events, a single 13-Amp fused RCD plug feeding into a multi-way distribution board (four or six sockets) is fine, as the total current can't exceed the 13-Amp fuse rating in your RCD plug. From this distribution board you should try, where possible, to connect direct to equipment, or feed the equipment in logical groups. Normally, you can take the initial feed from a socket at the back of the stage and run all your backline straight from this, with one feed going off to the PA system. If you need to use more than one socket in a small venue, ensure that all your signal connections are balanced, and never, ever remove an earth connection to get rid of hum or noise. Also take care when using those 'flying saucer' extension reels. They are very useful and neat, but remember that their maximum current-carrying rating only applies when the cable is fully unwound.

Care & Maintenance

All leads, connectors and equipment should always be checked before use, even if this is a quick visual check for any obvious signs of damage. If it's your own gear, you'll know it's all correctly fused, but it's best to check if you're not sure. Cables should be undamaged along their entire length and plugs should be securely clamped on, with no inner conductors visible. Cables with moulded plugs are a common sight nowadays, but these plugs cannot ever be re-used, and if damaged or removed for any reason they must be thrown away — preferably after destroying them so that an unaware person can't find one and plug it in. If anything looks faulty, then it probably is. Remove it from service and make sure it can't be used again until it has been repaired and tested.

All electrical equipment, including cables and connectors should be stored and used in dry conditions unless it is designed for outdoor wet weather use and carries an appropriate IP rating (for mains connectors this will usually mean industrial 'Ceeform' types — coloured blue — either rated IP44 (which is splashproof) or IP67 (which is waterproof).
Never be tempted to 'lift' the earth wire for any reason. If you find a plug like this one, with the earth disconnected, don't use it. 

Never be tempted to 'lift' the earth wire for any reason. If you find a plug like this one, with the earth disconnected, don't use it.Photo: Mike Crofts

Summing Up

We've covered the basics of finding a suitable supply and connecting the gear to it, but there are other things to consider when rigging. Cable runs need to be thought out to avoid or minimise trip hazards, and a generally neat cabling job will be much easier to troubleshoot than a spaghetti surprise. Don't forget the rule 'signal before mains'. Connect the power leads last and switch on after everything has been connected in the signal path (with the master levels down, of course). Turn on your power amps last of all, and switch them off first when powering down the system.

In this article I've taken a very basic and superficial look at the power side of live sound. There's a lot of additional good advice to be found, and it's well worth taking a professional approach and discovering as much as you can. After all, if you were going to jump out of an aeroplane you would, presumably, want to know that your parachute was (a) of the correct type; (b) correctly installed on your person; and (c) recently tested! Electrical power is a serious business, so if in doubt, ask a qualified electrician. If you don't know one personally, someone you know will, or you can look one up in the phone book.
There are also plenty of useful pages on the Internet. The UK's Health and Safety Executive web site, for example, has a lot of relevant information and links to some very good guidance publications. Check out www.hse.gov.uk.

Safety Checks

Your visual examination, before connecting any equipment, every time you're about to use it, should include checks for:
All portable electrical equipment should be periodically tested for electrical safety, and 'PAT' (Portable Appliance Testing) records kept. Some venues will not allow you to use anything which hasn't been properly tested. Get a quote from your local electrician for testing; it's not expensive and is well worth it for the peace of mind. 

All portable electrical equipment should be periodically tested for electrical safety, and 'PAT' (Portable Appliance Testing) records kept. Some venues will not allow you to use anything which hasn't been properly tested. Get a quote from your local electrician for testing; it's not expensive and is well worth it for the peace of mind.Photo: Mike Crofts
  • Damage to cables or plugs, including cuts, cracks, abrasions, bent or missing pins.
  • Previous repairs or modifications, including exposed or taped-up cable joins and unsuitable connectors.
  • Exposed inner conductors where the cable enters the mains plug.
  • Signs of damage to casing and covers.
  • Obvious signs of previous problems; for example, signs of water, moisture or heat damage.
A visual check on a regular basis (by a competent person, such as a qualified electrician or someone with appropriate training) should include taking the cover off each mains plug and checking that:
  • All wires are firmly attached (screws nice and tight) to the correct terminals, with no bare wires showing.
  • The cable outer sheath is firmly gripped by the cord grip.
  • There is no debris or signs of damage internally.
Electrical testing on a regular basis (by a professionally-qualified and suitably trained person) normally includes all of the above, plus:
  • Additional testing of earth integrity and insulation.
  • Test results recorded and appropriate labels attached to the equipment.
  • Failed equipment identified for disposal or repair.
Published December 2005

Friday, August 19, 2016

Q. What is the difference between mono with one speaker and mono with two?


By Hugh Robjohns

A single speaker in a sealed enclosure is the classic means of monitoring in mono.I read recently that when top engineers check their mixes in mono, they don't just hit a mono switch, but instead route the mix through a single speaker to hear it in true mono. What's the difference between the two? 

A single speaker in a sealed enclosure is the classic means of monitoring in mono.

SOS Forum Post
Technical Editor Hugh Robjohns replies:

It's important to check the derived mono signal from a stereo mix to ensure that nothing unexpected or unacceptable will be heard by anyone listening in mono, as could be the case in poor FM radio reception areas, on portable radios, in clubs, on the Internet and so on. Mono compatibility, as it's called, is very important for commercial releases — the artist, producer and record company want the record to sound as good as possible in these less-than-ideal circumstances.

In addition to simply checking the finished product, mixing in mono — or regularly switching the monitoring to mono while mixing — is very useful and a good habit to get into. Summing to mono removes any misleading phasing between the left and right signals that can make a stereo mix sound artificially 'big'.

The crucial difference between auditioning the summed mono signal on a single speaker, as compared to a 'phantom' mono image between two speakers, relates to the perceived balance of the bass end of the frequency spectrum. When you listen to a mono signal on two speakers, you hear a false or 'phantom' image which seems to float midway between the speakers, but because both speakers are contributing to the sound, the impression is of a slightly over-inflated level of bass. Listening to mono via one speaker — the way everyone else will hear it — reveals the material in its true form!

Checking the derived mono is always best done in the monitoring section of the mixer or with a dedicated monitor controller. Although a mono signal can be derived in the output sections of a mixer (real or virtual), this is potentially dangerous — if you should forget to cancel the mono mixing, you'll end up with a very mono final mix. It does happen, believe me! Sadly, very few monitor controllers outside of broadcast desks and related equipment provide facilities to check mono on a single speaker. Most provide a phantom mono image, which is fine for checking imaging accuracy and phasing issues, but no good for checking the mono balance.

Published November 2005





Published November 2005