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Widen your horizons (Part 1)

FX tutorial

Article from The Mix, February 1995

Part one of Bob Dormon's effects tutorial


All you ever wanted to know about effects processing will be revealed in Bob Dormon's two-part guide. In the first installment, Bob explains the difference between a plate and a spring, and reveals some of their more unusual uses...

The ultimate reverb? – St Paul's Cathedral, The Vatican

Before music was actually recorded, performances would take place in buildings that had good acoustics. Churches are a fine example, and many concert halls were designed in order to give an impressive reverberant sound to the activity on centre stage.

However, a 'big' sound doesn't always make for beautiful music, and venues such as the Royal Festival Hall can actually change how reverberant they are, with mechanically-controlled sound absorption panels built into the walls. During the course of a performance, the walls actually move to create a complimentary acoustic atmosphere. Great stuff, but for the likes of you and me, an electronic simulation is by far the easiest way to go.

Many studios still sell time on the reputation of certain recording rooms. The sound of a room can add a certain something to a recording that an effects device simply can't emulate. It doesn't stop manufacturers of effects units from trying 'though, and for those of us on modest recording budgets, what they have to offer is the next best thing.

Using natural acoustics can be both a pleasure and a pain. For the home recording enthusiast and sound engineer alike, the pleasure lies in the relative simplicity of an acoustic recording session. The pain is that when you're stuck with a room that doesn't flatter what you're recording. Another constraint is noise. Singing in the bathroom might sound ideal, until the guy in the flat upstairs flushes the toilet and adds new meaning to the term a 'wet' effect.

In the 70s, it became fashionable in recording circles to try and eliminate as much natural ambience as possible, so that the engineer and producer could control the sounds exactly as they wanted, using the effects available in the studio. Drums had huge pads of wadding stuck on them to dampen their sound, and close-miking techniques were employed on most instruments.

These days anything goes, but generally speaking, if you're engineering a live band you might as well make the most of the room acoustics. Techno and dance styles will probably involve a mixture of both, as the drum loops employed will invariably be from a bygone era, and these are likely to have been recorded using the natural acoustics of the studio. What goes on top is almost certainly going to be processed either from on-board keyboard effects, or outboard multi-effects.

Types of Reverb



1. Real Acoustic Rooms and Chambers

It's worth considering that even though all your recording may be based around synthesizers and samplers, you can still use any rooms at your disposal. I've worked with people who've wanted to try give a track more than just a preset from an effects unit. So if you want something that no-one else has, then why not send that bass drum, bass guitar or organ sample out to an amp in a room, plug in a mic and have a listen to the sound? You could even add it to the original untreated sound, and use the acoustic amp/mic set-up as an effects return.

The BBC go one better. In the basement of Broadcasting House they have a room (or acoustic chamber) with a similar configuration to that mentioned above, only that they can vary the size of the room by moving the walls! A simpler, more common method of acoustic sound control is to use acoustic partitions. Often called 'flats', they are like huge thick doors on wheels that are used to provide a degree of isolation between musicians, but together with good microphone technique, flats can also limit how reverberant a room is.

Partitions are just too big to be practical for a home set-up, so it's curtains for you... quite literally. You can use curtains to make a room less reflective, and they're good for reducing brightness in acoustic as well as aesthetic terms. Curtains also help to keep some of the noises outside at bay.

2. Plate Reverbs

Before digital reverbs became as common in studios as colour tellies in sitting rooms, there were only two electronic alternatives readily available: Spring reverbs and plate reverbs. So if you've ever wondered what 'Vocal Plate' meant on a digital effects preset, then rest assured it's nothing to do with what your singer likes for breakfast. The EMT 140 TS plate reverb was the Lexicon PCM 70 of the era, and can still be found in studios today.

It's probably got more to do with the size and weight of the thing than its sonic performance. At eight feet long, four foot high and nearly a foot thick, the EMT 140 houses a rectangular steel plate six feet by three feet, and 1/64 inch thick. The plate is suspended under tension, and sound waves are propagated through the plate from a moving coil driver that responds to signals sent to it from the mixing console.

One or two (for a stereo effect) contact mics are placed on the plate at slightly different distances from the driver, and return the ripples of sound induced in the plate to the mixing desk. There's a simpler example of how this works. For sound effects in theatres, a metal plate is wiggled, to imitate the decaying sound of thunder. Similarly, old 500 gallon oil tanks also make a great sound if you give them a thump. But with plate reverbs, you have far more control. The reverberation time can be varied from less than a second to around 4 seconds. This is achieved by moving another plate that's covered in acoustic damping material nearer to the main reverb plate (for a shorter time), or farther away (for a longer time). This movement was later electronically controlled, but used to be done manually with a handwheel.

Plate reverbs are also sensitive to external vibrations, and because of their size, are frequently stored out-of-sight, in machine rooms or tape stores, where degenerate tape-ops soon discovered they were the perfect height for rolling joints on... allegedly.

This is the room where Phil Collins gets his famous drum sound.


3. Spring Reverbs

Working on a similar principle to plate reverbs, spring reverbs are far more compact in comparison. Unfortunately, most spring reverbs don't sound as good as their plate counterparts, and employ equalisation circuitry to compensate. Once again a signal driver is used, and placed at one end of a spring. A transducer at the other end, receives the undulations of the spring. Usually, at a least two springs are used, and their sound is determined by the thickness of the wire, the number of turns per inch and the overall length of the spring. On more sophisticated models, damping methods akin to those on plate reverbs are used.

The Great British Spring was a company that were devotees of this type of reverberation, as were AKG. AKG made a variety of models, amongst them the BX25E, which had a variable reverb time between 2 to 4.5 seconds, and not only had remote control but was extraordinarily shockproof. Being the size of a small fridge, it was considered portable, and suitable for location work! Spring reverbs may be old-fashioned, but still have a part to play in the life of your average guitarist. My old Peavey Studio Pro has one, which lies at the back of the amp like an upside-down aluminium ice tray. I've heard better, but like so many things, it's nice to have the option.

4. Digital Reverbs

Digital reverbs are like a virtual reality in an acoustic world. No longer limited to just level, EQ and decay time, you can now set predelays, EQ crossovers, density, reflections, just about everything except the colour of the wallpaper. Like experimenting with different guitar tunings, you can tweak a digital reverb to produce entirely new sounds. How it's all done is by initially converting the analogue sounds at the input into digital information that can then be manipulated by the computing power of the digital signal processor or DSP. Once in the digital domain, it's really down to the designers as to what they feel you'll need to get a great sound. The capabilities of the DSP also play a part, so you tend to get a menu of effect types, or algorithms as they are generally known. These algorithms frequently include emulations of plate reverbs, as well as rooms and halls. Within each algorithm are editable parameters, which you can change to tailor a reverb type to suit your music. Typical parameters are:-

(i) Pre-delay
This parameter is measured in milliseconds, and decides the length of the gap between the original unaffected sound and hearing the effected sound.

(ii) Size
It's not the size that counts, it's what you do with it. And indeed, while it may be tempting to go for a biggie, it's the interaction with all the other functions that make the difference between a gloomy room or a tall hall. Size is measured in metres, and sets the dimensions of your acoustic architecture.

(iii) Reverb time or Decay
This is shown either in seconds, milliseconds or just an arbitrary decay figure. Adjusting this parameter varies the time it takes for a reverb sound to die away.

(iv) Crossover
Not all reverbs have this facility, as the crossover sets a threshold below which frequencies will be treated differently from those above it. It is used to split the low frequencies from the mid frequencies, so that they can have different reverberation times for a more accurate acoustic simulation. Usually, the low frequencies w ill last longer than the mid frequencies.

(v) High Pass Filter (HPF) & Low Pass Filter (LPF)
A high pass filter will let the high frequencies through, and reduce the bass frequencies. Adjusting this can help reduce the boominess of a reverb. Conversely, if your reverb is sizzling in the high frequencies, or sounds too cold, then using the low pass filter will help decrease the amount of top, while letting the low frequencies through unaffected.

(vi) Density, Diffusion & Liveness
These parameters vary in different algorithms and devices, but essentially they alter the how the initial echoes of the reverb gradually radiate over time. They affect the overall clarity of the reverb, yet their effect is largely determined by other parameters such as room size and reverb time.

You only have to go back ten years to find a time when a decent digital reverb would set you back thousands. The Lexicon 224XL and AMS RMX 16 were the industry standard then. However, Yamaha's SPX 90 multi-effects device changed the digital effects world for good, and the Alesis Midiverb – sniffed at by many – set the tone for a company that has made affordable digital technology their business.

The Roland D50 was the first synth to feature on-board effects.


Types of Echo Devices



1. Tape-based Echo Units

There can't be many guitarists who haven't heard of the legendary WEM Copycat, a device that in its time brought to the masses the joys of effects devices. It's also fun to watch. The system is simply a tape recorder that uses a loop of tape, rather than a reel and three heads: Erase, record and play-back. What you put into it from your instrument is recorded, and heard shortly afterwards from the playback head.

You can mix your input with the play-back output, and get a slapback echo. On some versions of this type you could change the distance between the record and playback heads. Alternatively, varying tape speed alters the time between repeats. If you want more repeats, then the playback head can feed some of its output back into the record input together with any incoming 'untreated' signal.

This is then picked up again by the playback head. Ultimately, if you try to get too many repeats, noise creeps in and the signal deteriorates. Once again, there's a certain sound characteristic to tape echos that continues to appeal to the anoraks. So as the taste for retro gear continues, perhaps will we see a revival of the WEM Copycat or Roland Space Echo, and a scarcity of tape loops..?

2. Digital Delays

If you can't afford a multi-effects device, then a digital delay is the next best thing. Although I'd recommend you look for one with at least a half second delay time, you can still do a whole lot with less. Digital delays sound cleaner than ever these days, yet older models have more knobs than a 90s synth, so their ease-of-use should not be overlooked. This was brought home to me recently, when preparing the effects tutorial on this month's Re:Mix CD. The Lexicon PCM 80 is indeed a great effects device, but in order to demonstrate phasing I gave up my fruitless meanderings through this new wonder-toy in favour of my ancient Fostex Model 3050 digital delay. While it can't compete with a Lexicon, it did have the sound I wanted, and that's what counts, isn't it?

Delays or echoes are a pretty obvious attribute of a digital delay. Typically, you'll get a display in milliseconds, enabling you to set specific intervals for your repeats, and the feedback control will determine how many repeats you get. Set it to one or zero, and you'll get a single slapback echo. Shorten the delay time to 100mS or less, and you too can be an Elvis impersonator. If you make the delay shorter still, then you begin to move away from distinct echoes and into the Twilight Zone.

This is when those three little words Chorus, Flanging and Phasing make an engaging appearance. To take full advantage of these particular effects, you have to become familiar with the Modulation Depth and Modulation Frequency (or speed) controls. The depth control alters the delay time slightly over time; extending and then shortening the original delay time. The regularity of these variations is governed by the frequency control. The combination of these two factors brings about a detuning effect. With longer delay times this detuning is more obvious, and hence a chorus effect has the longest delay time of the three types.

(i) Chorus
Set the delay time to around 20mS. You'll need to have a good balance between the original untreated sound and the chorus effect, as they work against each other to make the chorus sound. Altering the modulation depth will increase the detuning, and the modulation speed can be adjusted to get an appropriate oscillation beating. You can use the delay feedback control to make the sound more dense.

(ii) Flanging
Flanging is the halfway house between phasing and chorus. Flanging has the airiness of phasing, yet retains the colour of chorusing. Using the same controls, set a shorter delay to around 10mS or less.

(iii) Phasing
Very short delay times are used for phasing, even less than 1mS can produce searing results, but if you start around 2-3mS and have the original sound well balanced, then the effect should be noticeable instantly. To make the phasing sweep, alter the modulation frequency control to about 0.5Hz, and add a healthy dose of modulation depth.

Diffusion conclusion



Although new delay and reverb devices are becoming increasingly easy to use, with even better presets on offer, if you want to get the best out of your equipment, then tailoring the sound to your needs is the only way to give your music some individuality. By having a grasp of the theory and thinking behind these devices, you'll be better prepared to use them more creatively. And if it's the creative applications of effects that lights your fire, then Part 2 of this feature next month, will probably have just the trick you've been looking for.

(NB Apologies to Bob Dormon for not crediting him for the DCC/MiniDisc feature last month).

On the RE:MIX CD

An accompanying audio tutorial can be found on the CD this month, so you can hear exactly what Bob's talking about. Bob makes extensive use of the Lexicon PCM 80, which is reviewed elsewhere in this issue.


Reverberation explanation

If you think about a snooker ball bouncing off the cushions of a snooker table into another cushion and around the table, then you've got a rough idea of how sound reflections occur in a room. Okay, so the ball has to travel round the table, and with sound it goes to all the 'cushions' and bounces off in different directions all at once. However, some sides will take longer to reach than others, and depending on where you're situated, the reflections will reach you at different times.

When you clap your hands in a large hall, the first sounds you'll hear are the early reflections. These are echoes from the walls nearest to you, and as the sound ricochets around the hall, the individual reflections become harder to identify as the echoes cascade, blending into an explosion of sound that eventually fades away. Reverb is essentially a chaotic mixture of thousands of short echoes. How those echoes disperse and how long they last, is totally dependent on the acoustics of the environment. And how the acoustics perform has to do with the room size, how reflective the surfaces are (walls, floor and ceiling) and what those surfaces are covered in.

(a) Multiple reflections within a room


Reverberations exist everywhere, and how a place sounds can actually affect how we feel about it. Yet there is one place where reverberant sounds are kept to a minimum, and that is in an anechoic chamber. Anechoic chambers are used for testing hearing, and are very unnatural sounding environments. You actually feel uncomfortable in one because there's nothing about them that your ears can hook into. It's extremely claustrophobic, as all you can hear is yourself breathing – the audio equivalent of deep-sea diving.

In music, reverb plays an enormous role in giving instruments impact, sustain and general earhole-friendliness. Is it any wonder that engineers, designers and musicians devote so much time to this superfluous acoustic component? When you select a reverb for a sound, you're really fashioning that sound to a particular audio image that you have. In a sense, you're dressing it up the way you want it to appear, in the hope that your sense of style will appeal to others and still endear itself to you in the months and years to come.

(b) Reflection build-up from single percussive sound


Series - "Widen Your Horizons"

All parts in this series:

This is the only part of this series active so far.


More with this topic


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Effects Processing



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Mixed Media

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On the beat


Publisher: The Mix - Music Maker Publications (UK), Future Publishing.

The current copyright owner/s of this content may differ from the originally published copyright notice.
More details on copyright ownership...

 

The Mix - Feb 1995

Donated by: Colin Potter

Coverdisc: Mike Gorman

Sound Advice

Topic:

Effects Processing


Series:

Widen Your Horizons

This is the only part of this series active so far.


Re:Mix #8 Tracklisting:

08 Bob Dorman's patches


This disk has been archived in full and disk images and further downloads are available at Archive.org - Re:Mix #8.

Feature by Bob Dormon

Previous article in this issue:

> Mixed Media

Next article in this issue:

> On the beat


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