GRAPHIC EQ

The basic idea behind the 'graphic equaliser' is that it contains a set of frequency response shaping filters, covering the whole audio bandwidth. These filters are controlled by vertical sliders which, by their position, give a 'graphic' display of the cut or boost applied to each frequency band, and hence a display of the frequency response curve obtained. Graphic EQs are very popular, and even portable stereos seem to need them these days.

As an alternative to standard mixing console EQ (treble/mid/bass), a graphic can be useful with as few as eight or 10 frequency bands covering the audio range. But to equalise a PA system, or to really fine tune the sound of an instrument or a mix, some 27 to 30 bands are needed. A graphic with this number of bands is known as a 'third-octave' graphic equaliser. In this, each frequency band covers precisely one third of an octave. The response of each band will be carefully tailored to fit in with its neighbours, and when all the bands are set to flat, the input signal should sound exactly the same at the output whether the EQ is switched in or out.

One slight problem common to all graphic equalisers is that the 'graphic' display given by the sliders is something of an illusion. The display will show you that, say, both the 1 kHz and 1.25kHz bands (which are adjacent) are set to 6dB boost. What it doesn't show is that a frequency halfway in between (eg. 1.12kHz) will be boosted by something more like 9dB, due to the interaction between the filters. This is not to say that the graphic display isn't useful. It isn't accurate, but it is worth having - as long as you can interpret what it is trying to tell you.

REAL-TIME ANALYSIS

Real-time analysis, or 'spectrum analysis' to give it its alternative name, is a method of observing and measuring the frequency content of any audio signal as it occurs. To achieve this, a number of filters split the signal into third-octave segments, each of which is sent to a level display, which is usually a column of LED indicators. Using a music signal as the source for the analysis, you can see the different frequencies bobbing up and down in level according to which instruments are playing. It's an interesting way of seeing, as well as hearing, which frequency bands have been cut or boosted when you listen to a recording.

Using the real-time analyser with a music source isn't really anything more than interesting. But when used with an appropriate test signal, it can give an instant check on the response of a system - usually a PA system, but it could also be used to check monitor loudspeakers.

The test signal in this case is usually pink noise, but before I explain that, let me explain white noise first...

White noise is really more of a concept than a useful tool. By analogy with white light, which can be shown to contain all the colours of the rainbow by splitting it up into its component parts with a glass prism, white noise contains all the sound frequencies there are. Defining it more tightly, it contains every frequency - in a given band - at the same level. White noise is produced by the amplification of completely random motion of electrons in a conductor.

Pink noise is filtered white noise, and is biased towards the low frequency end of the spectrum. In each third-octave band - or octave band for that matter - it contains the same amount of energy. The energy in white noise actually increases as you move up the frequency scale, simply because there are more frequencies. Between 100Hz and 200Hz, to explain simply, there are 100 frequencies, each at 1Hz intervals. Between 10,000Hz (10kHz) and 20,000Hz (20kHz) there are 10,000 frequencies, each 1 Hz apart. The ear doesn't hear things that way, however, which is why we divide our frequencies up into octaves for musical use. Pink noise, with equal energy in each octave (or third-octave) is considered 'flat' to our ears.

The result of all this is that if pink noise is fed into a real-time analyser, all the LED columns in the display will come up to the same level. Simple, really. Now if the pink noise is fed through any kind of audio system that doesn't have a flat frequency response, it will show up immediately in the display. Taking this one step further, if a graphic equaliser is placed in the audio chain, it can be used to correct any anomalies and bring the display back to flat.

This kind of analysis is most commonly used in PA systems, and is sometimes referred to as 'pinking out' the system. Pink noise is fed to the loudspeakers and picked up by a calibrated reference microphone. The mic's output is fed to the analyser, which shows the system's response, and the graphic equaliser is used to correct it.

I said earlier that the curve of a graphic equaliser couldn't be accurately read by looking at the slider positions without intelligent interpretation. This is also true of real-time analysis. The third-octave response is only an approximation of the response of the combined loudspeaker/room system. You still have to listen to what's happening while you adjust the graphic. Despite this, it is still useful. It is the difference between being in total darkness and having a torch to throw some light on the scene. It may not be broad daylight, but it's a hell of a lot better than nothing.

ENTER THE AUTOGRAPH...

After a lengthy introduction on a subject not previously covered in Sound On Sound, it's time to move onto the Peavey Autograph itself.

As a graphic equaliser, the Autograph has 28 bands on third-octave centres, the top and bottom bands having a shelving characteristic to cover the entire audio range. Apart from the two extreme bands, the filters have the property of 'constant Q' - something I shall explain in a moment. Rather than utilising 28 old-fashioned slider controls that you actually have to physically push up and down - perish the thought! - there is an LCD display which gives a graphic representation of the slider positions. Adjustment is by way of up/down nudge buttons.

As a real-time analyser (RTA), the Autograph has an internal pink noise generator and an input socket for a microphone, which provides 12 volt phantom power. A mic is not supplied with the unit, but Peavey have suitable types in their range and also explain in their manual how any good quality omnidirectional mic that is supplied with a calibration chart may be used. The real-time analyser uses the same type of display as the graphic. It is even possible to get a display of the EQ and RTA at the same time.

Setting the graphic itself is done using a display such as that in Figure 1. As shown, a boost has been set centred around the 4kHz band. The available degree of cut or boost, by the way, is up to 12dB. Below the 'sliders' is a cursor. The cursor in this instance is positioned at 1.6kHz, and the level of that slider is 0dB, as the figures on the right show. There are alternative forms of the display, one of which is shown in Figure 2.

This shows the graphic sliders, set 'flat' in this example, and an RTA display of the incoming signal. This RTA display isn't intended for system equalisation, just for you to see what's going on frequency-wise. As you can see, the display is quite cramped, but you can move the 'window' up or down by adjusting the RTA sensitivity. Another type of display allows you to set the graphic's sliders with a resolution of half a decibel (dB), rather than the 1dB steps on the examples shown.

One of the bugbears of conventional graphic EQ is that if you want to store any favoured EQ setting, you have to write it down on paper (and reset the sliders). The programmable nature of the Autograph system means that it is possible to memorise complete sets of EQ settings. A generous 128 program memories are provided, which can be individually named. If the figure '128' reminds you of the 128 available MIDI Program Change numbers, then obviously the three MIDI sockets on the back panel have found a use. With the previous programmable graphic I used, I found it very useful to have a few different EQs stored. When I wanted to experiment with a sound, they were ready and waiting for immediate recall. Sometimes, using an EQ setting more or less at random can throw up effects that you wouldn't normally achieve by setting the graphic from scratch. Changes from one program to another are silent on the Autograph, making sequenced EQ changes during mixdown a tantalising possibility.

I mentioned conventional graphic bugbears at the top of the last paragraph, and I mention it again with emphasis. Have you ever used a pair of third-octave equalisers on a stereo signal? Yes? Then you'll know that the best way to find a good EQ setting is to mono the signal and set the EQ on just one channel, then switch back to stereo and copy the setting of each band onto the other channel's graphic. Time consuming? Not 'arf!

Peavey don't exactly shout about the feature I am about to explain. In fact, I tried in vain to find it mentioned explicitly in the manual - it is only hinted at, and its principal application not revealed. It goes without saying that you can dump the EQ settings of one Peavey Autograph unit into another Autograph; they do tell you that much. You can also dump the settings into a Peavey AEQ 2800, which is essentially an Autograph without the RTA facility. What they don't tell you is the really good bit: when you have your MIDI leads connected up and ready to go, if you adjust the settings on one unit, the other unit mimics those adjustments instantly. I had a suspicion that if someone was clever enough to make an automated graphic equaliser, they would be clever enough to include this real time-saver of a feature; it's lovely. It works, incidentally, by the transmission and reception of MIDI Continuous Controller messages, one for each frequency band. There must surely be other potential uses for this.

AUTO EQUALISATION

As I mentioned earlier, the Autograph can analyse the frequency response of a sound system and automatically provide suitable EQ. It's very clever and reasonably straightforward to use, as it must be if it is to find favour with PA engineers.

On entering the RTA facility, the EQ section is bypassed and the mic input activated. The first part of the procedure is to select which measurement microphone you intend to use. The choice is among a selection of four Peavey mics, a mic with a flat response, and two user-definable curves which should be set from the mic's frequency response chart. An omnidirectional mic is preferred for measuring the response of the entire room/loudspeaker system. I used a Peavey ERO-10 (see main photo), a small capsule electret omni, for my experiments.

The next step is to select the type of frequency response you want to end up with - which may not necessarily be flat, for PA applications. Once again there is a selection, including flat, with three user-definable curves.

With pink noise pumping through the speakers at a good level, discrete samples of the response are taken from various points in the room, which are all memorised - up to 128 samples, if you really feel you need that many! The Autograph averages all of these to come up with the response curve of the system. Continuous sampling is also available, which Peavey recommend for a final response 'touch up'.

From the response curve obtained by the RTA, a compensating EQ is worked out which hopefully gives the system a flat response, or the response you want. Some manual tweaking by the sound engineer should polish the automatically generated result.

DO I LIKE IT?

As a graphic equaliser, the Autograph is a very pleasant sounding unit. It's as quiet and well behaved as it should be. The 'constant Q' filters, I think, are an advantage, but opinions differ on this. Conventional filters have a 'Q' (sharpness of response) that varies according to how much cut or boost is applied, only reaching their nominal 'Q' value at maximum cut or boost. Constant 'Q' filters, in contrast, have the same sharpness at all degrees of cut and boost.

As an adjunct to the EQ sections of budget-priced mixing consoles, a programmable third-octave graphic equaliser really does give valuable extra control. Used on individual instruments, it is slower than console EQ to find the correct band and amount of boost, but you get there in the end, and can store the result. EQ-ing a full mix, with another unit for stereo capability, is a very pleasant experience, especially when the EQ on one unit is MIDI'ed straight over to the other.

As a real-time analyser, the only fault I could find with the Autograph was that the response of the display was a little on the slow side, even at its fastest setting. When I'm analysing a room/speaker system, I like to see whether the frequency bands are all decaying at the same speed, or whether some are 'hanging on' indicating a resonance. The display is too slow to show this. Apart from that, the RTA facilities are very good indeed and auto-EQ is a boon.

At the price indicated, the Autograph is a very competent unit and very good value for money. I like it.

FURTHER INFORMATION

Autograph £499; AEQ 2800 £369; PME4 £179 (all Inc VAT).

Peavey Electronics (UK) Ltd, (Contact Details).

I'm sure that there is a place in this world for a MIDI remote control unit; in fact, several different types probably. Here is one, the Peavey MIDI Director. It's battery powered (or from a 9 volt supply), small enough to hold in the hand, has a 15-button keypad and LED numerical display, and is cable connected to whatever MIDI unit you wish to control.

Its principal use is to select any of the 128 MIDI Program Change numbers on any of 16 MIDI channels. It can also select any of the 128 MIDI song numbers, which may be useful when using a sequenced MIDI system on stage. Other commands it recognises include Start, Stop and Continue.

I can't really see this unit being used as part of a MIDI system with a keyboard. At the very least, it ought to have a MIDI merge function so that you can combine MIDI Director data with MIDI keyboard data. Perhaps Peavey see one use as being the controller for a MIDI effects system. It certainly would be useful for this, switching between effects on a variety of units without moving from the mixing console.

If you have an application similar to the stage or effects control uses I have just outlined, then this unit is definitely worth considering. It could prove value for money in terms of convenience and time saved.

MIDI messages transmitted:
- Program Select 1-128.
- Song Select 1-128.
- Channel Select 1-16.
- Start.
- Stop.
- Continue.
- 1 X MIDI Out.

The Peavey AEQ 2800 is basically an Autograph without the real-time analysis facility. In fact, unless there is something I haven't spotted, it is an Autograph without the bells and whistles. It offers all the same MIDI functions of the Autograph and, indeed, can be used as its partner in a stereo system. At £369 inc VAT, it is certainly a very good unit and can be recommended.