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A&HB CMC 24 Mixer

Studio Test

Article from International Musician & Recording World, April 1985

The ultimate mixer for the semi pro? Tony Mills gets to grips with the computerised AHB CMC 24

The poor man's Solid State

You asked for it, and you got it. In fact, nobody thought to ask for it, and we're all quite surprised to get it. What is it? Quite simply, it's automated mixdown for less than a London telephone number.

Bet that caught your attention. Those years of poring over Solid State Logic catalogues in the vain hope that they'd make a mistake and deliver one to you while charging it to Mutt Lange may not have been wasted, for you are now clued in to the magic of computer mixdown, while the rest are still floundering with the concepts of subgroups and selectable auxiliary routing. If you fall in the former group, you can skip the next bit, although that would mean missing all sorts of witticisms and bits of gossip from the wacky world of A&HB — but perhaps that would be a good thing.

For the less experienced, a brief initiation into the delights of 16-track mixing. The CMC 24 looks like a 16-8-2, which it isn't, having in fact 16 group outputs. All the main channels visible are inputs — there are 16 complex inputs on the left of the mixer, and eight rather simpler inputs on the right, to a total of 24, all with very smoochy ALPS faders. Between these are the stereo outputs, and hidden away somewhere are 16 group outs. Total 24:16:2, probably the smallest any of us have ever seen and ideally suited to fitting into a converted bedroom or other home studio. You guessed it, the CMC24 is aimed at the personal or semi-pro studio, and specifically at the magical Fostex B16, which offers 16 tracks on half-inch tape for around £3,000.

So where did all the fancy bits usually associated with a 24:16:2 get to? Well, A&HB in their wisdom wanted to save space and so decided to store the routing assignments digitally rather than mechanically using large banks of selector buttons. Once you have a digital memory onboard (something which has become more and more economical over the last couple of years) all sorts of things are possible — but we'll discuss the automation features later.

The CMC 24 doesn't bear too much resemblance to A&HB's successful System 8 mixers. It's not so modular in construction, although there is an internal harness system for wiring and each channel is on an individual PCB, so larger or smaller versions (specifically 16 and 24 channels) are possible for the future. HHB are already stocking the CMC and find that it's creating a lot of interest even without the computer functions — in fact the mixer's astonishingly easy to set up and use. For a start, all the inputs and outputs are accessible from the top, so the mixer can be pushed right up against a wall. Inputs are on XLR's for mikes and otherwise on phono sockets for economy — compare to the TEAC range of mixers and the more recent Fostex units. The input channels are inline and have additional jack socket inputs on the underneath of the front panel. These are intended for insertion of guitars and are carefully balanced in level to cope with almost any likely input.

There's a three-band Eq system on each channel with a variable shelf on the top and bottom and a simple sweep on the mid. You can switch out the Eq and each channel has six auxiliary sends which have to be selected in advance — you can use up to four sends on any channel, and the simpler right-hand channels are ideal as effects returns. Alternatively you could use them to insert a multiple signal such as that from eight outputs of a drum machine — although they have some Eq, it's only high and low, and the routing options are obviously simplified. Each channel has an insert point which can be worked in 'Break' or 'Borrow' mode to add effects in whatever proportion you want.

The CMC works at -8dB, which seems odd, but apparently this presents no problems in conjunction with the +4 or 0 standards. The option of phantom power on mikes is there too at +48V if needed.

Easy access

Everything on the CMC is designed to be accessible at the push of a single switch. For instance, you can flick from record to remix as easily as from remix to monitor or mike to line inputs. One set of auxiliaries can be switched from pre-fade to post-fade, and the Normal/Mix function assigns channel routing either to computer control or straight to the stereo output mix. Channels can be muted or put on to solo, with an LED display showing whether either of these functions is operational. In fact the CMC is positively dripping with LED's — it looks great in the dark!

Everything about the CMC indicates that a great deal of thought and consideration for the end user have been put into the design. Is there a headphone output? No, there are two. Do you have to use up a channel for a talkback? No, if you have the optional LED ladder meter bridge you can plug a cheap cassette-type mike into a side socket. Why a cheap cassette mike? Because it's got a remote on/off switch built into it. Are the aux sends pre or post monitor? Take your pick — you can wire them yourself very easily. Do you want to connect both a reel-to-reel and a cassette deck to the mixer? There are two sets of rear panel connections to do just that. Want to connect a couple of channels or effects together? There's a mini patch field on the rear with sets of three phono plugs ganged together. In fact the CMC has lots of facilities you never knew you needed and probably couldn't live without after a couple of weeks.

That's dealt with the conventional mixer functions of the CMC, and as mentioned before, the digital memory aspects were initially introduced to save space. Rather than using large numbers of switches to route all the channels to output groups, you have to use the top right-hand field of buttons and associated LED's. Each channel comes up self-routed, which you mayor may not want to change.

Simply step along the group LED display until you're onto the group you want, and write it into the memory. When you've written all the channels — and whether they're muted or not on a similar display — you have one complete patch, which can be stored as one of 16 in the mixer's memory.

You can select any one of the memories in a fraction of a second, either by hand or using a footswitch which steps from one memory to the next. Remember that you don't actually have control over fades in volume because you don't have automated faders a lá AMEK or NECAM, just over whether a channel is muted and which subgroup it's routed to. Still, for a volume change (say for a guitar solo) you could just route your channel to a subgroup at a higher level, and route it back when the solo's over.

A Logical extension

Now for the computer bit. A large plug stuck in the side of the CMC gives control over to a Commodore 64 computer, assuming it's fitted with A&HB's CMI64 interface which carries a software package for mixdown functions. The interface is around £200, expensive perhaps, but remember that you don't have to load your software every time you use the computer — it's there in a fraction of a second.

The first display on the computer software asks which mixer you're using (so there must be some more to come!); you then enter the mains frequency, 50Hz or 60Hz (important for the clock functions to be described later) and then the date and time. You then see a Channel Index with the date and time at the top of the page; the Channel Index is simply a space for you to list what instruments or band members are connected to each channel for your reference. This may not be vital as you're working on a piece, but if you recall a piece from disk that you've been working on a week earlier, it's a lot more efficient than looking for some tatty bits of paper.

The Function keys of the computer let you get off the Channel Index onto something more exciting, such as the Track and Take Index — which is very much the same sort of thing. Next we come to the business end of the software, the Route Patching, which controls the channel to bus assignment. There are 56 Route Patch memories available (in seven banks of eight pages); when the computer's switched on, all the route patches are set to self routed condition. On the proposed CMC 32, additional numbers one to eight over channels 16 to 32 represent the last eight additional mixer channels.

Computer keys A to G select the page and odd and even routes are listed beside each channel — just hit CLR to self route everything. The mute patterns are on another page, a total of 1024 patterns with '/' representing Channel On and '*' representing Channel Muted. Simply place the cursor over the channel you want to change and press the space bar to alternate between open and muted. A copy function allows you to transfer large numbers of mutes to save time if you only want to make a couple of alterations.

Mute patterns can be dumped to disk using the 1/0 function — this can be used for cassette too, but it's not recommended unless you've got time to spare! You can put a name to each Channel Index, most obviously a song name — it would be displayed later as 'Song Name: C , and the Track Index would come out as 'Song Name: T', the Routes under 'Song Name: R', the Mutes under 'Song Name: M' and sequences under 'Song Name: S', so there's no need to use different names for the route and mute patterns of a particular song.

The most exciting part of CMC's software is the Sequencer. This allows you to enter a sequence of events, which in this case are mute or route patches, and step them along manually or from an internal or external clock. If you're using a digital drum machine and have an accurate tempo reading for your music you can enter your time signature (4/4 or 3/4 for instance) and the mixer should automatically keep track — but syncing up either to a drum machine or to tape through the interface discussed below will be a little more reliable.

Horn of plenty

Step through the patches with the space bar or a footswitch, or set off a short sequence of patches by footswitch for one particularly difficult part of a mix. You have a BPM display in the upper right corner and it's easy to edit sequences; you simply tell the computer which new patch to switch to at each important point, and since you can specify up to 2048 events you can go in for pretty spectacular Trevor Horn-type mixes. In fact, you could do all the alternative mixes — the 12 mix, the dub mix, the dance floor mix, the Abyssinian Gay mix — all from one memory.

The CMS 24 is the tape synchroniser which connects into another socket on the computer. It has phono inputs and output sockets for an FSK tape click, a Roland DIN sync socket for drum machines, a metronome click output and a footswitch stop-start socket. Obviously the technique is to lay down a click on one track of tape before you start recording, or in sync to your drum pattern, and thereafter you have a synchronised code for all subsequent mixing changes. You can't jump in on the middle of a mix and expect the computer to keep track — you'd need SMPTE time locking for that — but the effect of seeing all those little lights changing over with no human aid is still quite astounding.

So what if you can't actually do level changes? The CMC is fantastic value just as a 24-16-2, and with the addition of the computer functions it becomes unique in the field of home studio equipment. Inevitably it's going to become part of many forward-looking personal and semi-pro studios in time to come; I suspect that it marks just the beginning of a whole new trend in recording, and Allen and Heath will be in at the start.

A&HB CMC 24 MIXER — RRP: £2,900 approx

Also featuring gear in this article

(HSR May 85)

Browse category: Mixer > Allen & Heath

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Korg SDD1000

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Studio Of The Month

Publisher: International Musician & Recording World - Cover Publications Ltd, Northern & Shell Ltd.

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International Musician - Apr 1985

Donated & scanned by: Mike Gorman

Recording World

Gear in this article:

Mixer > Allen & Heath > CMC 24

Previous article in this issue:

> Korg SDD1000

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> Studio Of The Month

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