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AHB CMC 24


Computerised mixing is now a practical alternative for both semi-pro and home studios.

Not only does the CMC 24 offer computerised routing and muting but it is also one of the first in-line mixing consoles aimed at the personal multitrack market.


The word revolutionary seems to be applied all too liberally these days but this new product from AHB would appear to fully warrant it's use. Designed to be used in conjunction with one of the new generation of low-level multitrack machines, the CMC 24 is a 24 channel in-line mixing console which has the added advantage of digital routing and muting. This is a most useful combination of features in itself, but when used with the CMI 64 and CMS 64 interfaces and a Commodore 64 computer, it's capabilities can be extended to include sync to tape or drum machine (Roland standard DIN Sync) and muting patches may be compiled into a sequence to give a semi-automated mix-down.

At first sight, the CMC 24 might look like a 16 into 8 into 2 desk, and to anyone unfamiliar with the concept of an in-line mixer, this is an understandable misconception.

Falling in Line



Whereas a conventional mixer has a dedicated set of output groups, all the channels of an in-line desk may be used as input channels and are then routed to the multitrack mixer outputs by means of a mixing bus system. Routing can be performed either by conventional switching or by routing logic but AHB have opted for the latter system both to economise on panel space and to increase flexibility.

The CMC 24 has 24 input channels and 16 outputs, 16 of the input channels offering both mic and line inputs and the last eight offering fewer facilities and no mic inputs. As the mixer has six auxiliary send buses, it is reasonable to assume that most users would use these last eight channels as auxiliary returns during the final mix-down; the first 16 channels would be used to process the multitrack outputs.

Construction



Before delving deeper into the workings of the mixer, it's construction deserves further scrutiny because it represents a departure from convention in many areas. Like many mixers, including the AHB System 8 series, the CMC 24 uses a separate power supply both to reduce hum pick-up and to enable the mixer to follow a slim line design.

The control panel is fabricated from sheet steel but the bottom cover and display pod are formed from industrial grade ABS, a very tough plastic.

One glance at the photograph will show you that the cosmetic design is far less conservative than it's predecessors but there has also been a rethink in terms of operating ergonomics. Because a great many budget studios find themselves short of space, the mixer is often sited close to a wall or chimney breast and this makes it difficult to gain access to the connectors which are invariably mounted on the back panel. Recognising this problem, AHB have organised the CMC 24 in such a way that all the important connectors are within easy reach; the microphone inputs are mounted along the top of the front panel and the line inputs run along the front edge of the mixer, just beneath the ABS handrest, as do the insert points.

All semi-permanent connections such as those to the multitrack machine and the stereo mastering machine are achieved using high grade phono sockets and provision is made to connect in a second two track or cassette machine which may be selected from the mixer control panel. Additionally, an RIAA disc pre-amp is built in but this must be patched to a pair of input channels by means of extra leads before it can be used. Likewise, four groups of three paralleled phonos are provided for general patching. These connectors are again easily accessed from the top of the mixer and are located directly behind the display pod.

Alps switches and faders are used throughout for reliability and many of the controls are dual concentric types in order to maximise the use of panel space. Unless you buy the CMP 24 meter bridge, the only metering is on the stereo output and this is of the LED ladder type which may be switched to read VU or peak. The meters reside in the display pod which will be covered in the 'auto routing' section. This paucity of metering is not really a problem as most users at this end of the market can rely on the meters in their multitrack recorders. Output levels are 0VU = 300mv which corresponds to -8dBm. This matches most low level multitrack machines which operate on a nominal -10dBm system and it was designed to exactly complement the Fostex B16. The internal wiring harness allows any channel to be removed for servicing without desoldering.


Input/Output Channels



The first 16 channels are used as both input and remix channels. In common with most mixers, the first control is a gain control and this works in conjunction with a 'peak' LED which illuminates 3dB before the onset of clipping. Next comes a block of three switches which select line, mic or tape as the programme source and enables the EQ to be bypassed for quick A/B comparisons.

The EQ section itself is a three band arrangement which utilises dual concentric controls so that the HF and LF shelving frequencies may be varied. The mid control is a sweep equaliser where the outer part of the control knob is used to set the operating centre frequency and all three sections offer 12dB of cut or boost. Equaliser frequencies are as follows; HF 6kHz-12kHz, midF 300Hz-6KHz and LF 40Hz - 300Hz.

Auxiliary sends are arranged as two groups of two, each group being controlled by a dual concentric control and the auxiliaries are labelled 'a' to 'd'. 'a' may be selected to be pre or post fader but 'b' is a dedicated post fade send. By using a pair of push buttons, auxiliary 'a' and 'b' may be routed to the desired pair of auxiliary buses, 1, 2 - 3, 4 or 5, 6.

Auxiliaries 'c' and 'd' function in a similar way where 'c' is channel post-fade or monitor pre-level and 'd' is channel post-fade or monitor post-level. Next is the monitor source switch which selects either the multitrack input or multitrack output to be fed to the monitors and adjacent to this is the normal/mix button. This puts the mixer under computer routing control in 'Normal' and feeds the channels to the stereo output in 'Mix'.

Each channel has a monitor level and pan control, again configured as a dual concentric pair, and just below this is the monitor mute status LED, the mute function itself being under computer control. There is also a solo-in-place button for both the monitor and channel main outputs and these work when enabled by the master monitor and channel enable push buttons located on the master section. The difference between solo-in-place and the more usual PFL is that solo-in-place maintains it's special position as set up by the pan control.

The pan control itself may be used to position a signal in the final mix or to vary the odd/even balance when a signal is being routed to two output buses during recording. Just above the channel solo button is the channel mute LED and this is again under computer control.

Return Input Sections



These are the last eight channels which have no microphone inputs. The EQ is simplified to a two band fixed frequency system and only two auxiliary sends are provided, 'a' being selectable as pre or post fader and 'b' being a dedicated post fade send. No additional monitoring facilities are provided on these channels but each may be routed for record or remix and the mute function is again under computer control. Pan and channel solo-in-place controls are the same as on all the other channels.

Master Section



Located above the master stereo output faders are the auxiliary send master levels, the master monitor level and the headphone output level. Using manual routing buttons, the monitor section may be fed from the auxiliary outputs (in pairs), tape 1 or tape 2, and when all the switches are out, the master mix is automatically monitored.

Phantom powering of 48V DC is available on all or none of the mic inputs depending on the status of the 'phantom power' switch and in addition to the channel and monitor solo-in-place enable buttons, a further 'solo link' button allows these two systems to operate simultaneously.

All channels including the master stereo outputs have insert points which are on quarter inch jack sockets configured as ring send - tip return. So far so good but how does all this computerised routing and muting work?



Computer Control



Lurking beneath the front panel, just in front of the display pod is a simple but effective eight bit microprocessor which exercises it's control over the signal routing and channel muting via a set of CMOS multiplexer ICs configured as digitally controlled analogue switches. This microcomputer is operated from a small keypad just in front of the display pod. In it's unexpanded form, that is without the Commodore 64 interface the CMC 24 can store up to 32 combinations of routing and muting for instant recall. If these patches are set up and stored in the order that they will be required during a mix, they can be manually stepped through by means of the step button or an optional footswitch. In a normal mix, only the muting function would be used as the channels would be routed to feed the stereo output bus.

The display pod has a row of 16 red LEDs plus a further A/B LED so that the 32 patches can be stored and recalled as two groups of 16. During patch programming, these LEDs indicate the source channel whilst two groups of eight yellow LEDs further down the panel indicate the odd and even bus destinations. Again the source channels and their corresponding destinations can be examined by using the step button. Any mutes selected within the patch currently operative will cause the mute status LEDs on the front panel to light accordingly so that the operator can tell what the computer is up to.

Commodore 64



If the CMI 64 interface is used, a Commodore 64 can be incorporated into the system to provide additional control flexibility. Coupled to the CMC 24 via a simple ribbon cable, the CMI 64 interface contains the necessary software on EPROM so no disk or tape loading is needed.

The operating system consists of a menu followed by 'pages' and is very straightforward to understand and operate. It is envisaged that the CMC 24 will be one of a series of computer controlled mixers and so the software first asks you which mixer you have. As the same interface may be used on a system in the United States, the computer also asks if it's being fed 50 or 60Hz mains so for the UK you enter 50Hz. After this you can set the real time 12 hour clock and off you go.

The first option, which doesn't exist if the mixer is used alone, is the channel index which is really an on screen track sheet. Up to 15 characters may be used to describe each track and the completed index may be dumped to tape or cassette for future recall.

Route patching is possible from the computer and this offers two advantages over the stand alone system. Firstly, the number of available patches is increased to 56 and secondly, the source and target channels are all displayed simultaneously so you don't have to step through the display one channel at a time to find out whats going where: each route patch destination is identified by one digit (1 to 8) followed by a letter (A to G).

Mute Patching



At mix-down, the most useful facility will of course be the muting as channels carrying instruments that are not playing all the time can be shut down when not required to give a cleaner mix. Effect return lines may be muted and opened when required to control external effect processors and further tricks may be developed by incorporating some routing changes into a mix. Also, if a multitrack channel is hard patched to drive several channels simultaneously, these channels may be preset to different levels and switched in one at a time using the mutes. This gives the effect of a simple computerised gain control, though it is limited by the number of spare mixer channels. Because the mute patches are so useful, the computer will remember up to 1024 settings (0-1023) and a mute is programmed by simply putting an asterisk by the desired channel numbers on the screen. All the index, routing and muting data may be saved to tape or disk for subsequent reloading as it would otherwise be lost when the computer was switched off; data retained by the mixers internal computer will however be retained for several days after the power is removed.

Sync and Sequence



If the sync function is required, the CMS 64 interface is needed. For anyone involved in recording electronic music or for that matter any music involving a drum machine, these facilities are extremely valuable. Operating on the Roland 96 pulses per bar DIN sync system, the built in sequencer is capable of storing up to 2048 events and assigning any of the stored routing or muting patches to each event with a timing resolution of half a beat. The system will sync to your own drum machine if thats the way you want to work but you can also program a tempo of between 40 and 255 beats per minute and use the sync output to drive your drum machine via it's DIN sync input. As you might expect, these modes of synchronisation are known as 'Ext' and 'Int' respectively.

If you don't want to use a drum machine but still want the computer to find the right changeover points at the right time, there is a built in metronome function for those lucky enough to have a drummer who can keep time to one.

As you may want to play in some time signature other than 4/4, the beats per bar may be redefined as required. When a sequence is built up, it may include one loop which is defined by the loops start bar, end bar and number of repeats; if 99 is entered, the sequence will repeat indefinitely. Loop information does not dump to tape or disk with the other data and so must be entered as and when required.

In Use



The main difficulty when using this mixer for the first time is not as you might imagine driving the computer but getting used to the concept of the in-line format. Also the auxiliary routing is a little odd as all possible combinations of two push buttons are used to define which of the three pairs of auxiliary buses if any is to connect to which pair of auxiliary sends. Having a three band EQ with variable frequency points takes a little extra effort to set up but it does give you a lot of flexibility when you get into a tight corner. Other three band EQ systems can be like a cheap lager and not reach the parts you need to reach. Used without the Commodore 64, there is no way of checking the routing patch without stepping through all the channels one at a time so you might have to resort to a pencil and paper to keep things straight. The addition of the external computer does help a lot in this respect.

One disadvantage of the in-line system is that there is no easy way of subgrouping channels without external patching but on the other hand, you can get more use out of a limited number of channels. It may be wise to connect the channel inputs and outputs to a conventional jack patch bay as the phonos provided are not designed to be constantly plugged in and out and leads would soon start to fail.

A really good feature of this mixer is that the line inputs are high impedance (about 500k), so a guitar could be plugged directly into the mixer without the treble loss usually experienced due to excessive loading. Because of the limited amount of time I could spend with this mixer, I could not get the feeling of familiarity and assurance that I get with my own mixer but I learned enough to realise that this could be a very creative tool given a little thought.

The computer section on the other hand was easy enough to use almost immediately and the benefits to anyone creating 'synchronised' music are obvious, using the system even at it's most basic level means that songs can have clean starts and clean finishes as well as the luxury of tracks being switched on and off when required. During mixdown, the monitoring system is connected to the stereo mixing bus giving an extra 24 inputs if you need them.

Conclusions



Electrically, the mixer is quiet, the EQ is good and for once there are plenty of auxiliaries. The programmable signal routing could be omitted without being sorely missed and I have the feeling that it was included mainly to save space and the cost of dozens of push button routing switches.

On the other hand, the muting facility is extremely useful and really elevates the status of this mixer as a creative tool. Obviously I would like to have seen the channel level controls put under computer control for truly automated mixing but I realise that this is not really on at this price. In order to get the best out of the muting system, you really need the computer interface and the Commodore 64 but you can still use the system without; all you need to do is remember the points within your song where the patches need to be changed and then step through manually with a footswitch. Working in this way you must remember to store the patches in the order in which you intend to use them as you can only step through the memory one way.

The increase in flexibility when the Commodore 64 is added is well worth the modest expense of the CMI 64 and CMS 64 interfaces, especially as these open up the possibility of writing patch sequences to compliment the arrangement of the piece of music being mixed and the drum machine sync facility is a gift to anyone recording electronic music.

Ergonomically, the mixer is sensibly designed for small studio operation though the colour scheme and physical appearance will be loved by some and hated by others. An additional meter pod is available if you really need it but most users will live happily without it. Summing up then, this is another innovative British product and we at HSR look forward to seeing it succeed.

CMC24 £2,395.00
CMP24 - Peak-reading 16-way Meter Bridge £260.83
CMV24 - VU-reading 16-way Meter Bridge £260.83
CMI64 - Commodore 64 Computer Interface £130.39
CMS64 - Commodore 64 Sync interface £73.91

All prices excluding VAT.

Further information from AHB, (Contact Details).


Also featuring gear in this article



Previous Article in this issue

Cutec FX500 Flanger/Delay

Next article in this issue

TDK Jingle Writing Competition


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - May 1985

Donated & scanned by: Mike Gorman

Gear in this article:

Mixer > Allen & Heath > CMC 24

Review by Paul White

Previous article in this issue:

> Cutec FX500 Flanger/Delay

Next article in this issue:

> TDK Jingle Writing Competiti...


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