Within the last few years almost every aspect of recording has been influenced by the application of digital electronics of one sort or another.

Digital techniques have led to many signal processing devices such as pitch changers, delay systems, reverberators and effects which would either never have been possible or at least more difficult and/or expensive to achieve using analog techniques.

Currently, there are many systems for digital storage and generation of sound either available or in development. These range from video based formats, such as Sony F1 and 1610 formats, stationary head formats; both multi track and two track and also disk storage systems such as AMS AudioFile and the various sampling keyboards including Emulator, Fairlight, Synclavier products.

One area where computer technology has been making itself felt for some time, is in the control side of recording. Apart from various levels of automation embodied in mixing consoles, computers in the form of the microprocessor make time-code synchronisation a reality — and almost inevitably form the heart of any new tape machine design.

Although digital recording is now well and truly established there are many purposes for which analog tape machines are appropriate. With this in mind, manufacturers continue to design and develop replacement models for their existing products which embody ever increasing amounts of digital technology. Whilst the microprocessor takes over managing the mechanical aspects such as transport or motor control, and logic, it also provides other spin-offs such as integral time code synchronisation and allows much more flexible control facilities than previously deemed possible.

GIVEN THE PUSH

Given this new found flexibility, design engineers have been steadily replacing mechanical audio alignment controls with pushbutton alignment facilities.

Having replaced the old trim pots with a memory position for each audio parameter, one now has the facility to provide an almost limitless number of different alignment memories at insignificant expense, giving instant access for as many variations of speed, equalisation, reference, level, bias etc. as desired.

Having arrived at a state where electronic control of alignment is possible, the logical conclusion of this development is to add measurement capability into the system, together with control software, to allow the machine's processor to automatically adjust the audio electronics to the required operating levels.

Currently, the only machine which includes any form of "automatic" line-up in the professional field is the new Otari MTR-20, with Soundcraft indicating an intention to apply auto alignment on its Saturn multi track via an additional card. This leaves a second group of machines such as the Studer quarter inch range A810, A820, A812 and Sony APR 5000 series machines together with the Soundcraft Saturn multi track for the time being, all of which feature push-button lineup but have no internal self-alignment facility as yet.

OTARI MTR-20

When considering the design of a self aligning machine, the first stumbling block one encounters along the route is the many different reference levels used around the world. This mostly affects decisions on the reproduce side of the machine. Supposing that one tries to automate the replay adjustment procedure; the controlling processor will need to know where the various replay tones are located on the tape, not a significant problem when using a test tape, but certainly much more difficult when dealing with tones on the front of a previously recorded master.

Coupling this problem with the need to tell the unit the relationship between the fluxivity of the test-tones and the required operating level, it becomes easy to see why Otari, in the design of its machine, decided to adopt a manual set-up procedure for reproduce adjustments. After all, the idea of auto alignment is to simplify the process, not to introduce time-consuming keyboard activity.

Record alignment on the MTR-20 is realised through a module called, surprisingly, alignment. This module contains a programmable oscillator, 8085 microprocessor and associated RAM & EPROM memories, together with five digital to analog converters and analog circuitry for up to a maximum of four channels. Also included in this module is an analog to digital converter for sampling output levels together with a control keyboard for all the alignment facilities.

Having established the correct reproduce line-up the record alignment really only requires the user to indicate what over-bias level is required. This having been decided, the auto alignment system uses an eight-point sequence to establish the proper level for each of the five adjustments. These are; reference level, bias, mid-hi frequency, hi-frequency and phase compensation.

Firstly in the sequence is bias, the level of which is ramped up from minimum to peak level of repro output for each channel, one at a time. Having stored this peak point, if the level is not between 0.5 and 3.5 dB above 0 VU, then the system will align the record gain, mid-high and high-frequency parameters for 2dB above 0 VU and then repeat the bias line-up procedure, continuing this time to establish the request over-bias level and store the trimmer setting required.

Finally, the ramp-up procedure is repeated to allow for level fluctuations, drop-out etc. and this value, providing it is close to the first, is averaged with same and this averaged value is stored. Should the difference between first and second attempts be over a certain amount, a third ramp procedure is executed. This is then compared to the original two. The final value is then taken as the average of the closest two out of the three measurements.

Secondly, the system looks at record level. Initially, the input monitor signal level for 0 VU is measured and this value temporarily stored. Next, the system condenses the 255 possible trimmer setting levels into 16 steps and performs a ramp through these 16 levels storing the level equating to each of the trimmer positions. Finally, the trimmer setting at which the record level matched the input monitor level is calculated and stored in the record gain memory.

The third segment of alignment is mid hi-frequency where, initially, the frequency chosen for the record gain is utilised and the levels corresponding to a range of mid hi-frequency trimmer values are memorised. The frequency of the oscillator is then changed to a pre-instructed frequency for mid-hi adjustment, and the system then again sweeps the mid-hi trimmer and stores the levels equivalent to these alignment trimmer values. The processor then calculates the appropriate setting for flattest response and stores it in memory.

Step four is to align the hi-frequency trimmer using a similar technique to mid-hi except utilising a higher pair of frequencies. Once again the value is calculated for flattest response from the given set of measurements.

Subsequently, both mid-hi EQ and hi EQ adjustments are repeated to allow for any interaction between the two. Following this the record phase compensation is set for minimum overshoot when recording a square wave. This is achieved by ramping through the range of adjustment and comparing the measured values with those of the in-built oscillators' output. The closest value is then stored.

Finally, record adjustment is performed again to give the same level as measured from the input monitor. Once this has been completed the machine will sweep through all the internal oscillator frequencies to allow the response to be observed on the VU meters.

PUSH BUTTON MANUAL ALIGNMENT MACHINES STUDER A810/A820

The Studer A810 was probably the first on the market to feature digital alignment facilities.

Essentially, the main features of this machine were confined to extra line-up settings when compared to traditional machines. However, one unusual facility is that of being able to alter the time constant of the hi-frequency EQ, this proving useful to accommodate manufacturing tolerances in heads etc.

On the A810, access is available to all the line-up parameters via an RS-232 serial link, which allows automatic line-up via an external computer and automatic test equipment. Line-up data can also be stored on an audio track.

Studer's A820 features similar electronics to the A810 and a larger more rugged transport assembly. This machine also contains a switching mechanism which tells the audio electronics what headblock is fitted, and causes the audio line-up to switch over to the correct alignment parameters for the format in use. This facility seems to be one of the most useful yet to emerge from digital alignment, allowing as it does changes from quarter inch to half inch formats in very little time without having to totally lose the former set-up parameters.

SONY APR 5000

This soon to be released machine, features similar push button programmability to the Studer machines, although it has a number of extra parameters not normally seen.

The APR 5000 also features a head block with identifying switches fitted, to allow tape format changes to automatically recall appropriate line-up facilities.

The APR 5000 also has an alignment connection facility provided consisting of two BNC connectors, one of which routes to both the line inputs whilst the other monitors the output of whichever track is selected for alignment. Several parameters described as secondary record compensation are provided to allow for different tape formulations whilst two further settings allow compensation for heads with alternative gap widths. The APR 5000 series also features internal time code chase synchronisation and an integral time code generator.

SOUNDCRAFT SATURN MULTI TRACK

This latest machine from Soundcraft is the first multi track machine to employ push button alignment and will shortly be made available with complete self-alignment facility via the addition of another board.

As it stands, the machine can be push-button aligned for all audio parameters without leaving the engineer's chair — since the line-up controls are located on the remote package. An internal oscillator is provided which is also controlled from the remote box. Given the length of time consumed by lining up multi track machinery, it will certainly be interesting to see the self aligning variant of this recorder.

OPERATIONAL CONSIDERATIONS

Despite all the benefits which can be derived from the application of this sort of technology there still seems to remain a great deal of scepticism amongst those in the engineering walks of life about whether equivalent accuracy to a rotating trim pot can be achieved.

There are, however, overwhelming advantages to having different set-ups available.

Certainly, being able to walk up to a machine, press Tape Type 1 and know that a close to standard line-up is available is really useful, especially for those in a position of constantly having to align for tapes recorded elsewhere.

Speaking personally, I would like to see the Tape 1 alignment protected by a password or such like so as to provide control over what one can expect; something similar but slightly more sophisticated than that provided on the Studer A820, which can be disabled with the aid of a hex driver. Interestingly, two of the machines discussed only have one set of replay alignment parameters per speed/EQ despite having several record memories.

This, to my mind, should be altered forthwith, since being able to change standards without tears is really what these machines are all about — especially since one very rarely changes record line-up without doing replay too.