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Digitech TSR24

True Stereo Reverb & Effects Processor

Quantity of effects is no longer a major selling point — manufacturers are now striving to offer truly professional sound quality and performance at home recording prices; does Digitech's new flagship processor hit the right balance? Paul White finds out.

The era of DAT and affordable digital multitrack tape recorders like ADAT and the DA88 has made all of us more conscious of sound quality. Whether we're running a MIDI studio, a multitrack recording studio or simply recording live performances directly to DAT, we all want the end result to be transparent and squeaky clean. Currently available 'semi-pro' mixers and multitracks rival the performance of their professional counterparts, and until recently the nomination for weakest link in the audio chain had been shifted from the tape machine to the digital effects unit. The first generation of 16-bit multi-effects units is starting to sound just a little too dirty and there's clearly a demand for something better. Digitech have many years' experience in digital processor design and they clearly think that their TSR24 is the elusive 'something better' that we've been looking for.


The TSR24 is a two-in, four-out unit that can be configured in several ways; it can function as two discrete mono-in, stereo-out processors, or it can work as a mono or stereo in, quad out processor. It may also be used as a conventional mono or stereo in, stereo out unit. Like other multi-effects processors, the individual effects are linked in various configurations called algorithms, but unlike machines that provide only a fixed choice of factory algorithms, the TSR24 provides 20 preset algorithms and storage space for up to 32 additional user-defined algorithms. In other words, the user can create custom algorithms to connect any of the effects in any order, the only limitation being the number of simultaneous effects that the internal processing and RAM can support. The limit isn't always obvious, as some effects are processor intensive (such as the more complex equalisers) while others (such as delay and sampling), are very memory intensive. To make things a little easier, the manual translates the available processing power into RAM and CPU blocks and provides details of the RAM and CPU requirements of each effects module.

One nice aspect of being able to create your own algorithms is that effects combinations can be set up that are impossible on many other machines; for example, you could create a super-rich chorus by combining chorus, pitch shifting and flanging in the same algorithm.


After learning about the processing power of the TSR24 (see box), its physical appearance may come as something of an anti-climax. As 1U, rackmount processors go, this one is startlingly inauspicious, the overall feel being rather cramped due to the profusion of small, rubbery Sinclair-Spectrum-style buttons. There's a rotary Data Entry wheel and discrete level controls for the two mono inputs and the two stereo outputs, but other than that, everything is button and menu driven.

The display window comprises three sections: the meter display (one for each input channel), the Program Number window, and a 16-character wide LCD display which shows program names, parameters and their values. There's also a small symbol associated with each program name that shows the input/output configuration in use. The various buttons are logically set out in sections entitled: Access, Program, Parameter, FX library and Edit. The Access buttons are particularly useful; these are rather like the special function keys on a computer and can be programmed to give instant access to functions that might normally be buried deep within a menu hierarchy. There are four of these buttons and their functions are stored as part of a program, so if you have a reverb program and feel that you might need to adjust the reverb decay and high frequency damping on a regular basis, these can be assigned to two of the Access buttons. In other words, you can design your own quick edit mode for any program using up to four parameters.

Turning to the rear panel, the two input and four output jacks are all unbalanced, working at a nominal +4dBu. The Left input jack used on its own provides a mono feed to both channels, while the two output pairs are designated Main and Aux. The left Main output may be used to provide a mono output if required. MIDI In, Out and Thru connections are provided, as well as a footswitch jack. The MIDI Thru has switchable merging capabilities so that any MIDI input data can be merged with any MIDI data being generated by the unit. Finally, for all haters of 'carpet carbuncles', the mains power is provided by a standard IEC mains lead; no nasty external PSUs here.


When setting up an algorithm from scratch, the first thing to consider is input/output configuration. Any combination of mono/stereo in and one, two, three or four-channel output is possible, so it is important to connect the unit up appropriately for the type of algorithm in use. For example, it's no use setting up a quad-out algorithm and then only connecting two of the outputs, as part of the effect will be missing.

Existing algorithms may be used as the basis for creating new ones, but the TSR24 has a built-in security system to prevent the user getting into trouble. For example, you can't edit an algorithm that is being used in programs other than the current one, because if you were allowed to, all the other programs that contained that algorithm would be edited too — which could be disastrous. Instead, the system politely insists you save the edited algorithm under a new name, leaving the original intact. Algorithms may be deleted, but this effectively wipes all programs using that algorithm.

The most complicated part of setting up a program from scratch is creating a new algorithm, and I think I'd be inclined to use the existing ones or edited versions of them where possible. To set up a new algorithm, you'll need that most ancient of graphical user interfaces, the pencil and paper. First you draw a block diagram of the effects modules and mixer modules you wish to use, the order in which they are connected, and the input/output configuration to be used. It's also a good idea to check at this point that the available number of CPU and RAM blocks can cope with your demands. Mixer modules are available with 2-16 inputs and have either mono or stereo outs. Each mixer input has gain and pan facilities, and a mixer module must be used when one or more effects module outputs need to be combined. No mixer is required to split the output of an effects module if it is required to feed two destinations.

Once the algorithm has been created, the parameter values for the different effects blocks can be adjusted. Obviously, the same algorithm can be used in many different programs, with just the effect parameters or balance being different. There are, in all, 105 factory programs plus a further 128 user program locations, and scrolling through these from the front panel can be a little slow. External MIDI control is preferable in this respect.

"The pitch shifter based effects can be quite haunting, yet the side effects of the shifting process itself aren't unduly intrusive."


REVERBS: as usual, the main effect is reverb, and the TSR24 offers a choice of five types, each of which requires a different amount of processing power. Top of the list comes Gigaverb, a take-no-prisoners reverb that uses every last ounce of processing power the TSR24 has to offer. If you just need a great reverb, then blow all your cookies on this one!

The remaining reverbs use different amounts of processing power and are designed for use as part of a multi-effects algorithm, the most memory efficient reverb being MFX Reverb. Gigaverb, as you might expect, offers the most user-tweakable parameters (see box). There are 16 possible early reflections envelope shapes to choose from, and if natural reverb isn't what you need, then there are the usual gated and reverse settings available. In the case of the TSR24, gated reverb is created in the authentic manner by processing a reverb using a gate, the necessary reverb and gate being combined in a single effects module.

DELAYS: up to five seconds of mono delay is possible, though this is halved for stereo operation, and the more delay time you use, the fewer RAM blocks are left for other RAM intensive processes. The delays include one, two or four-tap versions with variable feedback and Hold for infinite looping.

MODULATION EFFECTS: these are available in profusion, with mono or stereo choruses, tremolo and flangers with a selection of modulation waveforms and a choice of single or multiple phase types giving a richer sound. Other modulation effects include Tremolo and Auto Panner.

SAMPLER: vying with the delay for the title of 'most memory intensive effect' is the Sampler. This is a simple monophonic sampler which may be triggered from an external switch or over MIDI. Various options are available (1-5 seconds mono and 1-2.5 seconds stereo). Recordings can be initiated using an audio trigger if desired and the sample start and end times can be trimmed after recording. All sampling is at a fixed sample rate of 48kHz, giving a full audio bandwidth; sound quality is very good.

PITCH SHIFTER: this works in mono or stereo to provide detune, shift and arpeggiator treatments.

EQUALISER: the EQ section can emulate anything from a simple high-pass filter to a five-band parametric or 15-band graphic.

NOISE GATES/NOISE REDUCTION: this section includes the DISC Silencer Noise Reduction, which is Digitech's proprietary single-ended noise reduction system.

MIXERS: This is the final module, providing a choice of mixers — vital components of any algorithm, as they allow the outputs from the various effects modules to be combined, panned and balanced.

"All the modulation effects are excellent; you couldn't wish for richer chorus sounds than you can produce with this unit."


Setting aside the complexity of creating algorithms from scratch, which you don't have to do if you don't want to, the TSR24 is quite straightforward and logical in use. Some of the procedures, such as mapping MIDI controllers or creating an assignment map, can be long-winded and tedious, but they're made as painless as can reasonably be expected short of giving the unit a proper graphical user interface.

The operational niceties I appreciated most were the Access keys, which are most useful as customisable quick edit keys, and the Test buttons, which can be used to audition individual effects modules in isolation. But as ever, the bottom line is the quality of the effects, and on this the TSR24 scores pretty high, given its mid-market price. The reverb settings, especially the Gigaverb, are extremely flexible, suitably dense and capable of emulating a variety of natural and not-so-natural environments. At longer decay times, I felt I could hear a trace of ringiness creeping into the sound, but careful parameter editing should keep this to a minimum. If anything, there are too many parameters available to the user and the differences made by changing some of them are very subtle indeed. Still — you don't have to tweak them all!

All the modulation effects are excellent; you couldn't wish for richer chorus sounds than you can produce with this unit. The eight-tap chorus and four-phase flange settings are capable of producing extremely lush washes of sound and work equally well on keyboards or guitar. All the delays are very clean, as is the panning, while the sampling option is very basic but still useful on occasions, especially if you don't have access to a proper sampler.

Most low and middle priced equipment is let down badly by the pitch shifter, but I have to concede that this one fares rather better than most. There is a slight metallic edge to the shifted sound and a short but audible delay between the original and shifted sounds, but in the context of a composite effect, these side effects are largely hidden. Some of the hybrid pitch shift effects are quite ethereal, and when used with guitar, they help create textures that sound much more like a synthesizer than a guitar. Indeed, I'd go so far as to say some of the treatments are almost Fripp-like.

Most of the effects are clean and quiet, but one or two that involve significant amounts of EQ boost or feedback tend to get a touch hissy. The inbuilt gate or Silencer can help here, but if the hiss level is too high, the effect of the hiss level going up and down is more irritating than a constant hiss. It helps to drive the input of the unit as hot as possible, but once the input stage is overloaded, the onset of distortion is both sudden and very obvious.

When switching from one patch to another, the output mutes for a second or so to prevent glitches, and this has to be kept in mind when organising patch changes that come within a piece of music. I know that arranging a seamless transition requires more internal memory and processing power, but it's something all effects manufacturers should ultimately be aiming for.


Taken individually, the effects available from the TSR24 are nothing out of the ordinary, and most programs are limited to a combination of four or five effects at a time, but some of the combination treatments that can be achieved are extraordinarily appealing. In this respect, it can honestly be said that the end result is far greater than the mere sum of the parts. The pitch shifter based effects can be quite haunting, yet the side effects of the shifting process itself aren't unduly intrusive.

Whether the impressive technical specification of the internal digital engine equates to the actual noise performance of this unit is harder to judge, because most of the noise we blame our effects units for is actually due to mix buss noise within our mixing consoles. Turn down the input to the effects unit and the noise disappears. Certainly patches that included lots of top boost or flanger feedback were, as you might expect, prone to noise, but the more sensible reverb, chorus, pitch shift and delay effects were very quiet by comparison. On reflection, I'd say the 90dB quoted is about fair.

Is the TSR24 worth the asking price? For some, the answer will be no, because they won't wish to take advantage of its immense programming flexibility. On the other hand, those who enjoy creating unique effect algorithms from scratch should relish the opportunities provided by the TSR24. The basic effect ingredients don't appear significantly different to those offered by the competition, but because of the almost infinite flexibility in how the effects can be ordered, interconnected and mixed, there are far fewer limitations to the TSR24 than might initially be expected. The quadraphonic mode of operation offered by the four-output option is very interesting for those mixing music for Dolby Surround TV or film, while the ability to configure the unit as two discrete mono-in, stereo-out units is appealing to those with fewer effects units than they would like. This isn't quite straightforward, as both effects have to be contained in the one algorithm, which might mean writing a new algorithm from scratch — you don't appear to be able to load in two different smaller algorithms at the same time which would have been easier.

This box really has hidden depths, not the least being its ability to double as a MIDI mapping controller for several external MIDI devices at once. The TSR24 represents a serious attempt to create a high quality effects unit at a semi-pro price, and though it might not be the sexiest looking effects processor on the market, I feel it's well worth checking out, especially if you're the adventurous type when it comes to effects creation.

Further Information

TSR24 £899 inc VAT.

John Hornby Skewes, (Contact Details).


Good basic sound quality.
Excellent degree of programming flexibility.
Variety of input/output configurations.
One of the best low-cost pitch shifters I've heard.

Bland styling.
Its very flexibility can be daunting.

A competent and well-conceived quality effects unit for those users who demand full control over their effects creation.


The TSR24 features an 18-bit, stereo delta-sigma input converter with 128 times oversampling — the kind of spec you'd expect from a good current DAT machine. Similarly, the output converters are 18-bit PCM devices, though the internal buss structure of the unit is rather generously specified, with a 24-bit signal path and an internal data path width of 48 bits. This latter provision makes sense, as many DSP functions involve multiplication, and to multiply one 24-bit number by another in one go, without running out of headroom, requires 48 bits. This means that the limiting factor in the hardware performance is set by the convertor resolution, which in this case, yields a dynamic range of just over 90dB before clipping. On paper, this isn't much of an improvement over existing units, but many older designs run out of internal processing headroom before the converters.

The sampling rate is 48kHz, presumably so that some of the hardware available for use in consumer DAT machines can be used here to keep the cost down. This yields a 20Hz to 20kHz audio bandwidth and a total harmonic distortion figure of less than 0.03% at 1kHz. Overall, the stage is certainly set for a technically competent piece of equipment.


Gigaverb is the TSR24's no-holds-barred, OTT reverb, offering control over lots of parameters, including:
Early Reflections Predelay
ER Spread
ER Shape
ER Stereo Blend
ER Diffusion
ER Front Level
ER Back Level
Reverb Predelay
Spread (determines how reverb density builds up)
Crossover Frequency (sets the point at which the response splits when the Bass and Mid Reverb times are adjusted)
Reverb Diffusion (controls the smoothness of the reverb)

Further control is available over the high, mid and low frequency decay characteristics, the Reverb Stereo Blend, Room Size, Reverb Front Level, Reverb Back Level and Reverb Back Delay. If this looks like overkill, I quite agree with you, but there's bound to be somebody who wants to have a go at tweaking them all!


The MIDI side of the TSR24 is tucked away in the Utilities menu; it's possible to set up the MIDI channel and assign Local and Global controllers for real-time MIDI control. As the names imply, Global controllers are always active, while Local controllers relate only to the currently active program. Up to four Local and an indefinite number of Global controllers may be linked to any of the TSR24's variable parameters and the high/low MIDI range of a pedal controller may be set to any desired values. Incoming program changes may be mapped to effects program numbers by means of a familiar assignment table; however, the TSR24 can also double as a sophisticated controller for other MIDI devices, as up to four other MIDI program change commands can be linked to each of the map assignments. This emulates the action of a MIDI controller that can simultaneously send several program change messages to several devices all on separate MIDI channels.

The entire contents of the TSR24 (or individual programs) may be dumped over MIDI, either to another TSR24 or to an external MIDI data storage device.

Previous Article in this issue

Practical Studio Design

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Soft Options

Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


Sound On Sound - Dec 1993

Donated by: Rob Hodder

Gear in this article:

Studio FX > Digitech > TSR24

Gear Tags:

Digital FX

Review by Paul White

Previous article in this issue:

> Practical Studio Design

Next article in this issue:

> Soft Options

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