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TFPro P8

Multi-mode Stereo Compressor
Published April 2004
By Hugh Robjohns

TFPro P8Photo: Mike Cameron

Ted Fletcher's new processor packs four different well-known compression types into a single box.

TFPro's audio processors and preamps may still retain some similarities with the familiar green Joemeek range (which was also spawned on Ted Fletcher's workbench), but there is a whole new raft of ideas and technology behind them, and a new 'love it or hate it' colour scheme. The subject of this review is the P8 multi-mode compressor, or Edward The Compressor as it is has been named by the company!

Multi-mode Compression

The name of this 2U rackmount unit (measuring only 120mm from front to back) is apparently derived from a rather tenuous link between Edward the Confessor — a Saxon king of England renowned for both his wisdom and unshakeable religious faith (and depicted on the Bayeux Tapestry too) — and Ted Fletcher, an audio designer renowned for his... well, you get the idea!

This is a true stereo processor with, at its heart, what is claimed to be 'the most versatile optical compression circuit ever developed' — based around the latest type of light-dependent resistors which apparently react far faster than anything previously available. The basic design concept for the P8 is for an opto-compressor which is configurable to mimic the sonic effect of some of the best-known classic compressor designs, by

emulating the fundamental characteristics of four different compressor topologies. Additionally, Edward incorporates Transient Release and Stereo Width controls, as seen on the later Joemeek SC2 compressors — part of Ted's own distinctive optical compression sound.

Photos: Mike Cameron

Construction & Circuitry

The P8 uses a Cadmium Sulphide (CdS) light-dependent resistor as the gain-controlling element for each channel, illuminated via an LED driven from a high-current servo circuit. Unusually, the bulk of the stereo signal path actually operates in an M&S (Middle and Sides) mode, rather than the more conventional left-right configuration. The normal left-right stereo input signal is received by Ted's latest Current Mode Superbal input buffer circuits, and then processed in a sum-and-difference matrix to derive the M&S equivalent signals. These then pass through the optical compressor circuitry before being matrixed back into the left-right format and output via impedance-balanced line drivers. The Middle signal (left and right summed) is used to drive the side-chain, which incorporates sophisticated servo techniques to provide the necessary control signals for the gain-controlling element.

All of the circuitry is constructed using conventional components — there is no surfacemount technology here — with bi-FET op amps used for all the audio processing (TL072s and TL074s). The inputs and outputs are accommodated only via XLRs, but balanced or unbalanced connections can be made equally well. While I'm mentioning the rear panel, the IEC mains inlet incorporates both a fuse and a 115V/230V selector switch.

The reason for adopting the unusual M&S signal processing approach is essentially because it is notoriously difficult to make two opto-compressor circuits match each other perfectly in terms of the instantaneous amount of gain reduction being applied. The sensitivity and linearity of the CdS opto-resistors tends to vary, so that finding a pair with identical characteristics is virtually impossible. In other words, this kind of device inherently tends to mistrack when used in stereo applications — a problem which would normally manifest itself as stereo image shifts or unstable imaging. Our hearing is particularly acute to this effect, so even relatively small tracking errors quickly become very obvious.

By processing stereo signals in the M&S format, any level imbalance or mistracking between the two sides of the compressor results in image width variations (rather than image centre shifts), which are far less noticeable to most people. A useful side-effect of this approach is that the signal's stereo width can be easily adjusted while in the M&S mode, providing the user with another creative tool, effectively for free. In the case of the P8, a control is provided which allows the stereo width at the output of the unit to be adjusted from zero (mono) through 100 percent (stereo) and on to about 150 percent.

Front-panel Controls

Thanks to the spacious 2U front panel, the P8's controls are large and well separated, providing an uncluttered user interface. Starting at the left-hand side, the first knob is a large input-level control, calibrated such that unity gain is provided with the control set to slightly over five (on a scale of one to 11). The unit is set up for a nominal operating level of +4dBu, with a maximum balanced output level of +26dBu. The input gain control ranges from off when fully anticlockwise to +20dB, so the unit can be easily integrated with -10dBV systems as well as professional-level equipment.

The next two controls are both four-position rotary switches. The bottom one determines the 'Slope' or ratio in four discrete steps spanning roughly 1.5:1 up to 8:1. However, the control is actually marked simply as positions 1-4, the reason for the vagueness being that the actual value of the slope or ratio changes with the different operating modes, which are determined by the switch above. There are four options here: Green Box (in other words the Joemeek SC2 opto-compressor), LA2A ('40s Teletronix valve opto-compressor), 1176 (Universal Audio's FET-based solid-state compressor), and a generic solid-state VCA mode.

Of course, all four configurations are actually implemented using the opto-compressor circuitry but, because the new LED/LDR system reacts relatively quickly and the control voltage is managed with a servo, it has been possible to emulate the linearity and some of the faster attack response times typical of generic VCA designs and the FET-based 1176.

To the right of the large, square VU meter is the Width control mentioned earlier, and a Compression control. This effectively adjusts both the Threshold and output gain at the same time, to maintain the perceived signal level, but since the effective threshold level also changes with the operating mode, it has been calibrated simply from one to eleven. A pair of buttons are placed either side of the Compression control. The left one switches the meter to show either gain reduction or the compressed signal (prior to the output gain control). The unmarked right-hand button provides a bypass and is accompanied by a pair of LEDs, red for bypass and blue for when the unit is active. In bypass mode the input and output sockets are connected directly together, so the input and output level controls have no effect.

The next three controls determine the dynamic aspects of the compressor, setting the Attack, Release and Transient Release time constants. Again, the precise values vary depending on the operating mode, so all three controls are marked simply 'F' and 'S' at opposite ends of their continuously variable control ranges. (The Transient Release control is only operational when the unit is switched to the Green Box mode.) Finally, an Output Gain control ranges from fully off to +6dB, with a marked unity gain position. A rocker switch provides the main power switching.

Second Opinion

The guys at TFPro have always had a quirky sense of humour, so when they launched a product called Edward The Compressor, I wasn't entirely surprised! On vocals, all four compressor settings provided gain control without dulling, though I felt the two vintage compressor settings were better at doing this transparently, and in most instances they seemed to produce a bigger, fuller sound. For example, applying a lot of gain reduction with a regular VCA compressor can create the illusion that the singer is backing away from the mic, whereas, with the 1176 and LA2A emulations, the impression was of the singer maintaining position and singing at a more consistent level. To be fair, the Green Box setting also comes out well on this test, though the tonality and texture of the vocal sound is subtly but noticeably different across all three settings.

The differences become more apparent when drums are processed, with the Green Box setting being particularly good for adding definition, but tending to rob the sound of weight unless the attack and release times are set carefully. Again the two vintage options created a bigger, more weighty sound, and were more tolerant of the control settings, though this weightiness could almost be matched by the Green setting after careful adjustment of the Attack, Release and Transient Release controls. Transient Release is particularly effective on single drum hits, as it allows the character of the initial transient to be varied considerably.

As expected, the VCA setting was the most predictable, but again it didn't rob the sound of high-end detail like some compressors do, presumably because the opto circuitry used can't deliver the same fast attack as a true VCA circuit. It is capable of providing tight dynamic control without overly affecting the sound quality, although, like some other VCA compressors, the sound can thin out a little during heavy compression.

The Width control could be useful in both mastering and mixing situations, and, because it works by balancing the Middle and Side components, any adjustments remain mono compatible. Ted Fletcher has sensibly limited the stereo width maximum setting to 150 percent, so there's little chance of using it excessively.

The two vintage modes are great for creating solid, stable sounds, whether on vocals, drums, bass, or guitar, and each offers a slightly different character to the other. The Green Box option exhibits elements of the other compressors, but its attack and release characteristics are quite different, making it a useful alternative. Interestingly, although Edward is based on an opto circuit, its emulation of a classic VCA compressor is also extremely convincing, so for the price of one unit you really do get a taste of all the main compressor types. I wouldn't go so far as to say that Edward provided an exact simulation of the compressors modelled, other than perhaps the Green Box, but it does capture the essence of what sets the classic models apart from modern designs. Ultimately, it is a straightforward, musical-sounding compressor suitable for just about any kind of studio processing, so if you can't make up your mind what to get, Edward brings you the best of all worlds. Paul White

The Operating Modes

The Green Box setting provides the characteristically full sound typical of an opto-compressor, with all the 'musicality' associated with such devices. The SC2 on which this mode is based has a particularly complex dynamic characteristic, which naturally is replicated accurately here. For example, the attack phase starts very fast, but then slows after about 500µS to the setting determined by the attack control, and the release curve has a three-stage shape, recovering quickly for the first 20 percent of gain reduction, followed by a more linear period and then a final fast surge back to unity gain. The Transient Release control affects the recovery time for very brief transient signals, allowing them to be controlled separately from more sustained signals. This unique facility can have a pronounced effect on the sonic nature of the compressed signal, especially for percussive material.

The LA2A mode, being an emulation of another (albeit less sophisticated) optical compressor, is similar to the Green Box configuration. However, it seems to have generally slower attack times and less precise compression characteristics, including a modest degree of over-compression under some conditions, which are key aspects to the sonic character. This mode also has slightly simpler release characteristics, with an initially fast recovery slowing gently as the gain reduction approaches zero. The compression slope has a very gentle knee and steepens continuously as the signal level rises above the threshold. The P8 replicates these nonlinear dynamic characteristics with a clever servo feedback system which controls and modifies the side-chain current driving the TFPro P8Photo: Mike CameronLEDs which illuminate the light-dependent resistors.

The 1176 mode also relies on the side-chain servo system's controlled nonlinearities to obtain the required (very) fast attack and multi-stage release dynamics from the opto-compressor system. Some deliberate filtering is also applied to the side-chain signal to tailor the unit's response slightly, so that the amount of compression is dependent to a small but characteristic degree on the spectral content of the source signal. The 1176 mode has a relatively sharp knee, so the onset of compression is quite hard — a key element of the sound character of this famous unit.

It is interesting to note that, because the gain-controlling stage uses an LDR rather than a FET, the amount of distortion introduced by the P8, even when compressing hard, is significantly less than for the original 1176 design. As a result, the emulation tends to sound a little cleaner and smoother — but the dynamic characteristics are similar and it can certainly be used to emulate the 1176 sound on appropriate source material.

The VCA mode is the gentlest and most subtle of all the settings — you might almost say lacking in character — with a soft knee rolling into a well-controlled and constant-ratio slope. This setting is most appropriate for fairly accurate but transparent level-control duties — for example when squashing a complete mix, rather than individual instruments.

Compression Confessions

The P8 — Edward now that I know him better! — is very easy to get along with, and extraordinarily amenable. I was both surprised and impressed with the versatility of this machine. It's ability to replicate the characteristics of the SC2 and LA2A opto-compressors was to be expected — although the way in which the dynamic behaviour and some of the nonlinearities have been captured is impressive. However, it is remarkable that the P8 is also able to provide — using only LDRs — the relatively fast and precise characteristics expected of solid-state VCA and FET designs.

Regardless of the technical, numerical accuracy of its emulations, the P8 does manage to endow signals with the same kinds of sonic watermarks as the classic processors its seeks to ape, and delivers the same characteristics that made those units popular in the first place. And that's the point here — precision isn't really what this is all about. It's more a case of being able to process the sound from various instruments to obtain specific sonic effects and attributes, and the four operating modes provide four alternative 'flavours' of sound to achieve just that.

I found that with some sources with relatively small dynamic ranges (spoken voice being one), the differences between the four modes was often quite subtle. However, with sources with a wide dynamic range (bass guitars and percussion being good examples), and with more extreme settings, the different sound characters of the four modes became far more obvious, and the range of compression effects and characters that could be achieved with the P8 was surprisingly broad and musically useful.

There are a lot of compressors on the market today, but few offer this level of versatility, and even fewer at the UK price of the P8. Ideally, we would all have a rack full of genuine 1176s, LA2As, and SC2s, but in the real world we have to be pragmatic and make compromises! Some prefer to go the software route, but if analogue outboard compression floats your boat, Edward does a good job of providing very similar characteristics to these classic devices. If you only have the space or budget for one compressor, Edward could well be your man!

Published April 2004