JoeMeek SC4

Joemeek combine the old and the new in their opto‑electronic analogue compressor with 24‑bit digital I/O and M&S signal path. Paul White tries it out.

The Joemeek SC4 is designed as a high‑end stereo compressor based on the company's existing optical compressor system, but utilising their new M&S topography. As in their low‑cost C2 stereo compressor (see last month's review), the stereo input signal is matrixed into a mono 'mid' signal and a 'side' difference signal prior to compression. After compression, the signals are recombined via a sum‑and‑difference matrix to produce conventional left/right stereo. One of the benefits of working in this way is that the stereo imaging always remains properly balanced; in the SC4, the user is also supplied with insert points that provide individual access to the mid (sum) and side (difference) signals. By applying different processing to the mid and side components of the signal, it is possible to manipulate the centre of the stereo image more than the left/right extremes or vice versa. This opens up interesting possibilities in mastering, where external processors such as equalisers and enhancers can be used to change the perceived stereo width of the programme material. Furthermore, the M&S coding part of the circuit can be bypassed using a front‑panel switch, enabling the SC4 to be used for decoding M&S recordings — see box.

In addition to balanced analogue inputs and outputs, the SC4 is also fitted with 24‑bit digital I/O in both AES‑EBU (XLR) and optical format, with a choice of 44.1kHz and 48kHz sample rates. The SC4 can therefore sync to external digital sources or convert an analogue input into either 44.1kHz or 48kHz digital formats. Because the input and output converters are separated by analogue circuitry, it's also possible to go in at one sample rate and out at another without worrying about the quality of sample‑rate conversion.

The Package

Housed in the familiar green case, the SC4 is a 2U rackmount processor with balanced analogue I/O on XLRs only. TRS jacks wired tip send, ring return are used as inserts to provide access to the 'middle' and 'side' signal paths. Metering is via large, illuminated moving‑coil meters, where the left‑hand meter reads the stereo volume and the right‑hand meter shows the amount of gain reduction taking place. These meters are not labelled, but it's fairly clear what is going on, as the gain‑reduction meter normally rests at the 0dB position unless compression is taking place.

The signal first encounters an input gain control, which controls the gain of both the analogue and digital inputs up to a maximum of +18dB of gain. I don't know what films Joemeek designer Ted Fletcher has been watching recently, but his level control goes up to 11! An adjacent button selects M&S decoding mode and, as with all buttons relating to the compressor, is accompanied by a large status LED.

As with the other compressors in the range, the gain‑reduction curve has a soft‑knee characteristic, but in this model, a five‑position Slope switch allows the user to set a gentle curve with a low ratio or progressively increase this to a tighter curve with a higher ratio. The tightest curve, at setting 5, offers a ratio of around 8:1 at a nominal operating level. Separate Attack and Release controls are fitted along with a single Compression knob, but it's hard to be precise about the time‑constant settings, as these are dynamically modified by the input signal in order to provide a faster release time after short transients, such as drum hits. An In/Out button disables the side‑chain circuitry to allow the compressor sound to be A/B'd with the uncompressed sound, but it isn't a true bypass — a 2dB gain adjustment is built in so that the relative levels of the compressed and uncompressed sounds are comparable.

By varying the mix of M and S signals when recombining to stereo, the stereo width can be modified, so the design includes a stereo width control. Turned anticlockwise, the signal can be made entirely mono, while turning beyond the centre position enhances the stereo width. The last control is Output Gain, which provides up to 6dB of make‑up gain after compression.

Digits

On the digital side, there is little to adjust as the unit automatically locks on to the clock of an external digital source, providing the Local/Remote Sync button is set. A front‑panel LED indicates when the input is locked to an external source. The reason this button is present at all is so that analogue inputs can be output via the converter at either sample rate without having to disconnect the digital input. Though the converters are 24‑bit, they will happily parse low bit depths (but there is no internal dithering), and a rear‑panel buttons switches both the XLR ins and outs to AES‑EBU or consumer S/PDIF mode as required. An adaptor cable for XLR to phono (linking XLR pins 1 and 3 to the cable screen) is all that's required for S/PDIF use. Conventional TOSLink optical digital connectors are also provided, so the SC4 can double as a co‑axial‑to‑optical converter (with sample rate conversion) or vice versa (albeit with the compressor's analogue circuitry always in line). In consumer mode, the copy‑protect bit remains set to 'copy OK' status.

Used in analogue mode, the signal path has a frequency response 1dB down at 7Hz and 30kHz. In digital mode, this is limited by the sample rate to around 20kHz. The 128x oversampling converters have a dynamic range capability of 115dB and in most practical applications, the dynamic range is claimed to exceed 110dB. The maximum analogue output level is +26dBu, which provides plenty of margin for level‑hungry digital recorders, and the nominal distortion is claimed to be better than 0.01%, but with the deliberate introduction of second‑harmonic distortion during compression.

In Use

Like the other Joemeek compressors, the optical circuitry of the SC4 has a somewhat unorthodox control law, which is one of the reasons the design has such a distinctive sound. Unlike conventional compressors that attempt to achieve a specific ratio of compression at or above the threshold, with the Joemeek circuitry you get to a point as the input level continues to increase where the gain‑reduction actually drops off again and the response becomes more linear. This still results in an overall drop in level but is said to be one of the reasons why the Joemeek circuit doesn't deaden or choke the sound in the way conventional compressors can. Designer Ted Fletcher explains that the human ear is in fact a very non‑linear device, so any processing designed to fool it must also been non‑linear in a similar way. Furthermore, because the SC4 is designed primarily as a mastering compressor, the circuitry has been tweaked to make the compression kinder to complex mixes — Ted says he aimed for an 'early Fairchild' kind of sound.

Whatever the design philosophy, the SC4 certainly works well on mixes, keeping the level under control without losing the sense of openness. In fact it's possible to apply almost ridiculous amounts of gain reduction and still leaving the mix sounding musically acceptable, but as with most mastering devices, it's best used with a bit of restraint. Even acoustic music sounds great through this compressor, but if you want rock and roll‑style pumping, you can get it by shortening the time constants, using a high ratio and cranking up the compression. Even then, the result is musical, not a sonic mess as is often the case when you push things to extremes.

The M&S insert points are actually very useful in a mastering context. I patched an old graphic EQ into the 'Difference' side‑chain insert point, with interesting results. In this case, the side signal level increased slightly when I patched the EQ in, but this was easily compensated by turning down the Width control to restore the original level. Adding top boost to the Difference signal brightens anything that's panned left or right in a mix, including stereo effects, but has virtually no effect on centrally panned sounds such as vocals, bass or kick drums. It's particularly interesting to see how the vocal reverb can be changed without affecting the dry part of the vocal sound. Conversely, it's possible to EQ the Sum signal to affect only centrally panned sounds. In a mastering situation, this allows you to make changes to a mix that are quite impossible by conventional means.

The digital I/O will also be appreciated in a mastering context, and though it's fairly basic in terms of the facilities it offers (no dithering or external word clock), it sounds fine and is simple to operate. However, the lack of S/PDIF phono sockets means that you have to make up special XLR‑to‑Phono cables, which is a bit of a pain. I cheated and used my EMO cable tester as a socket conversion box! More professional users may miss not having a word clock input but you can't have everything at this price.

Summing Up

Though the SC4 may seem like a fairly simple device in terms of the controls provided, it is actually a very flexible and sweet‑sounding compressor that can flatter without giving the game away. It has the classic Joemeek sound, albeit tweaked a little for mastering, and the M&S topography gives rock‑steady stereo imaging. The ability to process the middle and side signals separate via the Sum and Difference insert points makes this a very powerful post‑production tool, and having digital I/O enables it to patch easily into a digital workstation environment. Furthermore, the simple stereo width control allows stereo signals to be expanded beyond their normal limits without in any way compromising their mono compatibility. As a simple solution to making mixes sound good, the SC4 must be judged a success.