The one certainty of the march of digital audio is that everyone needs converters to translate real-world analogue source signals into the digital realm, and back again to be auditioned. Although the physics of such conversions are well defined and understood, different manufacturers and different end users seem to have slightly different views on just how to achieve these A-D and D-A conversions. One outcome of this debate is that people like to select their own favoured brand of converter as an artistic decision, much like selecting monitors or mixing desks — because they 'sound good'.
Apogee have long been regarded as a company which produce converters that 'sound good' — from the very earliest days of digital audio, in fact. The company was also one of the first to realise the significance of jitter and to find ways of addressing its causes. They also pioneered the UV22HR dithering algorithm for word-length reduction (which has attracted many disciples over the years) as well as the use of soft limiting to minimise the risk of A-D overloads.
Photos: Mike Cameron
The Rosetta 800 under review here is an eight-channel unit combining A-D and D-A conversion into a single 1U rackmounting unit which measures around 300mm from front to back. The standard model accommodates sample rates up to 96kHz, but there is a Rosetta 800 192 version which can work at up to 192kHz sample rates if required. UV22HR word-length reduction is included to provide 16-bit word lengths for those systems than can't handle the standard 24-bit signals, and Apogee's soft limiting can be applied to the analogue inputs.
Like many Apogee products, the Rosetta 800 includes various interfaces as standard, with independent routing options to allow a degree of format conversion. All inputs and outputs carry eight channels, and include ADAT lightpipe, AES, balanced analogue, and a variety of other interfaces via optional cards. The ADAT interface includes pairs of I/O ports to support the SMux mode, which carries four channels at 96kHz on each lightpipe connector. Similarly, the AES interface can support eight channels at all sample rates, or four channels if the double-wire mode is used. Currently, there is only one option card available, which is an interface for the Pro Tools 24 Mix system (connecting directly to Core and Farm hardware cards). However, a similar interface is expected imminently for the Pro Tools HD Core and Accel cards, and a Firewire interface for direct connection to suitably equipped computers is scheduled for release soon too.
The internal routing consists of two independent source selectors, each accessing all four input interfaces: analogue, ADAT, AES and the option card (if fitted). One selector feeds the analogue outputs, providing simple D-A conversion for all eight audio channels. The other feeds all three digital output interfaces simultaneously, providing either A-D conversion for the analogue inputs, or digital format conversion from the selected digital input. It is also possible to select in pairs some channels from the analogue inputs and some from a digital source, which is useful when overdubbing with a computer-based recorder, for example. Although sample-rate conversion is not possible, the output word length can be reduced from the default 24 bits to 16 bits using the UV22HR algorithm — but only when operating at 44.1kHz or 48kHz sample rates. The assumption is that all high-sample-rate systems can accommodate 24-bit data, which seems logical enough! Note also that all three digital outputs always share the same word length — you can't send 24-bit signals to the option card and 16-bit signals to the ADAT ports, for example.
With all these interfaces, the rear panel is not as busy as you might expect. There is a simple IEC mains inlet feeding a switched-mode power-supply unit, which accommodates any mains voltage between 100V and 240V. The unit consumes 45W of power and there is no external fuse. A pair of BNC connectors provide an unterminated word-clock input and word-clock output, while a 25-way D-Sub socket provides eight channels of AES-EBU in and out (four channels if configured for dual-wire operation). The absence of termination on the word-clock input is a little odd — it would to have had a switch to select terminated or unterminated modes.
Two pairs of shuttered Toslink sockets provide ADAT in and out. Only one input port works when operating with standard ADAT signals, but the second comes into play for SMux operation, with channels one to four on the first port and channels five to eight on the second. Both output ports carry all eight channels in normal use, and also split 1-4/5-8 for SMux mode. As standard, the analogue I/O is factory aligned to the SMPTE standard (0dBu is -20dBFS), but internal jumpers offer two alternative settings to provide maximum analogue levels of +20dBu, +24dBu or +6dBV. The analogue D-Sub connectors conform with the ubiquitous Tascam eight-channel wiring convention. The AES D-Sub port is wired in accordance with the Yamaha eight-channel standard.
The option card slot is located above the ADAT ports, behind a removable panel, but as no interface cards were supplied with the review model, I am unable to comment further on this facility. Two more 25-way D-Sub sockets — and they are both sockets — cater for the eight channels of balanced, line-level analogue in and out.
The front panel is as deceptively simple as on most Apogee converters — by which I mean that there are few controls and displays, controlling more functionality and options than might be obvious. To the left is a button which operates a mains power relay inside, followed by the sample-rate and clock-source buttons. In any converter the stability of the digital clock source is critical. Any clock instability will manifest as jitter and degraded audio quality in terms of both noise and image resolution. The Rosetta 800 features Apogee's latest dual-stage clock circuits to help ensure the most stable clocking arrangements at all times. Repeated presses of the first button cycle through the available clock rates: 44.1, 48, 88.2, 96, 176.4 and 192kHz, and the external clock rate. The 176.4kHz and 192kHz sample rates can only be selected if the unit has been suitably upgraded, naturally, and neither the ADAT nor the SMux ADAT interfaces support such high sample rates.
If external clocking is selected, the second button determines the clock source from ADAT, option card, AES or external word clock. When dual-wire AES signals are used, the unit must be suitably configured, as the sample rate on each AES input is effectively half the required rate. To select the dual-wire input mode, the Sample Rate button must be held down and the Clock Source button pressed to toggle AES dual-wire mode on or off. Presumably in the interests of presenting an uncluttered front panel, this 'hidden' set-up function is not marked, and will be easy to forget if not used regularly — a problem I often encounter with similar hidden functions on my PSX100.
It is also possible to configure the unit to clock to a sample rate which is a multiple of the sample rate required. For example, a 44.1kHz master clock can be used when working with an 88.2kHz digital input, by holding the Clock Source button down and then pressing the Sample Rate button to select the target sample rate (88.2kHz in this case). Both the 88.2kHz an 96kHz lights illuminate to warn of this non-standard configuration. A pair of LEDs in the form of an exclamation mark show the stability of the conversion, with the red one indicating the best lock.
The next button controls the first of the two routers, and is labelled Source To Digital Outs. Repeated presses of the button select each of the digital sources (skipping the option card if not installed), followed by the analogue source, and then the analogue source together with each of the digital sources. In the latter modes, it is then possible to swap pairs of the selected digital source inputs for the corresponding analogue sources. By holding down the Source To Digital Outs button, the current routing status is displayed on a row of eight Digital Out LEDs to the right — illuminated LEDs represent analogue channel selections. The current selection can be changed by pressing the Soft Limit button while still holding the STDO button — each press cycles to the next pair of analogue inputs — and then pressing the Output Resolution button to confirm the selection. It sounds complicated, but it's a useful facility nonetheless!
In normal operation, the Soft Limit button simply toggles this function on or off for the analogue A-D conversion, and the Output Resolution button activates the UV22HR algorithm to output a 16-bit word length. Should the selected input not be synchronous with the selected clock source, it's LED will flash as a warning, and the SMux and DW (dual-wire) LEDs associated with the ADAT and AES sources respectively are only indicators of the input format, not mode selectors.
Moving on across the control panel, an array of 24 LEDs is used to indicate the status of the unit. The top row indicates A-D conversion overloads and a channel's LED illuminates when three or more consecutive samples are detected at 0dBFS. The row below is labelled Digital Out, and below that Analog Out, and these both illuminate to show audio on the corresponding output channels, as well as being used to configure the output routing as described above.
The last button is labelled Source To Analog Outputs and, logically enough, selects the source feeding the analogue outputs. Once again, pairs of channels can be dropped from the selected digital source and replaced with the corresponding analogue inputs. Source selection is performed as before in conjunction with the Soft Limit and Output Resolution buttons.
The Rosetta 800 is easy to configure and use, although the more elaborate routing options are a little contrived. I found the sonic performance of both A-D and D-A sections to be excellent, and noticeably better than my usual PSX100 — although this is to be expected given the improvements Apogee have made to the clocking and converter circuitry. The Rosetta 800 sounded slightly quieter and smoother, a little fuller in the bass, with a clearer and less congested mid-range — although such observations are highly subjective, as the changes are extremely small. For most of us, the differences between high-quality converters like these are completely swamped by the inadequacies of the rest of our equipment or recording techniques!
The soft-limiting and dithering worked as expected, and although I prefer to leave headroom rather than squeeze transient peaks, the soft limiter has a certain analogue tape-like quality about it which I know many users appreciate. Noise and distortion seem better than the quoted specs (105dBA for the A-D and 103dBA for the D-A), and I certainly had no complaints whatever. The review unit was not equipped with the 176.4kHz/192kHz sample-rate option, but at standard and double rates it performed superbly, providing clean, transparent and very stable conversions in all modes.
With the options for direct connection with Pro Tools systems and Firewire-linked computer DAWs, this eight-channel dual converter makes a very flexible and extremely high-quality front end. Although more expensive in the UK than the Digidesign 192 I/O and 96 I/O equivalents, the Rosetta 800 is more flexible and Apogee aficionados may well decide the additional cost to be a worthwhile investment. On the other hand, the Apogee Rosetta 800 192 version is half the price of the Prism Sound ADA8 — widely regarded as one of the very best multi-channel dual converters on the market — and while the ADA8 might have a small quality advantage edge, the Rosetta 800 is no slouch! So, as a compact high-performance interface to almost any DAW, this remains an impressive, flexible and therefore cost-effective unit. Well worthy of individual appraisal.