We often take many parts of our digital audio arsenal for granted, including our converters; most of us simply use whatever comes standard on our equipment. However, just like with the preamps on your mixer, you can achieve a noticeable improvement in sound over your built-in converters by using dedicated outboard ADCs and DACs. A 2-channel digital converter with a variety of input and output options is especially handy-you can use it with your DAT, ADAT, CD-R, and hard-disk recorder.
Apogee Electronics offers a pair of 2-channel converters that you can customize to a number of specific studio applications while using digital converters similar to those in Apogee's flagship 8-channel converter, the AD-8000.
The Rosetta is a 24-bit, 2-channel A/D converter that comes in two "speeds": one that records at the common sampling rates of 44.1 and 48 kHz, and a high-speed "96" version that adds 88.2 and 96 kHz to the list. (An upgrade to the 96 kHz version costs $795.) The PSX-100 is a 2-channel A/D and D/A converter. Each converter has its own clock so that they can be used independently.
Although the Rosetta and PSX-100 are meant for slightly different applications, they share a number of handy features that make them stand out from other outboard converters.
DYNAMIC CONTROLFor music with a dangerously wide dynamic range, the Rosetta and PSX-100 include Apogee's proprietary Soft Limit. This acts as a subtle, yet transparent, compressor/limiter. Soft Limit kicks in as the source material approaches the digital ceiling of 0 dBFS, avoiding overs to let you tape a hotter signal. I found Soft Limit to be useful for mixing and mastering as well as for going direct to DAT in the field (where I used it most). If you're recording instruments with an extreme dynamic range (in my case, it was the organ at St. Mary's Cathedral in San Francisco), Soft Limit is invaluable.
Speaking of levels, I found the Rosetta's horizontal level meters to be quite helpful, and I actually preferred them to the shorter, vertical ones on the PSX-100. The Rosetta meters seemed to give a better visual indication of what I was hearing.
WHITHER GOEST DITHERBoth units begin by converting the analog signal to a 24-bit word length. To get an output of a shorter word length, you have two options: select 16- or 20-bit with UV22 using the Output Res button, or leave the unit in 24-bit mode and allow the recording device to truncate the word by dropping the least significant bits.
UV22 is Apogee's answer to using dither and noise shaping in the reduction of word lengths. UV22 places a high-frequency (22 kHz) "clump of energy" that acts as a bias taking the noise floor down to the theoretical 16-bit level. The result is the ability to hear sounds disappear smoothly as their levels go below the noise floor-similar to how quiet sounds move into the noise floor on analog tape.
UV22 is transparent and gives a remarkable clarity to 16-bit recordings at 44.1 or 48 kHz. (You cannot use UV22 on the faster sampling rates of 88.2 or 96 kHz.)
You may be wondering, then, why anyone would want to record without it. Because UV22 is centered around 22 kHz, any music destined for downward pitch changing risks exposing this added sonic energy. For sources destined for downward pitch shifting, or extreme data-compression schemes, Apogee warns against using UV22.
APOGEE BIT SPLITTINGIn order to get 24-bit word lengths and 88.2 or 96 kHz sampling rates to work on a modular digital multitrack machine, the Rosetta and PSX-100 use Apogee Bit Splitting (ABS 96), which spreads the 2-channel, high-resolution signal across all eight tracks of the MDM. To listen back to a recording done with ABS 96, you will need the D/A of the PSX-100 for decoding. Because the Rosetta is an A/D converter only, you will not be able to monitor ABS 96 with the same high resolution you are recording with.
You can, however, use the S/PDIF outputs (coaxial and optical) for monitoring purposes. These outputs send every other sample to the D/A, which translates to 44.1 kHz at the 88.2 kHz rate, and 48 kHz at the 96 kHz rate. But without the proper filters, the resulting audio will not be suitable for recording because of possible aliasing and other audio artifacts. On the PSX-100, you get similar results when you monitor from the aux output.
There are two possible ways to use the AES/EBU outputs for ABS 96: double-wide and double-fast. The one that you choose will depend on your recording device. Double-wide, which is the standard setup in the Rosetta and PSX-100, sends half of the high-resolution signal through AES/EBU output 1, and the other half through AES/EBU output 2.
Double-fast requires only one of the AES/EBU outputs because it sends the data twice as fast. Originally, Apogee didn't support double-fast on the Rosetta and PSX-100, but now it does, via an upgrade (prices to be announced).
EVERYBODY OUTThe PSX-100 and Rosetta allow you to send digital signals to multiple devices at once; on both units, all of the digital outputs are on at the same time. If you're recording at 44.1 or 48 kHz on the Rosetta, for example, this gives you two AES/EBU outputs, an optical output (selectable between ADAT or S/PDIF), TDIF, and a coaxial S/PDIF output.
On the PSX-100, things get a bit more interesting. It not only acts as a format converter, but it can also give you outputs at two different resolutions at the same time, because a 24-bit signal is always available from the AES/EBU aux output. This allowed me to send a 24-bit, 44.1 kHz signal to the DA-45HR (Tascam's 24-bit DAT machine) while simultaneously making a 16-bit, 44.1 kHz backup on a Panasonic SV-3700.
DON'T FORGET THE DIPBack-panel DIP switches on both devices allow you to customize each unit for specific applications. The switches are designed more for a "set it and forget it" situation, thus freeing the user from unnecessary menu levels.
For instance, switch 1 on the Rosetta sets the input to run at either -10 or +4 dBu. On the PSX-100 you can set the input and output separately to operate at -10 or +4 dBu. The DIP switches also allow you, among other things, to determine the number of consecutive full-scale samples (selectable from 1 to 4) it takes to make the level meters indicate an over.
Aside from these shared features, the Rosetta and the PSX-100 differ greatly in their feature depth and implementation.
DECIPHERING THE ROSETTAAs a 24-bit A/D converter running at 44.1 or 48 kHz, the Rosetta couldn't be more user-friendly. Simply plug in your analog inputs, connect your digital recorder to the appropriate Rosetta outputs, choose your sampling rate, pick the bit resolution you need, and you're ready to roll. It doesn't get any easier than this.
The only other thing to consider is whether you want S/PDIF or ADAT information sent to the optical output. Once you've decided this point, you're ready to record with increased resolution, even to a 16-bit recorder such as DAT or CD-R.
The Rosetta is a little trickier to figure out when you want to use the higher sampling rates of 88.2 and 96 kHz on an ADAT. Although the information is clearly laid out for the Tascam DA series recorders, it took a bit of sleuthing in the manual to get all the information I needed for using the ADAT. But once I got it up and running at 96 kHz, everything worked reliably.
The Rosetta's front panel is easy to navigate (see Fig. 1a). From left to right, there are only six buttons. Next to the power switch, which is nice to have on the front panel, is the Sample Rate selector (44.1 through 96 kHz). Next is the Resolution selector for choosing the bit depth (16- and 20-bit with UV22, or 24-bit), followed by the Optical Out selector, which you can use to determine whether S/PDIF or ADAT protocol is sent over the optical cable. The last two buttons are the Soft Limit selector and the Meter Clear button, which clears the red "over" LEDs. The Soft Limit button also does double duty: hold it down for two seconds and it puts the unit into calibration mode.
There is also an LED bar graph and overload light for each channel, as well as adjustable trim controls for setting the level going to the A/D converter.
The back panel is also fairly straightforward (see Fig. 1b). A pair of XLR connectors that can accept either +4 dBu or -10 dBV serve as the analog inputs. Digital outputs include jacks for a 25-pin TDIF cable, coaxial and optical jacks, and a pair of AES/EBU connectors. There is also a BNC jack for word-clock output. A set of DIP switches and the connector for the IEC power cable complete the scene.
INSIDE THE PSX-100Although the PSX-100 has many more features than the Rosetta, the overall implementation is just as logical. Like the Rosetta, the PSX-100 includes buttons for output resolution, Soft Limit, and optical output format (see Fig. 2a). Additional buttons include A/D Sync for selecting the sync source, individual mute switches for each channel, buttons to select ABS and Digital Copy mode, an A/D or D/A metering selector, and the MDM Input selector for choosing which tracks of the MDM you wish to monitor.
Most of the buttons on the PSX-100 serve double duty. Press them quickly to get the results printed on the front panel; press and hold a button for two seconds to get to another layer of parameters. For example, to return the unit to its default settings (DIP switches excluded), press and hold the Output Res button during power-up; to go into Fast mode for operation at 88.2 and 96 kHz, press and hold the A/D Sync button. Memorizing which buttons had the press-and-hold features that I needed was easy.
The PSX-100 can cover a wide range of applications, and is easy to use once you set it up for your own studio situation. Once everything is plugged into it and the DIP switches are configured (see Fig. 2b), the PSX-100 becomes almost invisible during your work.
For example, you can send an analog signal into the A/D converter and send the results from a digital output to a recorder, while at the same time converting a digital input to an analog signal through the D/A output.
THREE OPERATING MODESThe PSX-100 has three modes that determine how A/D and D/A converters interact with each other: Confidence Monitor, Analog Monitor, and Digital Copy. In Confidence Monitor mode, the A/D does not go directly to the D/A converter. Rather, the A/D and D/A converter act independently of one another, with each using its own low-jitter clock. Besides being able to use the PSX-100 as if it were two separate converters (you can even run them at different sampling rates if you like), you can do what Apogee calls confidence monitoring: taking the digital output of the device you're recording to and running it through the PSX-100's D/A converter. This lets you hear exactly what's being recorded.
In Analog Monitor mode, the A/D converter sends a signal directly to the D/A output, while also feeding each of the digital outputs. This means you are monitoring what is going to the A/D converter rather than what the recording device has recorded.
In Digital Copy mode, a signal received in one of the digital inputs is routed to all of the digital outputs as well as the analog output. This allows you to do digital format conversions, such as sending an optical S/PDIF to AES/EBU and coaxial S/PDIF simultaneously. I have to admit that I didn't think I would use the PSX-100 much as a format converter. But in the time I've had the unit, converting formats has been one of the primary uses of the device in my studio.
A NEW RESOLUTIONFeatures and convenience aside, the sound quality of recordings made with the PSX-100 or Rosetta at the front end was astounding. In every case, they proved to be a vast improvement over the internal converters on each recorder that I used.
Both the Rosetta and the PSX-100 recorded a 24-bit, 96 kHz signal on an ADAT XT with startling results. Immediately, I noticed the extended dynamic range the high-resolution recording promises, not to mention a high end that is clear and true, with greater detail in the low-level information.
Using the PSX-100 with the Tascam DA-45HR, 24-bit DAT machine was equally revealing. The 16-bit, UV22 recordings made with the Rosetta had more detail and dimensionality than the 16-bit tracks made with the DA-45HR's own converters (which, by the way, also begin as 24-bit words). But putting the PSX-100's 24-bit converters on either end of the DA-45HR (which internally has a 24-bit A/D, but a 20-bit D/A) made the biggest difference. The results were a remarkable smoothness throughout the frequency spectrum, quicker transients, and an even greater dynamic range. By comparison, the other, lower-resolution recordings were harsh and difficult to listen to.
Recordings made to standard 16-bit media saw an improvement with the Rosetta and PSX-100 as well. When I used the 16-bit, UV22 combination and also took the direct 24-bit output from the PSX-100's Aux out, it made other 16-bit recordings sound two-dimensional in comparison.
PREACH TO THE CONVERTERI must admit, I didn't think that I would find enough of a difference between the two units to talk about in a review. At the beginning, I thought I would end up using the PSX-100 the most because, among other things, it has the D/A converter and does digital format conversion. In other words, it has everything I might need in a 2-channel digital converter.
However, I found myself taking the Rosetta on the road for direct-to-DAT recordings and leaving the PSX-100 in the studio because the Rosetta is simple to configure and use. The Soft Limit feature allows me to leave my compressor/limiter at home, and since I'm recording a 16-bit, UV22 signal, I don't need a D/A converter. Perhaps the biggest advantage of the Rosetta is that the 44.1/48 kHz version is less than half the price of the PSX-100.
If you want the awesome sound and flexibility of Apogee's AD-8000 and 88.2 and 96 kHz capabilities, but don't need all eight channels, the PSX-100 will suit your needs for many years and projects to come. However, if you just want to improve the sound of your recordings, whether your medium of choice is 16-bit DAT and CD-R or 24-bit MDM and DAW, the Rosetta is the perfect choice. But if you're serious about mobile stereo recording and need to keep your studio configuration intact, you'll just have to buckle down and buy both.
Gino Robair is an associate editor at EM. Special thanks to Dave Hatt, Karen Stackpole, and Myles Boisen.