RME Fireface 800
by Mark W. Hibben
The Good: studio grade 192 kHz A/D and D/A conversion with eight channels of analog inputs and outputs, Firewire interface to either PC or Mac, usable with advanced audio software.
The Bad: PC audio management is a mess requiring frequent tweaking and resetting of system audio parameters.
The Ugly: At $1700, only the price.
Analog vs. Digital
Ever since Sony and Philips introduced the compact disc in the early ‘80s, a debate has raged among lovers of recorded music over the superiority of analog vs. digital recordings. The fact that vinyl records are beginning to make a comeback indicates that the debate is far from over. Analog enthusiasts claim that analog sounds smoother and more natural and decry the harsh synthetic quality of digital recordings. I myself have many recordings in both vinyl and digital formats and have listened to them side by side to try to discern the difference. Through a high quality audio system, the differences are subtle, but one thing is clear: there is a difference. I myself have come full circle as well, finding that I prefer the sound of analog compared to CD, assuming that the recording is in good condition and provides reasonable frequency response and low background noise. Analog compared to the more advanced digital sound of which the Fireface 800 is capable, is another matter altogether.
As an engineer I’ve given some thought to why this is. After all, isn’t “CD Quality” supposed to be the ultimate in digital recording? A thorough treatment of this topic is beyond the scope of this article, but suffice it to say that CD quality is not considered sufficient for professional digital recording use.
Professional recording studios typically record at the Fireface 800's capability of 192 kHz or 96 kHz sample rate and 24 bits/sample, whereas the CD provides 44.1 kHz sample rate and 16 bits/sample. At these high sample rates, multi-track digital recording is frequently performed through an external audio interface that provides multi-channel A/D and D/A conversion, interfaced to a PC or Mac through USB or Firewire (IEEE 1394 a or b). The personal computer serves as the digital repository for the audio tracks, and as a digital mixing board and effects processor for generating final stereo recordings. The RME Fireface 800 is a Firewire based audio interface intended for professional recording use that can serve as a superb digital interface for a multi-channel personal computer based home audio or home theater system. Specs are shown at right.
Although it’s not cheap, it provides excellent value compared to many “audiophile” digital or analog components. RME is a German company, and the Fireface 800 is made in Germany. The people at RME are top notch when it comes to professional digital audio, as can be seen from their web site: http://www.rme-audio.de/en_index.php The Fireface 800 is in the category of professional audio components rather than “audiophile” components that I prefer to use for my home audio systems. Professional components are intended for a different audience: professional recording engineers, musicians, and music producers.
Pro components are usually extremely high quality in both manufacture and performance, but lack the cosmetic appeal of home audio components, and the RME is no exception. The front face has a practical instrument-grade appeal like the old HP instruments. Often these components are somewhat more challenging to use as well, but a much better value overall than “audiophile” components. At the high end of the audiophile market, the prices being charged are truly staggering. The best example I can think of is the $150,000 Clearaudio Statement turntable. That’s right, it’s a record player. I’m sure there is a level of income and passion for things analog where the Statement makes perfect sense, but most of us (myself included) will never know such heights. For the rest of us, professional audio gear is a great alternative to audiophile, providing high performance as well as excellent value for the discerning listener. And although I appreciate the unique attributes of analog audio, digital is part of my life, as it is for most people. Nothing can really replace the portability, convenience and durability of digital media. The RME provides a way to “have your cake, and eat it, too”. The digital sound of the RME is so good, so much better than “CD quality”, that I think of it as “virtual analog”.
How it Works
The key function provided by the RME is A/D and D/A conversion using recording studio grade converters. Digital data is routed to and from the computer via a Firewire 800 interface. Drivers and software in the computer provide control over the RME from the computer, either PC or Mac. Up to 8 analog channels of inputs and outputs are supported at the maximum sample rate of 192 kHz and 24 bits/sample. The 8 analog outputs can be configured as a digital surround sound 7.1 multi-channel output by assigning specific RME outputs to specific surround channels as will be discussed in more detail below. By taking the analog converters out of the computer, RME can achieve higher levels of performance than personal computer analog audio. Background noise is inaudible with the Fireface 800, whereas I have found that background noise of computer analog outputs to be distinctly audible, especially for built-in analog audio outputs. The interior of a computer is a digitally noisy environment, and the analog electronics are often inadequately shielded.
Because the Fireface 800 is intended for studio use, it also has many features that the home user may or may not need, such as microphone and musical instrument (guitar) inputs, MIDI inputs and outputs, as well as additional digital inputs and outputs. Since I have musical interests as well, these were all welcome to me, but if the home user can dispense with some or all of these functions, RME provides lower priced alternatives such as the Fireface 400 and Fireface UC, which uses USB 2.0 rather than Firewire.
Another key function of the Fireface 800 is digital mixing and routing of inputs and outputs. This function is performed by digital signal processors (DSPs) within the Fireface, with the computer serving as the user interface. Once the mixing configuration is set up, it can be saved to memory in the Fireface so that the Fireface mixing functions are usable even without a computer running. The mixing functionality means that all inputs and outputs can be simultaneously active up to bandwidth limits imposed by the sample rate. Thus you can connect multiple audio sources such as analog phono or tape, digital CD or DVD, and, once configured, they will all be playable without “selecting” them. Although, if you have multiple sources going at once, you will get a digital mix of those sources. Because the mixing is digital, there’s no problem with cross-talk or noise. Mixing is performed at the internal (user defined) sample rate and all sources are digitized or up-converted to this rate. I usually use 96 kHz, mainly because it provides more I/O channels than going to the higher 192 kHz rate. At 96 kHz, both the analog and digital channels can be active, whereas at 192 kHz, you basically have to choose between them.
Up-conversion of CD audio occurs flawlessly, and seems to noticeably improve the audio quality of CDs. Of course, as more and more of my digital media have gotten moved into the computer, the less important the other digital interfaces of the Fireface have become, but they’re there when I need them: optical Toslink I/O, and electrical SPDIF I/O using RCA connectors.
Setting up the mixing functions does require some learning curve, especially if you're not familiar with recording studios and mixing boards. The Fireface user interface adopts a mixing board GUI which is fairly intuitive once you get used to it. There's also the Matrix, which is essentially a digital patch panel where you can route any input to any output simply by clicking on the point of intersection in the Matrix.
By using the Matrix, you can switch components such as CD players or tape recorders into and out of your system, but in general, once you get used to the mixing capability, you won't bother, you'll just leave everything switched on. The Matrix can be a little confusing at first, but with a little experimentation, you get used to it quickly.
Setting things up for digital 5.1 or 7.1 output can be a pain in Windows. First and foremost, forget trying to do this with anything less than Vista or 7. The audio capabilities of earlier systems just aren't up to the task. Even with the more modern systems, you will still need to tell Windows what sample rate the Fireface is using in the Sound control panel. Whenever you make a change in the Fireface control panel, you also need to make a change in the Windows Sound control panel, and this is also true for Mac OS. Also, you will need to use the configuration control panel to set up the multi-channel system and test to make sure that audio is being properly routed. Here you may need to make changes in your Matrix set up, since there's no way to predict where Windows will send a particular surround sound channel (in terms of the Fireface output channel. In the next panel I show a typical matrix surround 5.1 set up configuration in the Matrix. Outputs 1 and 2 are front left and right channels, output 3 is the center speaker, which I send to analog output 3 as well as mix into the other front channels. Output 4 is the sub-woofer output, which I mix equally into the front left and right channels, since I don't have a separate sub-woofer. Outputs 5 and 6 are the rear left and right channels, which I route to analog outputs 5 and 6.
Note that I had complete freedom to route the outputs differently, and tended to use the same number analog output as computer (internal) output just for mnemonic convenience. The upside of this is that once it's set up, you won't need to make changes to the Matrix or Total Mix. Windows is another matter. If your PC has built-in audio capability, you're better off disabling it, since Windows has a tendency to switch away from the RME to the on-board audio with no apparent provocation. This requires going back into the Sound control panel and selecting the Fireface as the default. My general impression is that sound device management is somewhat chaotic in Windows, due to the large number of sound devices and standards available.
Fireface Analog Audio Interface
One of the great strengths of the Fireface may be unfamiliar to the home audio enthusiast: differential analog inputs and outputs. Most home audio systems, even very expensive “audiophile” receivers and components, still use RCA connectors which are electrically “single ended”, having just one signal wire and a grounded shield. The disadvantage of single ended I/O is that the cables can serve as antennas for RF noise, especially 60 Hz noise emanating from home electrical wiring.
This noise can be amplified and generates background hum, sometime at annoying levels. With differential I/O, signals are transmitted and received through two wires (there’s also a grounded shield) where the signal in one wire is electrically inverted compared to the other wire. Thus, if a signal goes positive in one wire, it goes negative by the same amount in the other. The differential receiver circuit will subtract the negative signal from the positive, providing a resultant signal that is twice as large. The main advantage of differential I/O is common mode rejection. If noise is picked up by the analog cable, it will be canceled by the differential receiver as long as it is the same magnitude and sign in both signal lines. This works very well (though not perfectly) to confer noise immunity on differential signal (two wire plus grounded shield) cables compared to single wire RCA type cables. This approach is used almost exclusively in professional recording. For the home user, the benefits are that hum and ground loop problems go away almost completely.
Two different types of connectors are in common use for differential analog: the TRS and the XLR connectors. TRS, which stands for Tip, Ring, Shield, connectors are identical to ¼ inch stereo headphone jacks in common use in the US, but the two conductors are used for a single differential channel rather than stereo single ended channels. Usually, the cable wiring for these is heavier duty and better shielded than the standard headphone cable: the signal conductors are a twisted pair with a braided copper shield, standard cable for microphones.
The XLR connector uses two separate pins for the signal wires, plus a third grounded pin as well as a grounded metal shell. When making XLR cables, I usually tie the ground pin and the shell to the braided shield, and use the same type of cable as for the TRS connectors. XLR connectors come in male and female versions and are most often used for microphones and microphone cables.
I actually prefer the XLR connectors, since they provide a more positive contact with the signal pins than the TRS connector and can be locked in place, but they are bulky, and the Fireface uses TRS for the analog line I/O (except for the microphone inputs which are XLR), mostly because of a lack of back panel space for all the I/O ports it features. XLR connectors have also become de rigueur in the very highest end of the audiophile market, but TRS is an acceptable substitute, as well as easier to find in the US.
Adapting the differential outputs and inputs to the rest of your audio system may require some ingenuity. Fortunately, all the differential analog I/O can be used single ended without fear of damaging the electronics; you just lose the benefit of differential I/O. The simplest way to do this is substitute a mono ¼ inch headphone plug for the stereo plug, often called a TS (Tip, Shield) connector. Cables and adapters are commercially available that go from a mono ¼ inch TS plug to a female RCA. This is the easiest way to get the Fireface output into a conventional audio receiver. However, a better approach is to find an audio amplifier that accepts differential inputs. Here professional audio comes to the rescue, since almost all pro analog audio equipment is designed for differential inputs. The most commonly used is TRS, since the connectors are relatively inexpensive and readily available in the US. Numerous high quality amplifiers are available at prices that are a fraction of what is being paid for equivalent performance in the audiophile world. These are available through the same on-line retailers that sell the Fireface, some links to which are given at the end of this article. If you prefer an amplifier with XLR inputs, or happen to already have one, adapter cables between TRS and XLR are also readily available.
Other I/O Capability
Because of the studio nature of the device, Fireface 800 also offers 4 microphone inputs (XLR female inputs) as well as a single ended guitar or instrument input. I've successfully used the XLR microphone inputs as inputs for an Ortofon moving coil cartridge. Moving coil outputs are inherently differential, so it was relatively straight forward to ditch the RCA connectors and substitute XLR connectors for the cartridge output.
The microphone inputs have sufficient sensitivity and gain that they can be used without an intervening pre-amplifier. This is my preferred way to capture vinyl recordings to digital, and I'll discuss this in more depth in a future Tech Tips article. What about RIAA phono equalization? I prefer to apply this in digital post-processing. I find the results are far superior to analog RIAA equalization networks. I've tried it both ways, and I find that the analog equalization sounds muddy and less open in comparison to digital equalization. Last but not least, the Fireface offers MIDI I/O, perfect if you have a MIDI keyboard.
Digital Audio Workstation Software
Although the home user will typically not need the sophisticated capabilities provided by digital audio workstation (DAW) software, it's something that musicians may want to consider in purchasing the Fireface. The Fireface was intended to be used with DAW packages, and supports the ASIO standard. DAW programs for both Mac and PC can be used, since RME provides the necessary drivers for both. I use SONAR Producer for the PC, and find that the RME integrates almost seamlessly. All the inputs are controllable from the DAW, although certain hardware configuration items such as baseline sampling rate must be performed from within the RME control window.
For this article, I wanted to provide some independent verification of key performance specs such as signal/noise ratio (SNR) and total harmonic distortion (THD) of the analog I/O, but the specs are so good, that verifying them poses a considerable measurement challenge. The THD of my digitally synthesized signal generator was far in excess of the ratings for the Fireface.
Likewise, my digital spectrum analyzer lacked sufficient dynamic range and low enough noise to measure the SNR of the Fireface. Instead, I fell back on using the Fireface itself, combined with a PC spectrum analyzer program called True RTA.
I used my own instruments to partially verify the results of True RTA, so I have reasonable confidence in the calculation accuracy of that program. Essentially, I’ve tested the Fireface in loop-back mode, in which an analog output is routed back to an analog input using a TRS patch cable. In checking various combinations of outputs and inputs, I didn’t find any significant performance variation, so I report on only a single pair, output 1 to input 1. I use True RTA as the signal generator, as well as analyzer. In effect, I measure “round trip” SNR and THD with no ability to partition the contributions from input or output. But the measurement provides an upper bound upon either, since neither input nor output THD and SNR can be worse than the round trip measurement. As we'll see, the measured values are still very impressive, though not as good as RME's specifications. For these measurements, I set the sampling rate to 44.1 kHz. I found no performance difference at higher sampling rates, and this was mainly done as a convenience to restrict bandwidth for the RMS noise measurement.
The noise spectrum may look a little odd since it has this 10dB/decade upward slope. This is an artifact of the way True RTA displays frequency bins, since the numerical width of the bins increases linearly with frequency (1/24 octave). The noise floor is basically white (equal noise per unit frequency width), with some structure due to digital noise below 2 kHz. The white noise component of the system noise, I've estimated to be about -130 dBu/Hz.
When the signal generator is turned on, the noise floor is elevated. I've chosen to ignore this effect in the SNR calculations, since I believe it's largely due to noise on the digitally synthesized waveform due to computational error and quantization noise, and not noise imparted by the RME itself. This remains to be verified, of course.
On this and the following panel the spectra for the three different sine wave frequencies I used for the SNR and THD measurements: 100, 1000, 5000 Hz. Here, the signal level was set to the maximum the input could handle without clipping.
Summary of SNR and THD Results:
Frequency in Hz
Unweighted SNR in dB
As the reader may know, these are quite impressive numbers, even if they don't come up to the level of RME's specifications. This unit, which is now going on two years old, does appear to have degraded somewhat from its new condition.
Still, I was somewhat surprised to see a roughly 20 dB discrepancy in SNR compared to the RME specs. I'm going to continue to refine these numbers and try to round up some means of independent confirmation. But to put this in perspective, both the noise level and harmonic distortion are completely inaudible.
How Does it Sound?
Although the upscaling feature does help the sound of CDs, I find that they still sometimes seem overly metallic or harsh. I guess there's only so much sample rate upscaling can do. Otherwise, the effect of the RME is exquisitely neutral, with sound quality very much dependent on the source material. I have found that a good comparison to make is to compare pure analog playback of vinyl records to playback of digital recordings of the same records made through the RME. I find I much prefer playback through the RME, but mostly due to the superior digital equalization. I've also digitally recorded records with analog equalization. In that case, I can hear a slight difference between the analog, live, playback and the digital recording, but I can't really say which is better, as long as the sample rate is 96 kHz or above.
In day to day use, I've naturally gravitated to digital playback from the computer rather than from CD or analog. The reason is not merely convenience. Computer digital audio, even when ripped from CDs just seems to sound better than the original CDs. I don't know what the reason for this is, but I suspect that it has something to do with bit error rate (BER). The BER for optical disks can be rather high, due to scratches and imperfections (typically 1 bit in 10000 or BER = 10-4).
One of the key enabling technologies of the CD was bit error detection and correction. Reading from hard drives is much more reliable, and computer systems also can employ much more sophisticated error detection and correction. My suspicion is that BER is one of the key unmentioned disadvantages of the CD. Certainly, Sony and Philips never offered a performance metric to characterize the impact BER might have on the subjective impression of the audio for a listener, but always pointed to the lower THD and better SNR compared to analog media.
My experience suggests strongly that the complaints that analog music lovers have had about digital are mainly based on an obsolete digital format, the CD. I only wish that more 96 kHz, 24 bit content was available. DVD audio provides this higher quality, but the music selection is still somewhat limited. In the mean time, I generate most of my own content from my vinyl record collection, even using these to create MP3 versions I can carry around with me.
Where to Buy
If you live in a major urban area, your local Guitar Center or equivalent will carry RME products or be able to order them for you. I prefer to order on-line, and here's some links to on-line stores I've used with zero hassles: