The RME Fireface UC audio converter

This article is an adaptation of a document written by Alastair Gurtner.

The RME Fireface UC has all the features of a high quality audio device that is well suited for measurements with DIRAC.

  • Sample rate up to 192 kHz
  • Very robust USB communication using included ASIO and WDM drivers
  • Excellent technical data regarding SNR, THD and other important specifications
  • Small size, robust housing and reasonable weight
  • Floating signal ground
  • Supports 48V phantom microphone power
  • Line inputs will support ICP/IEPE microphones (using a separate power supply)
  • No potentiometers, the device settings are stored in the device itself with PC backup
RME Fireface UC

Installation is straightforward and explained in detail in the user manual. ASIO and standard Windows drivers are supported, and both can be used with DIRAC. Since RME has one driver for most of their audio converters, it will likely be maintained well into the future.

Please pay careful attention to section 9.1 of the Fireface user manual, stating that the Fireface should not be set as the default playback or recording device. Keep you PC's internal sound device as the default audio playback and capture device, so that system sounds or other applications do not interfere with your DIRAC measurements.
As DIRAC uses Windows WASAPI in exclusive mode, the corresponding option should be enabled for the Fireface in the playback and recorder properties.

Recorder properties

First you need to decide on your interfacing concept, if you have not yet have a suitable microphone. Professional class 1 microphones with metal diaphragm and precise frequency responses, have a very long expected lifetime when well treated, and are available in three basic interface versions:

  • The supply voltage of the so-called LEMO type preamplifier is 120 volt, giving these microphone types absolutely unique dynamics - up to about 140 dB. These microphones require high impedance inputs and a special power supply. Most audio converters are not well suited for such microphones - the inputs are too low impedance. LEMO microphone sets with supply cost several thousand Euros. The combination of DIRAC with LEMO style microphones will be typically a situation using an SLM with an AC output such as the B&K 2250 or 2270.
  • IEPE is a more modern preamplifier concept. Some well known implementations of this concept are ICP (PCB Piezotronics) and Deltatron (B&K). IEPE is the world interface standard for sensors and first got established for force and vibration sensors - some 30 or 40 years ago. Its limitations are due to a lower supply voltage of about 28 V, leading to a lower maximum output voltage swing of about 5 Vrms. Further these microphones require pre-polarized microphone capsules. A small power supply is also required, mostly available as battery driven unit, allowing field use. IEPE class 1 solutions are available from about € 1500.- (Microphone and power supply).
  • PA / recording studio phantom condenser microphones exist in many version; Low cost types, models for recording and studio work, and models using measurement microphone capsules - an invention started at B&K many years ago: DPA microphones. This last version can be connected to the microphone inputs of the RME Fireface UC without the need for an external power supply.

RME Fireface Input / Output capabilities
The table below gives an overview of the I/O system of the UC and its capabilities (measured values). The table describes the input behavior for the most sensitive setting, -10 dBV. Two less sensitive settings are available and when necessary, the sensitivity can be changed - but only device wide with this model. The calibration concept is based on the idea, to never change the attenuator settings of the audio converter - because the PC software will not know about this change in the calibration.

50 mV/Pa mic 15 mV/Pa mic
In / Out Use for Imp [Ohm] Lmin [dB] Lmax [dB] Lmin [dB] Lmax [dB]
Mic 1-2 XLR PA/Phantom (1) 900 (2)
6 118
Gain = 10 dB
11.5 123
Gain = 15 dB
Mic 1-2 TRS PA/Phantom (1) 3900 (3)
9.5 124
Gain = 15 dB
(1.7 V limit)
19.3 124
Gain = 25 dB
(500 mV limit)
In 3-4
No Pad, No Instr
10k 18.5 118
Gain = 12.5 dB
28.5 123
Gain = 18 dB
In 3-4
No Pad, Instr
470k 13 121
Gain = 0 dB

Gain = 12.5 dB
Gain = 0 dB
In 3-4
Pad, Instr
470k 19 122
Gain = 10 dB
In 5-8 IEPE 10k
12 122
Level set to -10 dBV
21.8 131
Level set to -10 dBV
Out 75
(1) Choose gain for optimum SNR. Take care not to accidentally change the gain after setup
(2) Can overload IEPE at signal levels > 1 V (including margin for long cables)
(3) Can overload IEPE at signal levels > 4 V (including margin for long cables)

The table shows the different connection options the device offers; first the microphone inputs supporting phantom power, then the three modes of the instrument inputs and on the last row the line inputs. As can be seen, one of the modes of the instrument inputs could be used for LEMO style microphones; this from the view of the impedance, but also regarding the noise floor of the input.

For IEPE type microphones the line inputs are well suited. Line inputs have a special quality which the other inputs don’t have: they have the same signal levels and attenuation steps as the line outputs, making loop-back measurements much easier.

Mixer setup
The next picture shows the mixer setup, where the first two line inputs (5, 6) are used together with the line outputs (1, 2). If you use an active loudspeaker, then the gain control AN1 (left bottom) can be used - or better yet the slider in DIRAC, the position of which can be fixed in a DIRAC measurement preset.

RME TotalMix FX

Do not forget to set the line reference levels to -10 dBV. This leaves 0 dBFS at +2 dBV with 12 dB headroom.

I/O levels

Sound device setup
In the sound device setup dialog you can either use the default (WASAPI) interface or the optional ASIO. Both will work fine, although ASIO may result in slightly better source-receiver distance measurements.

For accurate source-receiver distance measurements, DIRAC needs to determine the system's latency. To this end you need to perform a loopback test when prompted in the sound device setup dialog in DIRAC. Connect a cable between the selected analog output and analog input, and let DIRAC perform a loopback test. This will ensure that in future (synchronous) measurements the source-receiver distance can be determined from the impulse response.

Once the loopback test results are stored, you should perform a simple loopback measurement using this setup:

Loopback measurement

The resulting impulse response should look something like this:

Loopback impulse response

With the Dirac peak somewhere close to sample 0. Note that the peak may also be positioned at sample 1 or -1 = the right-most sample in the window.

Loopback zoomed