Empirical Audio Off-Ramp 4 User manual

Off-Ramp 4 User Manual
Empirical Audio Rev. 1.0
© 2010

Off-Ramp 4 User Manual
Empirical Audio Rev. 1.0
1. Function/Technology
The Off-Ramp 4 is a versatile hi-res compatible USB interface. The function of the Off-
Ramp 4 is to convert USB signaling into S/PDIF, AES and I2S so that a DAC can be
driven with a computer audio data stream. The Off-Ramp 4 reduces jitter in the digital
data stream by re-clocking using local oscillators. The Off-Ramp 4 incorporates our best
low-jitter technology and includes an array of technical advantages over other devices on
the market, including:
•Async USB, so expensive USB cable is not necessary
•Inputs are re-clocked and S/PDIF and AES outputs are re-clocked again
•External DC power source, allowing battery power
•Separate input and output power regulation
•High-quality discrete voltage regulation with Hynes regulator as option
•Precision impedance-matched high slew-rate S/PDIF and AES/EBU output
•Optimum clocking for best 24/176.4 and 24/192 performance
•I2S output interface for lowest jitter to Empirical Audio DACs
•Modular design to allow for clock, power and USB improvements and upgrades
These technical advantages allow the Off-Ramp 4 to be a world-class digital source,
leaving even the most expensive CD transports and USB converters in the dust.
The sound of the Off-Ramp 4 is unlike any other digital source. The clarity and imaging is
unsurpassed, allowing you to “see” into the soundstage more than ever before. Width
and depth are improved significantly. It is a totally immersive experience, the way that
music playback should be. More like vinyl, only better.
1.1. What is jitter?
Jitter is the number one problem limiting the performance of digital audio. Jitter creates
an unnatural, harsh and sibilant sound that is pervasive in digital audio. Jitter causes
listening fatigue with both CD players and computer audio devices. It’s only in the last
three years that the audio industry has become sensitive to jitter and started actively
addressing this. The sound of jitter is like looking through a window with a film on it.
When jitter is reduced, it is like cleaning the window and finally seeing the scenery
outside with crystal clarity and vividness.
Jitter is the inaccuracy in the timing of the digital data. It is not data errors, it is
inaccuracies in the timing of the presentation of the data to the D/A converter. This is
different than sending digital data to a printer for instance, or even a hard-disk. These
data transfers are not “real-time”. Streaming audio data from a CD transport or a
computer source is real-time because the D/A converter creates a real-time analog signal
from the data each time a new data word is presented. If the timing of the presentation of
each word is inaccurate, this causes frequency modulation distortion. There is nothing
that can be done about the samples in the recording and the clock that was used to make
those samples, but playback jitter can be minimized. This is the role of the Off-Ramp 4.
The Off-Ramp 4 virtually eliminates the sibilance and harshness due to jitter, making your
digital playback closer to analog. The Off-Ramp 4 is a truly unique and remarkable
product. Once jitter is minimized, the clarity, image focus and dynamics are improved
significantly. Even bass-tightness improves. Recordings of rain, thunderstorms, violins,
cellos, bongos and timpani are finally lifelike.
1.2. Theory of Operation
The primary function of the Off-Ramp 4 is to convert USB streaming audio format into S/
PDIF, AES/EBU or I2S formats and provide hardware interfaces in order to be compatible

with DACs. The Off-Ramp 4 is a re-clocker, meaning that the timing of the data flow into
the device is retimed using internal clocks. This has nothing to do with the timing or
tempo of the music. Because it utilizes an asynchronous USB interface, the master or
system clock for your audio system is located in the Off-Ramp 4. This clock provides the
timing for the dataflow from the computer source. This is the optimum way to achieve
low jitter.
The Off-Ramp 4 reclocker inserts between your computer or USB source and your DAC.
The data stream is not modified or upsampled in any way, it only passes through. The
only cable that is critical to performance is the cable between the Off-Ramp 4 and your
DAC. This should be a high-quality S/PDIF coax, AES/EBU or I2S cable to minimize jitter.
1.3. Performance of different outputs
The three outputs all deliver world-class performance, however in highly resolving
systems some differences may be audible.
The I2S output delivers the lowest jitter, for those that have DACs with this type of input.
Next best in performance is the S/PDIF or “coax” output. Finally the AES/EBU output is
last because it requires an additional buffer to achieve the higher AES voltage spec.
For non-Empirical Audio DACs, we recommend using the S/PDIF output with a high-
quality S/PDIF coax cable, such as the Empirical Audio Bitmeister.
1.4. Internal clocks and clock quality
The Off-Ramp 4 comes standard with two clocks or free-running oscillators. Two clocks
are required because the asynchronous USB interface uses free-running clocks rather
than clocks that are synthesized from a single oscillator frequency. One clock is required
for the sample-rate group 44.1/88.2/176.4kHz and a second for the 48/96/192kHz group
because the two sample-rate groups are not related.
These clocks can be upgraded, improving the jitter of either one or both of the sample-
rate groups. One or two Ultraclocks or one or two Superclock4s can be added. Other
clock options may be offered in the future. These clock upgrades are not user-installable.

2. Front-Panel:
2.1.1. Power LED
Green indicator indicates DC power is applied to the DC input on back-panel.
2.1.2. Data Error
Yellow indicator indicates that data stream is corrupted or not PCM.
2.1.3. USB Fault
Red indicator indicates that USB cable is not connected or the USB port at the
computer is not active.

3. Back-Panel
3.1. 12VDC Input
This is the power input for the Off-Ramp 4. The power from this input goes directly to
the voltage regulators, and any installed Superclocks or Ultraclocks, so the quality of this
power is critical. Upgrades here make an immediate difference in jitter. Average power
consumption is up to 500ma, depending on clock options, but much higher transient
currents are possible. The standard AC adapter included with the Off-Ramp 4 supplies
2.5 Amps @ 12VDC. The connector is center positive, 2.1mm and protected from
reverse voltage. To power-on the Off-Ramp 4, plug in the AC adapter and then connect
the DC power cable to the Off-Ramp 4. The included AC adapter is floating with respect
to the AC power ground, but grounded AC adapters can also be used.
3.2. USB Input
The USB input connects with a standard A-B USB cable to a computer or other
streaming USB audio source. The streaming rate limit is 24-bits, 192kHz. The USB
interface is asynchronous, so USB 2.0 compliant ports on the computer must be used.
The USB cable can be as long as 5 meters or 16 feet. To prevent application and
system crashes, it is best to close the music player application, such as iTunes or
Foobar BEFORE disconnecting the USB cable. Make sure that you load the USB driver
into the computer before connecting the USB cable for the first time.
It is recommended that you close your player application (such as Amarra, Pure Music,
Foobar etc.), before you unplug the USB cable from the Off-Ramp 4 or computer. Then
restart the application after the USB cable is reconnected.