User manual

3.3. Toslink Input
The Toslink input (optical) is for use in Modes 1,2 or 3. It should be connected to the
source device Toslink output. This supports only PCM data, not AC3 surround. All
sample-rates are supported, but must be selected in two groups using the Sample-Rate
switch on the back-panel. The Toslink has a door on it that flips up inside when you
insert the optical cable end. Some cable ends are too large and can damage the door,
so if it is difficult to insert, don’t force it. Instead, use a small knife to trim the ridges on
the cable connector a bit to prevent damage to the door when mating.
3.4. USB input
The USB input connects to the source computer, either a PC or a Mac. The USB cable
can be as long as 5 meters and the quality is not important. The 5V power in the cable
is used to power the USB interface in the Pace-Car USB. The USB port on the
computer should preferably be 2.0 compliant. It is recommended that you disconnect
the USB cable only after stopping music playing on the computer. 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.
3.5. S/PDIF Coax Output
The S/PDIF coax output (optionally AES/EBU output) is the primary output that will
connect to most DACs. The return signal on this connector is tied to earth ground and
chassis ground, so the ground used for the Pace-Car USB AC power adapter should be
the same ground used for the DAC. Same AC circuit and phase also. The output
impedance is 75 ohms if the connector is S/PDIF and 110 ohms if AES/EBU. The cable
that connects from this connector to your DAC should preferably be of highest quality
and match the impedance, 75 ohms for S/PDIF or 110 ohms for AES/EBU. The RCA
jack is made of high-quality direct-gold plated copper with Teflon insulator, so it is better
than most BNC connectors. Optionally, a Canare BNC connector can be supplied here,
which is also excellent quality. The cable attached to this connector should be at least
1.0m in length, but minimize the length beyond that. 1.5m is usually perfect and will
work best with other gear.
3.6. I2S output
I2S is a more direct D/A interface than S/PDIF. It was developed by Philips when Philips
and Sony created the CD format and the S/PDIF interface specification. I2S is a three or
four-signal interface that includes clocks. All four signals are provided at 3.3V logic levels
at the Pace-Car USB output, but 5V logic levels are available optionally. I2S is the
native interface for most D/A chips. Therefore if one can drive I2S directly and avoid S/
PDIF translation, there is the opportunity to reduce jitter even more. Empirical Audio
standard I2S interface uses 4 signals and 4 returns and the RJ-45 connector. It is
compatible with I2S on several other manufacturers DACs, including Stello, Perpetual
Technologies and Northstar, as well as our own Overdrive DAC. Other manufacturers
DACs require an adapter cable to make the connection since they don’t all use RJ-45
and the pinouts are not compatible. Most of the other manufacturers DACs also require
the 5V logic level option. Make sure the I2S connectors are mated at both ends of
the cable before powering-on either the Pace-Car USB or the DAC. Likewise,
power both down before disconnecting the I2S cable.
3.7. Sample-Rate Selector Switch
This two-way toggle switch selects between two groups of sample-rates when the Pace-
Car USB is used in Modes 1, 2 or 3. It has no effect when the USB input is selected. It
is possible to customize these groups for additional fee, such as combining 44.1/96/192
into one group.