User manual
The Pace-Car USB is a reclocker, meaning that the timing of the data flow into the device
is retimed using an internal clock. The Pace-Car USB reclocker inserts between your
source device or computer and your DAC. The Pace-Car USB buffers the music data
stream in a memory and then reclocks it out with an ultra-low jitter clock. The data is not
modified an any way, it just passes through. There is a small amount of data that is
always stored in the memory, but it will not cause any audible delay in the music start-up.
The only cable that is critical to performance is the cable between the Pace-Car USB and
your DAC. This should be a high-quality S/PDIF coax, AES/EBU or I2S cable to minimize
jitter.
1.3. Modes of Operation
The Pace-Car USB can be used in a variety of different ways, each achieving identical
sound quality:
1. USB interface from a PC or Mac computer
2. Mode 1 – Word-Clock driving a stock external source device
3. Mode 2 – Master-Clock driving a modded external source device
4. Mode 3 – Pace-Car USB clocks are tuned to a stock external source device
All sources use the clocks inside the Pace-Car USB and the data-stream passes through
the Pace-Car USB. All modes can be used in the same Pace-Car USB, with some Mode
3 sample-rate limitations. The Pace-Car USB uses two separate clocks to support all
sample-rates. Both standard clocks are present in the stock product. One clock is used
for 44.1, 88.2 and 176.4. A second clock is used for 96 and 192. 48 can be optionally
added, but may slightly affect the quality of 192. Upgrades of one or two Superclocks or
one or two Ultraclocks can replace the standard clocks for even better jitter performance.
1.3.1. USB:
The USB interface is asynchronous, meaning that it does not rely on the timing of
the clock in the computer. This enables great results with inexpensive USB
cables and computers. A USB 2.0 port on the computer is recommended. On
playback, the Pace-Car USB automatically selects the correct clock for each
sample rate. The USB module is isolated from the Pace-Car USB power and
powered from the USB cable. With USB, you basically connect it and forget it.
The Sample-Rate switch on the back-panel has no effect when in USB mode.
1.3.2. Mode 1:
For Mode 1, the source device must have a Word-Clock input, such as some
high-end transports like the Marantz SA-11S2, some Firewire devices such as
the Fireface400, PCI cards such as the Lynx AES16 or the Transporter. Word-
Clock is a low frequency clock that matches the sample-rate, such as 44.1 kHz.
A Word-Clock cable is connected between the Pace-Car USB and the source
device. The Word-Clock cable is ordered separately and usually terminates into a
50 ohm BNC male connector. This BNC connects Word-Clock to your source
device. The Word-Clock output can be set for 5V or 3.3V logic levels depending
on the source device. This is done at the time of ordering. The customer also
provides two digital cables, one inexpensive cable for input to the Pace-Car USB
and a high-quality cable for output to the DAC. These cables route the digital
data in and out of the Pace-Car USB.
The Word-Clock output on the Pace-Car USB changes automatically to match
the sample-rate of the incoming data-stream within a few clock cycles. The
source device must also respond fairly quickly to changes in the Word-Clock
frequency. If this change takes 30 seconds or more, it is likely that the Pace-Car
USB will overrrun or underrun its buffer. If devices have this delay problem, the
Pace-Car USB can optionally output a fixed Word-Clock frequency. This is a
custom order.