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The Pace-Car USB has been discontinued and is replaced by the Syncro-Mesh

Empirical Audio Pace-Car USB

The Pace-Car USB is a reference quality 24/192 reclocker that will challenge the very best and most expensive CD transports and music servers. It is a window into the music that puts you in the front-row seat at the concert. With a good DAC, such as the Overdrive Signature, you will hear differences in cables, power cords, formats and playback software. It masks nothing. The clarity is uncanny. Even older 44.1kHz CD rips that you found unlistenable may now be enjoyable. Since we completed the Pace-Car USB, we have been doing much less up-sampling of 44.1kHz CD tracks because they sound so good as-is.

The Pace Car is simply stellar at getting the music out into the room in a way that equals that heard from spinning CDs directly with either the Cary or the EMM.

...the Pace Car clearly leads the pack.

Dave Clark, Editor, Positive-Feedback Online Issue 39

The digital equivalent of a Swiss-Army knife.

The Pace-Car USB has an asynchronous USB input that supports sample-rates up to 24/192 max. Asynchronous USB means that the clock in the computer is a slave to the clock inside the Pace-Car. Slaving the computer enables the lowest jitter result. The asynchronous interface also has the advantage that an inexpensive USB cable can used. The S/PDIF or I2S output cable is still important and should be of highest quality

It can also take other standard digital inputs, including Toslink, S/PDIF coax and optionally AES/EBU. Devices that drive these inputs must operate in Modes 1 or 2 as described below, or the Pace-Car must be tuned to the device so it can operate in Mode 3. The Pace-Car USB provides an ultra low jitter output from S/PDIF (optional AES/EBU) and I2S (Empirical Proprietary) to feed your DAC. The Pace-Car USB allows you to choose even the most inexpensive device and transform it into a world-class digital source. Have it all – ease of use and the best digital sound quality.

The Pace-Car USB is plug-and-play with a number of CD transports and computer audio sources:

  1. PC and Mac
  2. Marantz, Esoteric and other CD players with Word-Clock inputs
  3. Lynx AES16 and AES16a PCI cards
  4. Logitech Transporter
  5. All source devices with Word-Clock inputs

The Pace-Car USB is a true reclocker, not an asynchronous upsampler. The Pace-Car USB reclocker inserts between your source device and your DAC, 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.

There are no controls or switches except for the sample-rate select, which is only functional in non-USB modes. You never have to think about the Pace-Car USB. It just works.

Sample rates and clock upgrades

Sample rates of 44.1kHz, 88.2kHz, 176.4kHz, 96kHz and 192kHz are supported.

Two Standard clocks are included and required with the Pace-Car USB. They are very good clocks, custom-made for Empirical Audio. They are better than what most other vendors are using. However, they can be upgraded to Superclock4, which is more musical or Ultraclock which is the most live-sounding. Two clocks are required to cover all sample-rates, one for 44.1/88.2/176.4 and a second for 96/192. You can upgrade both or just one clock.

48kHz support is optional. If the option is selected, the quality of the 192kHz playback may be slightly compromised, so we decided to let it be optional since it is less common.

The Pace Car USB automatically switches all sample-rates when the new track is played, so you just connect it and then forget it.

Pace-Car USB operation modes

The Pace-Car USB can operate in four modes, depending on the source device:

Mode 1: Synchronous word-clock

The source device must have a word-clock input, such as some high-end transports, some PCI cards such as the Lynx AES16 or the Transporter. A word-clock cable is connected between the Pace-Car USB and the source device. The customer provides two digital cables.

Mode 2: Synchronous master-clock

The source device must have a slave-clock input to use this mode. This can be added to Sonos, Squeezebox, Duet, Tascam US-144, EMU 0404 and other devices with a small modification by Cullen Circuits to add the Slave-Clock input. A master/slave-clock cable is connected between the Pace-Car USB and the source device.

Mode 3: Asynchronous

The Pace-Car USB is tuned to the source computer or device and tracks the rate of the incoming digital stream. Asynchronous devices are Mac Mini, Macbook, AirPort Express, iPod, Wadia iTransport and Apple TV. The customer must ship the source device to Empirical Audio so that we can tune the Pace-Car USB. This requires one day and no modifications are made to your source. There is an additional cost for tuning and only Superclock4 can be used. With a tuned Superclock, the Pace-Car USB can still be used in Mode 1 or 2. A Toslink cable is included.

Mode 4: USB

The USB interface is galvanically isolated, both power and ground. This can only be done in a Pace-Car type reclocker to avoid adding jitter. This isolation eliminates the ground-loop between the computer and the audio system, even when using the I2S output.

The Pace-Car USB has been tested on Mac with iTunes/Amarra at all sample-rates and using a PC with XP and Foobar2000 with Kernel Streaming plug-in. The sound quality is virtually identical in both cases. Drivers are included on a CDROM for both Mac and PC.


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 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.

The Pace-Car USB virtually eliminates the sibilance and harshness due to jitter, making your digital closer to analog. The Pace-Car USB is a truly unique and remarkable product. Once jitter is minimized, the clarity, image focus and dynamics are improved significantly. Recordings of rain, thunderstorms, violins, cellos, bongos and timpani are finally lifelike. The Pace-Car USB is so good at reducing jitter that it is used by the editors of Positive-Feedback and Stereo Times.

USB option compatibility

The Pace-Car with USB option still maintains full functionality of original Pace-Car for use in Mode 1, 2 and 3, so other devices can also be used with it.

The USB interface is not present when Mode 1/2 configuration and Mode 3 configuration are selected at purchase time, so the USB connector on the back panel is omitted.

The standard Pace-Car USB also can work in Modes 1 and 2. However, it is possible to custom-order a Pace-Car USB that also works in Mode 3. Please email us for this kind of order as it has some limitations.

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. This 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.


Pace-Car User ManualDownload User Manual


  1. USB
  2. Toslink
  3. S/PDIF coax
  4. AES/EBU

Outputs (all outputs are active)

  1. S/PDIF coax or AES/EBU
  2. I2S on RJ-45, Empirical Audio standard (3.3V or optional 5V)
  3. Word-Clock output – switchable between 44.1 and 96 if both clocks are present
  4. Master Clock output for 44.1 (if clock is present)
  5. Master Clock output for 96 (if clock is present)


  1. Input Select switch for S/PDIF or Toslink
  2. Select switch for 44.1kHz word-clock output or 96kHz word-clock output


The Pace-Car USB is designed to work with all DACs and SS receivers/processors. The I2S output can be converted to 5V optionally for DACs such as Stello or Northstar.

Sample-rates supported in USB and Mode 1 and 2: 44.1, 88.2, 96, 176.4, 192. 48 is optional.

Sample-rates supported in Mode 3: 44.1, 88.2 and 96


  1. Pace-Car USB
  2. 12VDC wall-wart power supply
  3. Toslink cable with Pace-Car USB tuned to Macbook, Mac Mini, Apple TV or AirPort Express


  1. Audiocom Superclock4 (required for tuned applications)
  2. Second Audiocom Superclock4
  3. Audiocom Ultraclock – not for tuned applications
  4. BNC or NextGen output connectors rather than RCA
  5. 5 Volt I2S output for Northstar and other DACs
  6. Revelation Labs Cryo-Silver Reference I2S cable
  7. Word-Clock cable
  8. Master-Clock cable

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(541) 595-1001 - 10:00am to 6:00pm Pacific Time