The single cable solution for delivering pristine, high-resolution digital audio from your computer to your digital audio playback system.



With the new Bridge from Devilsound Labs, it is possible to get a high resolution, ultra low jitter signal straight from your computer, with a single, cable.  Completely plug-and-play on both PC and Mac, the Bridge allows for 24 bit resolution at sampling rates up to 96 kHz, and works with virtually any file type and player.



The Bridge is build upon the solid foundation of the sophisticated USB audio receiver code, Streamlength, by Wavelength Audio. Streamlength allows the extremely low phase noise clocks inside the Bridge to run as the master audio clock, resulting in extraordinarily low jitter output (timing errors). Also, since re-clocking circuitry is eliminated, the footprint of the circuit becomes tiny - the Bridge runs smarter, not bigger.

Inside, multiple layers of power filtering create clean power rails for the internal circuitry. A high quality output transformer allows for complete isolation of the output signal.   

But the design isn't merely for convenience of a single plug-and-play cable. Eliminating the output cable preserves signal integrity, keeping jitter to a bare minimum.




Technical Specifications

• Jitter - < 100 ps {RMS deviation from ideal over 1 ms period}
• Power - supplied from USB connection. PI filtered, and double down regulated; separate power rails for digital circuitry and master clock.
• Drivers - Plug and Play, using Streamlength. Allows for 44.1, 48, 88.2, and 96 kHz (user selectable), at 24/16 bit resolution.
Isolation - output is fully isolated using a high quality digital audio transformer.
• Connection - 75 ohm, available in BNC and RCA versions.
• Construction - CNC milled, anodised aluminium enclosure. Double sided, four layer printed circuit board.
• Designed & manufactured in the USA.

Design
S/PDIF - "Sony / Philips Digital Interface" - is a widely used standard for transmitting digital audio.  The information is encoded and transmitted into single line, using bi-phase mark encoding.  There are two main physical transmission modes:.  transferred by one of two physical formats:


75 ohm impedance coaxial cable.  The signal is carried by a voltage (typically 1 volt peak-to-peak, unloaded) down a controlled impedance cable, which is terminated at both ends in 75 ohms to avoid reflections.  This cables is best terminated by a 75 ohm BNC connector.  Although RCA connectors and jacks are frequently used, a standard RCA jack cannot carry a 75 ohm signal, due to the geometry of the connector.  (Some manufacturers do 75 ohm connectors, which must be mated with a specially designed 75 ohm RCA jack to keep the correct impedance.)


Optical fiber (Toslink).  Since the coaxial transmission is much more commonly used in high end audio, we opted to use the coaxial instead.
 


The goal of any S/PDIF converter is to convey the exact bits in the audio signal, with the least amount of timing errors (jitter) possible.  Pretty much all S/PDIF devices can deliver the correct bits, although it is important that the device have the resolution needed to play the file.
 


However, the timing is a different matter.  Timing errors in the signal can come from a number of different sources: noisy power supplies or clocks, timing errors already present on the audio signal being encoded, and noise in the transmission line, just to name a few.


Most S/PDIF receiver devices will add at least some jitter reduction, especially in the higher frequencies, since the PLL (phase-locked loop) circuitry used to recover the signal typically requires filtering in order to work.  In addition, some more advanced digital to analog converters incorporate additional reclocking circuitry, which reduces the jitter noise throughout the audio band.


Still, attenuation of high frequency jitter is not perfect, and there is always a lower frequency limit below which the re-clocking circuitry will not work.  For these reasons, it is critical the S/PDIF signal itself have very low timing deviation, through and below the audio band.


USB Receiver
The transit is based around the champion of USB receiver codes, Streamlength by Wavelength Audio.  The Streamlength firmware, running on the TAS1020B, allows for several key advantages:
 
Streamlength is completely plug-and-play.  For a single cable design, not having to deal with installing drivers is critical.  Streamlength also automatically allows the user to select the output frequency of the device - 44.1, 48, 88.2, or 96 kHz.  This allows audio to be run at the native sampling rate, or can be used to add an additional level of oversampling.  (For instance, selecting 88.2 kHz when playing an audio CD will give 2x oversampling, before the signal even reaches the DAC.)
Streamlength allows for 24-bit audio, at sampling rates of 44.1, 48, 88.2, and 96 kHz.  This allows for playing a huge range of files, from CDs (or rips of CDs) to newer, high-res audio files.


Most crucially, Streamlength allows the audio device itself - in this case, the Bridge - to run the master timing of the audio system. So, rather than the computer running the master clock, and the audio device slaving to this, the main system clock is on the device, and the rest of the system slaves to this.
 
This last feature is what makes Streamlength unique among plug and play devices, and gives it an edge.



Power Supply
Critical to any high-end audio design is the power supply.  Since the Bridge is powered through the USB connection, the power coming from the computer is supplied at 5 volts, though is typically not terribly clean, due to noise in the computer and other attached peripherals, such as hard drives.


In order to supply clean power to the on-board circuitry, the Bridge uses a combination of power supply filtering and multiple stages of down regulation. Power coming into the device is first PI filtered (CLC), which gives a two-pole attenuation for noise above roughly 10 kHz.  This works to eliminate high frequency noise, which down-regulators are typically not as good at rejecting.

The filtered 5 V signal is then down-regulated to 4V.  Note that down-regulators tend to be excellent at rejecting noise at DC and lower frequencies, the rejection ratio falls off at higher frequencies.  The combination of an initial CLC filter and an initial down-regulation stage gives a complementary technique to removing power supply noise, before it even reaches the main down-regulator chips.



The 4V power rail is then used by two additional down-regulators to provide 3.3V for running the digital circuitry, along with a separate 3V power dedicated to the master clock.  



S/PDIF Encoding and Output Stage
The output from the TAS1020B is in I2S, which is converted to the bi-phase encoded (S/PDIF) format with a WM8804 transceiver chip [http://www.wolfsonmicro.com/products/WM8804].  In order to insure that the chip has not added any jitter, the output from this device is clocked a final time by the original master clock of the system, using a XX D-type flip-flop XX.  



Output Transformer
In order to isolate the output from the (potentially noisy) computer ground, and to avoid the possibility of ground loop noise, SPDIF commonly employs an output transformer.  We use a high-quality American made output transformer, the SC947-02, which allows excellent isolation and signal integrity in a small package.  

(One side note about size - in no case in the Bridge were parts selected for small size; it just so happens that these components tend to be quite small, anyway, and the high-quality parts can be fit into a small board with careful design and layout.)

Physical construction
A key feature of the Bridge is the small size.  The small size wound up being somewhat of an after-though in design - the original plan was to build the best possible circuit, and it turns out that we were able to build it quite small.



The enclosure is custom, CNC machined from billet aluminium. The BNC connector version is a standard 75 ohm BNC connector (gold pin, of course), custom milled so it can connect to the enclosure during assembly.