Halide Design Bridge

With the new S/PDIF Bridge from Halide Design, it is possible to get a high resolution, ultra low jitter digital audio signal straight from your computer, with a single, plug-and-play 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 extraordinarily low jitter output. Since there is no resampling or reclocking, the footprint of the circuit becomes tiny - the Bridge runs smarter, not bigger.

Inside, highly filtered ultralow noise power regulation provides clean power rails for the internal circuitry. A high quality output transformer further isolated the output signal from any computer noise.

"This simple, small, single cable solution incorporates innovative design and the current holy grail of USB implementations asynchronous transfer mode ... There are some fabulous DACs around without USB or FireWire input that will benefit greatly from the Halide Design Bridge. I highly recommend The Bridge from Halide Design ... " — Chris Connaker, ComputerAudiophile.com.

The S/PDIF Bridge was awarded the coveted "class A" recommended component status from Stereophile magazine.

From the April 2011 issue:

"The Bridge ... produced a very clean datastream free from timing uncertainty; and with it's relaxed, grain-free sound, the Bridge excelled at coneying recorded ambiance and low-level detail, said JA" (Vol. 33 No. 12 WWW)

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. The signal can be transmitted 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 cable is best terminated by a 75 ohm BNC connector. Although RCA connectors and jacks are frequently used, a standard RCA jack cannot maintain a true 75 ohm impedance, 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 common for high end audio, optical will not be discussed here. The goal of any S/PDIF converter is to convey the exact bits in the audio signal, with the least amount of 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. However, a typical S/PDIF receiver usually has little or no jitter attenuation throughout the audio band and below, meaning much of the audible jitter in the incoming signal is simply passed into the output digital signal (typically a three-line audio signal, such as I2S or right-justified).

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

USB receiver

The Bridge is based around the sophisticated USB receiver code, 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 (Note: 88.2 may not be supported under Windows Vista/7). 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.

The last feature, which is frequently referred to by the technical name "asynchonous" (as opposed to "isosynchronous"), is the key to obtaining ultra-low jitter on USB devices. With asynchonous USB receivers, the jitter is essentially limited only by the clocks on the audio device, plus any [typically very small] timing errors from additional logic gates that the clock signal must travel through.

For more on asynchronous USB receiver codes, check out the computeraudiophile.com article called Asynchronicity: A USB Audio Primer 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 an newly released power down regulater. Power coming into the device is first PI filtered (CLC), which gives a two-pole attenuation for noise above roughly 3 kHz. This works to eliminate high frequency noise, which down-regulators are typically not as good at rejecting.

This filtered signal, which is slightly less than 5 V (due to resistive elements in the passive filter), is down regulated to 3.3 V for the digital electronics, and a separate 3 V line for the clocks.

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 LC filter and a regulator with high PSRR (70 dB at 10 Hz, a reduction factor of over 3,000), ultralow noise regulator insure that the clocks and the digital circuitry can operate as accurately as possible.

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 S/PDIF transceiver chip. 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 D-type flip-flop.

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 small, high-quality output transformer, which allows excellent isolation and signal integrity in a small package.

Physical construction

A key feature of the Bridge is the small size. The small size wound up being somewhat of an after-thought 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. Much of this has to do with the use of Streamlength™. Because the core components are quite small and require no additional jitter reduction circuitry (such as asynchronous resampling), which can take up quite a bit of board space, the entire device can be built into the plug.

The enclosure is custom manufactured by CNC from high quality aluminum. 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. Alternatively, the device is available with an RCA connection. This is the silver bullet plug, by Eichman.

◦Power - supplied from USB connection. LC filtered (two pole passive) and down regulated; separate power rails for digital circuitry and master clock.

◦Drivers - Plug and Play, using Streamlength™.

◦Supported rates - 44.1, 48, 88.2, and 96 kHz (user selectable), at 24 bit resolution. (Note: Windows Vista / 7 currently requires WASAPI for non-resampled 88.2 kHz playback by plug-n-play USB audio products)

◦Isolation - output is fully isolated using a high quality digital audio transformer.

◦Connection - 75 ohm, available in BNC (gold center pin) and RCA (Eichmann silver bullet) versions.

◦Construction - CNC milled anodized aluminum enclosure, laser engraved. Double sided, four layer printed circuit board.

◦Length - 6 feet, extends to 16 feet with optional extension cable.

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