Sonic Frontiers Transport 3 CD transport & Processor 3 D/A processor I2S: Two Don't Always Tango

Sidebar 2: I2S: Two Don't Always Tango

One of the key elements of Sonic Frontiers' new Series 3 digital components is the inclusion of an UltraAnalog-designed I2S-Enhanced interface. The original I2S (Inter-IC-Sound) bus found in nearly all CD players provides discrete pathways for the master, word, and bit clocks, as well as audio data signals as they travel between processing sections within a player, and eliminates the need for the transmission and receiver circuitry mandated by the AES/EBU or S/PDIF transmission schemes to connect two or more digital components, which is prone to jitter due to the fact that the word clock is embedded within the data.

The recent offerings of I2S interface options for two-box systems are derivations of this bus applied externally for inter-component, rather than inter-IC, communication. For example, the critical reference clocks and audio data each travel from a transport to a digital processor intact, on discrete lines, without suffering the inevitable degradation caused by bi-phase encoding, transmission, and subsequent decoding of the combined clock and data signals, as occurs with traditional interfaces. The result is a dramatic reduction in interface jitter.

Good examples of external I2S interfaces include the I2S-E and the original 13W3-I2S standard developed by Kevin Halverson of Muse, and now offered to other manufacturers by Digital Axiom Corporation as a ready-made solution in the form of drop-in modules. By using either scheme, the relative lack of interface jitter allows a designer to fully realize all the advantages of two-box construction. These include the isolation of critical decoding circuitry and the analog output stage from interaction with transport servos and unwanted power-supply coupling.

For the past year and a half I've listened extensively to systems equipped with I2S interfaces using 13W3 hardware. These include two separate Muse systems, and now the Sonic Frontiers pair. In each case I've heard a significant enhancement of low-level detail and dynamic contrast, better focus and transient definition, and a notable increase in the perceived effortlessness of expression compared with any version of an AES/EBU or S/PDIF connection I listened to—so far, all great news.

However, as mentioned in the body of the review, there's a compatibility issue prospective buyers must be aware of. UltraAnalog branched off from the original 13W3-I2S specification to design their own version, as was certainly their prerogative. However, UA's employment of the same 13W3 cable and connectors (probably because they're ideally suited to the task) ensured the kind of consumer confusion and incompatibility between the 13W3-I2S and I2S-E schemes that I experienced.

I don't have the space here to explore all the technical details distinguishing the two now-competing versions of 13W3-based I2S designs. Instead, I'll look at my experience with the Sonic Frontiers version, and a few practical differences of interest to the end user. I'll take a closer look at the technical merits of the 13W3-I2S interface in an upcoming review of the Muse Model 8 and 296 DVD/CD system.

I went through a considerable "debugging" period with the I2S-E system in the Processor 3. In December 1997 I was sent an SFT-1 transport equipped with the interface for use with the P-3. However, it wasn't yet ready for prime time—I couldn't get a clean signal lock. Sending the unit back to the factory failed to fix the problem, but by February '98 the bug had been fixed, and I installed a factory-supplied modification to the I2S-E circuitry within the P-3. Since then, the interface has worked great.

In May, Murphy's law returned in the form of a bug unrelated to the I2S issue: The SFT-1's front-panel controls began to consistently lock up, forcing me to use another transport—until the T-3 arrived. Since then, I've not had a lick of trouble with the T-3 or the P-3.

As no digital rigs contain examples of the I2S-E and 13W3-I2S interfaces, it's impossible to directly compare the audible differences, if any, between them. Despite any claims to the contrary, when both systems are properly implemented, using their respective highest levels (in which the master clock is located in the processor and the transport is the slave), there should be little if any theoretical or practical sonic differences resulting from interface issues. In any event, what really matters to the listener is the overall experience from a given complete system, as discussed in the body of the review.

Curiously, the version of I2S found in the expensive SF gear uses the lower of two performance levels offered in the I2S-E specs; ie, with a transport-based clock and the processor in slave mode. Chris Johnson said they felt this was sufficient for the demands of CD, and that they would implement "Level One"—with the master clock in the processor—when the P-3 is updated for 24-bit/96kHz capability.

Regardless, you can rest assured that both the I2S-E and 13W3-I2S schemes work extremely well with CDs. However, since the Muse system plays DVDs and AADs as well, the 13W3-I2S specs have been optimized to address a number of that format's performance and legal requirements. The current specs of the I2S-E interface will need updating to address these issues prior to use in DVD-based systems. Just keep in mind that a 13W3-I2S interface found in other digital components will not work with Sonic Frontiers' new gear. Here's hoping they'll find a way to include compatibility in future iterations of their 13W3-based I2S interface.—Shannon Dickson

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