In our increasingly digital world, the term &#;digital interface&#; has become an integral part of our daily vocabulary. From smartphones to computers, and smart home devices to industrial machinery, digital interfaces play a crucial role in enabling communication between users and machines. In this article, we will explore what a digital interface is, its various types, and its significance in the modern era.
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At its core, a digital interface is a point of interaction between a user and a digital device or system. It serves as a bridge that facilitates communication, allowing users to input commands or receive information from the digital realm. Digital interfaces can take various forms, ranging from the familiar touchscreens on smartphones to the graphical user interfaces (GUIs) on computers.
In the digital age, understanding and harnessing the power of digital interfaces is crucial for individuals, businesses, and industries alike. As technology continues to advance, the landscape of digital interfaces will evolve, offering new possibilities and shaping the way we interact with the digital world. Whether through touchscreens, voice commands, or immersive virtual environments, digital interfaces play a fundamental role in connecting us to the ever-expanding realm of digital technology.
Most d/a converters offer more than one connection. At the back, at least a coaxial and optical connection can be found. And in many cases that has now been expanded to include a USB connection. If it is a more luxurious model, you may find another AES, BNC or some form of I2S connection. The question is: what is the best interface or digital connection? We walk you through the pros and cons of each.
What perhaps not everyone knows is that S/PDIF (Sony / Philips Digital Interface) is a protocol. Specifically, it is a data link layer protocol. IT nerds now know what OSI layer we are on :-).
S/PDIF is uni-directional and thus has no error correction, as a data-network does. Also, S/PDIF in stereo mode (PCM) is not package based: it is a serial data stream. But now we go more into the protocol and that was not really the focus of this article. However, it is useful to know that the S/PDIF connection has no error correction and is in fact a simple serial data stream.
The coaxial, S/PDIF connection is suitable for stereo audio (originally up to 20 bit / 48 KHz, now also 24 bit / 192 KHz) and also compressed multi-channel audio. Of course, the d/a converter or receiver must accept that, since Dolby and DTS use their own codecs.
For the connection, Sony and Philips have agreed to use a 75 Ohm cable. When introduced, these cables featured an orange connector (in contrast to red and white for analog cables).
The &#;electrical&#; S/PDIF interface is a pretty robust interface. However, it can suffer from EMI, RFI or other forms of interference. The cable &#; in our experience &#; has quite an impact on playback quality. Also, the characteristic impedance is crucial, as reflection can occur if it is not perfectly 75 Ohms (and RCA connectors never are&#;). In short: yes&#; the coaxial interface works technically well, but most RCA coaxial cables not optimal.
In the table above, you can see a jitter measurement on a couple of cables. The two cables that score very badly are not in fact coaxial cables. So, not only does the cable need to be a true coaxial cable; the connector has an influence as well.
We don&#;t see the BNC interface very often. And that&#;s a shame, because the BNC connector is many times better than the RCA variant. Why? Simple: the BNC connector is perfectly 75 (or 50) Ohm. And that avoids a lot of problems, as described above.
If you can choose between a &#;regular&#; coaxial cable with an RCA connector or the same cable with a BNC connector, we strongly recommend you choose the BNC variant. It will produce audibly better results. Also, we have also been able to measure that BNC works better. They use BNC in studios and laboratories for a reason.
The AES/EBU interface is basically a balanced version of the coaxial S/PDIF interface. Both interfaces are based on the S/PDIF standard, however, the AES/EBU interface has chosen a balanced design and its characteristic impedance is 110 Ohms, rather than 75 Ohms. Also, the channel-state bit differs compared to the coaxial interface.
Because of the balanced setup, you essentially no longer suffer from EMI or RFI, and the characteristic impedance is also not an issue with the balanced connectors (&#;XLR&#;). Bottom line: if you have the ability to connect the source and d/a converter via an AES/EBU interface, this is the preferred choice.
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The optical link we use is officially called TOSLINK. That stems from Toshiba Link. Toshiba developed this interface to connect their CD players to receivers. The software layer is based on the S/PDIF standard, but they chose fiber optics for the physical layer, rather than the 75 Ohm coaxial copper cable that Sony and Philips used.
The official documentation states that TOSLINK works up to 20 bit / 48 KHz, however, we know that modern implementations work up to at least 24 bit / 96 KHz and even 192 KHz. The optical interface also works with Dolby and DTS standards.
Qualitatively, an optical interface should be equivalent to the coaxial S/PDIF interface. However, our experience is that coaxial usually sounds slightly better. There can be two reasons for this: conversion is required at both the source and the receiver. Initially from an electrical signal to light and back again. Conversions are almost never completely lossless.
A second issue is the quality of the cable itself. The core of a fiber optic cable is by no means always made of glass. Actually, it is rarely a real glass core. Most often it is plastic. The quality of the core and of the connector itself is extremely important, because scattering or reflection of light quickly occurs. And that, in turn, can introduce jitter.
An advantage of the TOSLINK connection, however, is that no ground loops or other forms of electrical interference can occur. After all, light does not suffer from EMI or RFI.
We move away from the S/PDIF protocol and go to I2S: Inter IC Sound. (pronounced &#;eye squared ess&#;).
Basically, I2S splits the various signals joined together in S/PDIF, namely data and clock. S/PDIF requires clock recovery, but I2S does not, which has its advantages in terms of clocking.
There are at least three signal lines in I2S: Bit Clock, Word Clock and Multiplexed Data. There are more elaborate I2S implementations where another Master Clock and perhaps another multiplexed data line for upload are added.
Now I2S is actually intended as an &#;Inter IC&#; protocol, meaning that it is intended for short distances between two ICs. However, manufacturers saw possibilities to use I2S also between CD drives and dacs.
Now this is not just a matter of creating some in- and outputs, since I2S was not designed for use with cables at all, and thus major problems can arise if this is not corrected in the design. Think of mismatches in impedance and partly because of that, synchronization problems between clock and data. Fortunately, all major manufacturers keep this in mind.
The I2S interface can provide quite a jump in quality if the implementation is done properly. Just know that every manufacturer does have their own spin on this interface. Thus, a source with I2S interface will probably not work with d/a converters from other brands.
The USB interface is quite controversial. And there is a good reason for that. Of course, we know the USB interface from our laptops, PCs and Macs. When USB connectivity was added to dacs, it opened up a whole new world. By connecting a laptop to the hi-fi system, we could essentially play all the music we wanted!
But it soon became apparent that it didn&#;t sound that good. The reason: noise. A laptop, PC or Mac is not a clean source in terms of electrical noise. The USB interface is simply pretty dirty.
But manufacturers don&#;t just let themselves get off the hook and soon these implementations got better and effective filters, such as the Audioquest Jitterbug, appeared. Simply put, these are energy filters for the USB port. And yes: they work.
USB audio, unlike S/PDIF, does not work with a simple bitstream, but package-based, meaning that packets of audio data are sent from source to the d/a converter. However, as with S/PDIF, error correction is not possible. There is error detection, but no correction is possible.
There are basically two modes within USB-Audio:
In the high-end audio world, asynchronous is preferred because it disconnects the source&#;s clock from the d/a converter. With synchronous &#; and adaptive &#; the source is leading, as the dac&#;s clock links to the USB-SOF (Start of Frame). However: in most cases, the d/a converter will have a better clock than the source. By making the d/a converter &#;master,&#; a better result can be achieved.
Now a computer often works with drivers. So what about when you connect your d/a converter to the computer via USB? That&#;s surprisingly easy these days. In most cases &#; if we&#;re talking about a modern Mac, Linux or Windows system &#; no drivers are needed. And should drivers be needed, they can be installed just fine.
In the part about USB we explicitly mention that the asynchronous mode is usually preferred, because then the d/a converter keeps control over the clock; the dac is &#;master&#;.
This is a rather important point to mention because it works differently with S/PDIF. With S/PDIF, the source is &#;master&#;. This is because the source sends the clock along with it. This clock is extracted from the S/PDIF signal and has an influence on playback quality. Yes: dacs reclock and pull all kinds of tricks, but in most cases it is still crucial that the source has a decent clock, since it drives the data stream. This also applies to I2S, by the way.
So with USB it works differently, because the data transfer is asynchronous: the d/a converter asks for data, the data runs into a buffer and is converted so that the converter can start working with it. The converter then uses its own clock to convert things: the source&#;s clock plays no role at all. Unless, of course, you are using synchronous mode.
The type of digital interface undoubtedly plays a role in the quality of playback. We have tried to indicate the advantages and disadvantages of certain interfaces. It is up to you to know them off and try some connections. Again, rely on your own ears in doing so. They are wonderful measuring instruments!
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