USB audio converter module

The USB module transforms the afis or afi, in connection with a computer, into a full-fledged, very high-quality audio source device. The computer becomes a music player. The issues of "avoiding HF interference and clocking precision" are perfected here.

In principle, a computer is ideal for managing a digital music collection and playing back music with regard to its flexibility and operating convenience. Unfortunately from a sound point of view, a computer is the most inhospitable environment possible for an audiophile's enjoyment of music. Computer are machines that calculate at high clocking rates and run innumerable processes during music playback. As a result, they create a broad spectrum of high-frequency disruptions.

In order to keep this unavoidable interference from the computer away from the sound-sensitive audio section, the USB option has a two-part design:

1. The external USB-FOC module

for communication with the computer 


USBmodules 600

2. The internal USB module

built into afi or afis, which takes over all tasks of the music playback procedure which are relevant to the sound.

USB inputmodule

Both units are connected to each other with the help of three fibre optic cables (FOC) for maximum isolation from interference. Thanks to the optical coupling, there is no direct electrical pathway for HF interference from the computer to the audio section. Both units are completely galvanically isolated, mass flows are eliminated.

However, the most important difference between this and a conventional transmission of a digital audio signal via optical cable from the computer to a DA converter is the following: if a conventional digital signal is transmitted via toslink, then the clocking signal is embedded within it. The clocking information is at the same time the HF interference of the computer in the form of Jitter, and Jitter is very difficult to completely remove after the fact. Therefore, the optical audio connection does not eliminate the interference from the computer.

In contrast, the FOC connection of our USB module contains no clocking information that is relevant to the sound. Therefore there are a total of three FOCs. The external USB module receives the clocking information from an internal module. The reference oscillators sit in a perfect environment, fully decoupled from the computer, clocking the audio stream coming from the USB module in addition to the external unit. With regards to the clocking quality of the FOC connection itself, neither the clocking information going to the external USB module nor the audio stream from the external module have any influence on the audio quality. The double (duplex) FOC is responsible for both the clocking to the USB module and the audio from the USB module. The third FOC transmits the switching signal for each sampling rate family. Depending on whether the rate is 44.1 kHz multiples or 48 kHz multiples, the appropriate oscillator is activated.

AFI USB Modul Grafik1

The external USB-FOC module with a USB-B jack (1) to be connected to the computer, a duplex FOC connection (2) and FOC interconnection (3). Optionally, a separate 5-Volt power supply unit (1A) can be connected via the DC connector (8). With the sliding switch (7) you can switch between power supply via USB (Standard) and the DC connector. The RJ45-Ethercon output to afis or afi is either directly connected via RJ45-XLR cable (4) or via RJ45-RJ45 cable (5) plus adapter box (6) to the DAC.

USB module3 600

Further information:

Our sophisticated transmission concept in our USB module for Afis and Afi strives to keep this HF interference as separate from the audio signal processing as possible. There are several deciding factors here:

  1. USB is the most common interface today to connect audio devices with computers, at least in home audio. But USB is actually the worst interface imaginable for this, because USB cannot be galvanically isolated. Galvanic isolation is - often mistakenly assumed - not a cure-all for HF interference. But this is an important component to achieve interference suppression at all. But because this is not possible, all interference generated in the computer is to a large extent transmitted to the USB interface. Therefore, the small external USB module is far away from the audio electronics.  
  2. USB interfaces also work with a micro-controller and today such processors are sometimes enormously powerful, which again means a high potential for interference. This is also why the USB interface is placed in the external USB and thus far away from the audio electronics.
  3. The external USB module is connected with the actual Afi or Afis audio device via fibre optic cables. This connection guarantees perfect galvanic isolation, maximum interference suppression full immunity against disturbances and the connection is also guaranteed not to disturb itself. These FOC are available in 2m and 6m. It is thus simple to keep the computer with the external USB module separated from the audio electronics. Distance is the easiest way to protect against interference.
  4. The clocking, which is so crucial for DA conversion, is generated in the Afi or Afis itself under optimal conditions, free from interference factors from the computer and from the USB interface. The audio signal arriving from the USB interface via fibre optic cables are then reclocked on the basis of this precision clocking and is thus free of any disruptive influences coming from the computer. The fibre optic cable connection protects the system from interference and the clocking is created in the Afi or Afis. The digital signal, which is sent from the output of the Afi or Afis to the DAC, has optimum clocking quality and no longer contains HF interference coming from the computer or the USB interface that could disrupt the clocking reference in the DA converter during the conversion process or the analog stages directly.

In short: the essential feature of the Afi(s) + USB, which separates it from all other solutions, is the complete isolation of the computer, including USB interface, from the audio electronics, thus suppressing HF interference from the computer extraordinarily well. A direct reciprocal effect on the sensitive audio electronics is thus largely suppressed.

FAQ - frequently asked questions:

- Doesn't a galvanic isolator for the USB interface do the same thing as the Afi(s) + USB?

No. Galvanic isolation is a good basis for the neutralisation of interference, but it does not necessarily suffice on its own. Depending on the composition of the coupling element, the physical isolation of both sides could only be for a certain frequency area. Some higher frequencies can bleed through despite the isolation. Therefore our network isolator GISO, for example, is much more effective than the built-in transformer in network interfaces. The coupling is much less at high frequencies. Another difficulty with USB is that the USB always requires a direct current connection to the computer in order to switch the various operating modes. Therefore, a purely passive decoupling such as with GISO is not possible here. USB isolators thus have to contain a relatively high amount of logic or small microcontrollers that actively control this switch to override the isolating elements. This explains the relatively high price of USB 2.0 isolators. Moreover, the secondary side also needs its own power supply. Either this is a small switch-mode power supply that can on its own again produce interference or the operating power is drawn from the primary side. In this case, interference can bleed through just the same. In any case, there are two potential sources of interference on the secondary side: the micro-controller and the power supply.

The USB interface and its potentially disruptive micro-controller remains in the audio interface and is thus a third source of interference located directly in the housing of the audio device.

All three sources of interference are eliminated through the Afi+USB concept.

- That sounds brilliant, the playback quality is also considerably better than without Afi(s) + USB, but despite this, I still hear subtle differences between the USB cables and when the PC is optimised or in the case of external power supply of the USB module.

This can happen and it is no contradiction. But it is a sign of the electromagnetic reciprocal effect of the computer directly affecting the other components of the audio equipment. As mentioned above, every cable coming from the computer is a potential broadcast antenna. This electromagnetic radiation affects the badly isolated parts of the equipment. The computer definitely feeds HF interference back to the electrical grid. This can influence other audio electronics via the power supply. The suppression of the direct primary path towards the audio signal stream considerably reduces the entire problem with HF interference. Performing some corrective measures to the computer can also improve the final result. It is important to have a well-constructed computer optimised for EMC; be careful with homemade computers. Mains filters are recommended. And cables of the equipment should be connected cleanly with regard to shielding. Whenever possible, do not use asymmetrical connections, in the case of XLRs do not connect the shield to the housing contact and only connect pin 1 if devices will not function together otherwise. Built-in isolated jacks are useful to avoid ground loops. But at the same time, this always opens the door to HF interference. Also, absolutely avoid connecting unshielded cables to audio devices! Then you will surely have HF in your device and such phenomena are bound to happen.

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