What is a network bridge and how does it work?

A network bridge in its purest form is essentially a digital audio connection away from the computer that connects to this computer over a network. Let’s have a closer look. Basically there are two ways to play music from the internet or from your computer over your stereo: connect your computer directly to a digital input on your stereo or use a network player. The problem with connecting a computer to audio equipment is twofold: the clock signal that sends out the digital music data usually is of low quality and the ground plane carries a lot of electrical noise. These two can cause severe distortion during the digital to analog conversion in your stereo. It is possible, applying a number of tweaks, to reduce both artefacts by using an audiophile power supply, using an audiophile USB on PCI card that uses an external audiophile power supply and so on.

01:08 - The alternative is to use an external interface that is built to audiophile specifications and connect that to the computer over an asynchronous connection. There are two ways of sending digital audio from one device to another: Isochronous and asynchronous. SPDIF, AES/EBU and TOSlink are isochronous, meaning that the sending device - the computer in this case - sends a steady stream of bits to the receiving device - in this case the DAC. The DAC has to follow the incoming stream of data, meaning that irregularities in the stream due to a poor performing oscillator in the source is also followed by the DAC resulting in jitter. Asynchronous connections don’t use a fixed speed.

01:54 - The source sends packets of data and the receiving devices stores the data in a buffer. When the buffer is full, the receiving device signals the sending device to stop sending until there is space again to receiving data. It then signals the sending device to send new data. In other words, the clock timing of the sending device - the computer - has become irrelevant after the data has been received by the receiving device. Two asynchronous connections are used for audio: USB and ethernet.

02:30 - USB knows many modes and two modes are used for audio. Audio Profile 1 uses isochronous data transport and Audio Profile 2 uses asynchronous data transport. It might be clear mode 2 should be used for high quality audio. Unfortunately all USB connections need to have a 5 volt DC line too, to activate the USB port on the receiving side and this voltage can - and almost always will - cary electric noise from the computer. Some DACs powers this function of the USB interface chip internally but this means that the computer will not recognise the DAC until it is switched on.

03:13 - The network connection doesn’t cary any other electric signal than needed for the bits to be transported. And network connections, true to IEEE 802 specifications, should have galvanic separation through small transformers. Still small anomalies occur through common mode signals. The network bridge is a device that is connected to the computer over the network. And since ethernet works asynchronous, the computer’s clock signal quality has become irrelevant.

03:50 - Music player software that is enabled to use network bridges sends the music data not to the SPDIF or USB output of the computer but to the network bridge using packets of data. The network bridge receives these packets, stores them in a buffer and plays them out against its own clock. The quality of this clock forms the basis of the quality of the digital signal sent to the DAC. But there are other factors too. I have spoken of a digital signal where I should have used the term ‘analog signal representing bits’. For what we normally call a digital signal is an analog square wave.

04:30 - And not even a perfect analog square wave since such a square wave requires an unlimited bandwidth. Even under the most ideal conditions this is not obtainable while real life conditions are far less optimal. In my video “Connecting your DAC #2: how digital can go wrong” I describe how that works for SPDIF signals but the principle is the same for all digital signals. So how can a network bridge do that better than the computer? To start with, a computer is not built for high-end audio application as where a network bridge is. A computer is a powerful machine using one or more powerful processors that will generate a lot of electric noise, polluting the ground plane.

05:22 - And the cheap crystal clock oscillators generate a lot of phase noise that causes jitter in the digital to analog conversion. A well designed network bridge basically is a computer too, but only as powerful as needed for its task. Therefore it demands far less power making the use of an audiophile, low-noise power supply affordable. Furthermore the electronics used for generating the digital signal to be sent to the DAC will be designed with audiophile demands in mind. What software can use network bridges. Most network bridges accept several protocols.

06:04 - For instance the Squeezebox protocol can be used. It needs the installation of the free Logitech Media Server program - LMS for short - on a computer or NAS. Versions for Windows, MacOS, Linux and the popular NASses are available. The music is chosen on the computer directly, on another computer - like a laptop - or using an app on a mobile device. Another popular protocol is DLNA and it’s twin brother UPnP AV. In practice it works roughly the same as the Squeezebox format. You install a DLNA or UPnP AV server program on a computer or NAS and select the music to play using a computer or mobile device. There are many server programs available for any OS and a great deal of them are free. Nowadays audiophile music players like JRiver Media Center and Audrivana 3+ support DLNA network bridges, which they call network renderers. Roon has its own protocol, named RAAT and many network bridges support it under the label Roon Ready.

07:12 - A network bridge that is Roon Ready is often called Roon Endpoint. HQPlayer, a player that is specialised in audio processing, has a proprietary protocol too and names network bridges Network Audio Adaptors, NAA for short. Apple Airplay can also be used as protocol, although it is limited to a maximum sampling frequency of 48 kHz. The basic idea of a network bridge is ‘a digital output at distance’. Most bridges use USB Audio Protocol 2, like the SOtM sMS-200 and the Sonore Rendu series.

07:53 - Some offer SPDIF and TOSlink too, like the Lindemann Bridge. But more and more other devices integrate the network bridge function into their products. Many network players can also function as network bridge. For instance all Bluesound products can function as Roon Endpoint as can the Auralic Aries series of streamers and many others. Then there are streaming DAC’s like the Mytek Brooklyn Bridge that are DAC’s with a network bridge integrated.

08:25 - There also are amplifiers and AV receivers with network bridge built- in like the Advance Paris Playstream A7 I reviewed recently and many AV receivers by Arcam, Denon, Marantz, Yamaha and others. Those that like the Raspberry Pi as basis for their audio player can also use it as a network bridge or streaming DAC. There is a large choice of sound cards and there is software for about any protocol you can think of. Using a good network bridge will result in a clearly better audio quality, compared to a computer directly connected to the DAC, with perhaps the exception of costly audiophile computers. The analog quality of the digital signal is - within broad limits - of no consequence as long as the signal remains digital but is of great importance during the digital to analog conversion.

09:25 - I will soon get back on this in a separate video. So the use of costly femtosecond precision crystal oscillators, clean ground planes, ultrafast flip-flops and very good circuit board design all lead to a cleaner digital signal and thus a better functioning DAC, resulting in a better sound. Also watch my video “An introduction to playing high quality music files”. It will be no surprise that taking care of the digital signal this way does cost money. Therefore the best network bridges will cost significantly more than a simple setup using a Raspberry Pi.

10:05 - But compared to the digital audio output of a normal computer - being it SPDIF, TOSlink or USB Audio 2 - any decent network bridge sounds better. The same goes for network streamers from specialist firms too. That’s it for this video with. There will be a new video next week, if all goes well at Friday 5 pm central European time. Since you don’t want to miss that, subscribe to this channel or follow me on the social media so you will be informed when new videos are out. If you liked this video, give it a thumbs up.

10:40 - Many thanks to those viewers that support this channel financially, it keeps me independent and thus trustworthy. If you like to support my work too, the links are in the comments below this video on Youtube. I am Hans Beekhuyzen, thank you for watching and see you in the next show or on theHBproject.com. And whatever you do, enjoy the music. And keep safe. .