Wireless control center
What a wireless control center! In terms of equipment, functionality and flexibility, the Stereo Hub easily exceeds even the highest expectations. The price is EUR 700, which doesn't sound particularly unusual. But the experts at Buchardt Audio continued to think where others stopped. So let's start with WiSa compatibility: Instead of relying on its own proprietary standard, the Stereo Hub can wirelessly supply up to 8 pairs of WiSa-enabled loudspeakers with stereo Sound.
The optically unobtrusive and compact hub can be placed anywhere and impresses with its uncompromising variety of connections. All sources, including PCs, notebooks, TV sets or even turntables, can be connected. The hub is conveniently controlled via the supplied remote control. And this does not even require visual contact with the hub, since it works via radio and not via infrared.
The best way to describe the variety of functions is with the word "gigantic". So let's embark on a long (equipment) journey and start: Bluetooth 5.0 with aptX, Chromecast (Roon endpoint), DLNA UpnP, WLAN, and as a result, cooperation with almost all popular music streaming services. By the way: The hub is currently working with Apple AirPlay, compatibility with Apple AirPlay 2 will be added at a later date via an update. This makes the Stereo Hub a fully-fledged, state-of-the-art and well-equipped entertainment center for the whole house, regardless of whether it’s streaming or about multiroom applications.
Thanks to the Lossless Advanced EQ and the highly developed Buchardt calibration system, there are first-class acoustic adjustment options for every user. The calibration, which works with special algorithms, can be carried out in a few minutes. The system works with Apple iOS devices from iPhone 6 upwards. Due to the particularly good suitability of the microphone built into Apple's iPhones, Buchardt Audio limits itself to compatibility with the iPhone. Of course, you don't necessarily have to own one. Since the calibration is carried out exactly once in the respective room, it is sufficient for someone in your circle of acquaintances/friends to briefly download the app onto their iPhone and carry out the calibration once. The computing power of the iPhone is completely sufficient to record the large amount of data determined and to create the corresponding filter curves from it. A PC or notebook is therefore practically not required.
Streaming from Android, Apple or PC
The Stereo Hub supports Airplay, Chromecast (Roon endpoint), Spotify Connect, DLNA UpNP and Bluetooth 5.2 and much more. Regardless of which music streaming service is used, it is always possible to send it to the hub.
Aluminum wireless remote Control
The enclosed wireless remote control made of aluminum is easy and intuitive to use. It turns itself on automatically as soon as a connected source device is switched on. Thanks to the wireless and display technology, the hub can also be operated from another room. The user still has an overview of the selected source and the volume. The remote always displays both the volume and the selected source as soon as it detects a change.
Extensive DSP functions
It is well known that the surrounding space plays a large role in the sound of loudspeakers. In listening rooms and home cinemas, room modes and reflections in particular are noticeable with their enormous influence on the sound. In order to compensate for these influences and thus optimize the sound, the Stereo Hub includes two different DSP functions: the automatic calibration system and the manual Lossless Advanced EQ.
Room correction by the automatic calibration system
In order to carry out the calibration with the calibration system, you need access to an iPhone (model 6s or newer). Since the calibration only has to be carried out once, it is not necessary for you to own such an iPhone yourself. It is sufficient if you can borrow it once from a relative or acquaintance. The calibration only has to be repeated if the position of the loudspeakers is changed.
The calibration system can only be carried out with iPhones, since the microphones on these models have a very low tolerance in terms of quality control. Therefore, the calibration algorithm can build on known microphone properties and carry out the measurement exactly. In order to reduce the scatter of the measurement results, the calibration system carries out several thousand (!) measurements within just one minute.
Please download the app from the App Store first. After starting the calibration via the app, pink noise is played through the speakers. The iPhone displays a 60-second countdown. During these 60 seconds you should walk around the room with the iPhone in your hand and cover as much space as possible with swinging arm movements. Please keep a distance of about one meter to the loudspeakers. When the 60 seconds are up, the app will show you the frequency response of your room. This will give you a good idea of what frequency ranges the room modes are in your room. The computing power of the iPhone is then used to summarize all the collected data and calculate the perfect DSP filter. This filter is tailored exactly to your room and the position of the speakers in the room. In the app you can now easily switch between "corrected" and "uncorrected" so that you can hear the difference in the measurement directly.
The correction implementation
The mathematical models and analysis tools used to analyze the CSS (Continuous Sound Sampling) data are very extensive. The programming of the algorithm has been optimized to such an extent that these calculation steps can be carried out with the iPhone within a few seconds. Therefore, no computer or notebook is required for the calibration. After being calculated by the smartphone CPU, the data can be transferred to the DSP and executed without additional system latency.
Room correction by manual Lossless Advanced EQ
In addition to automatic calibration, the Lossless Advanced EQ also offers extensive room correction options. These far-reaching DSP functionalities can be used independently of the automatic calibration. In addition to PEQ, high-pass, low-pass, high shelf and low shelf as well as other filter types are available for this purpose. The effect of the respective filter is displayed graphically and can be adjusted very easily in this way. Finally, the filters can be saved in three different presets, which can be easily activated or deactivated in the app. In this way, absolutely individual sound profiles can be created that perfectly match your listening taste.
Room Acoustics Basics
Room modes
When music is played in a room, the sound pressure reflects off the boundaries (walls, floor, ceiling) of the room, leading to a phenomenon called room modes. These are standing waves that occur between walls, ceiling/floor or even in several of these dimensions. They are present in every room depending on the size of the room. The larger the spatial dimensions, the lower the frequency of the standing waves. The figure below shows room modes in one dimension of the room, e.g. the room width. F1 shows the 1st order room mode. This is the lowest frequency that can form a standing wave in the given spatial dimension. The dimension of this dimension corresponds exactly to half the wavelength of the frequency. The self-interfering wave creates high sound pressure levels at the walls of the room and very low sound pressure levels in the middle of the room. If you move in such a room while playing the frequency of the 1st order room mode, from the wall to the middle, the initially high volume decreases more and more. If you now move further from the middle to the next wall, the volume increases again significantly. F2 shows the 2nd order spatial mode with twice the frequency of the 1st order spatial mode in the same dimension. Accordingly, the 3rd and 4th order spatial modes are also shown in f3 and f4. Theoretically, there are infinitely many orders of space modes. Theoretically, these occur at ever higher frequencies. This will be discussed in more detail later.
With increasing frequency, the deviations in the sound pressure level when walking through the room come closer and closer together. This makes it difficult to differentiate between the individual modes. Small changes in distance then lead to large changes in the sound pressure level. At this point, the behavior of the room changes from the mode-dominated area to the so-called diffuse sound field. This means that space is no longer dominated by well-defined room modes, but by a mixture of standing waves in all directions. Typically this transition is around 250-300Hz. This crossover frequency is called the Schröder frequency and is individual for each listening room depending on its dimensions. The figure above shows on the right how the transition from the mode-dominated area (normal modes) to the diffuse sound field (diffusion) takes place at the Schröder frequency.
Reflexions
Another space-related problem is reflections. These occur when a speaker is placed near a wall. Most loudspeakers are tuned more or less linearly ex works. However, this coordination usually takes place in a situation without walls, the so-called "free field". However, if there is a wall behind a speaker, for example, sound that hits that wall will be reflected and bounced back to the listener. This effect causes two audible artifacts: bass boost and cancellation.
High frequency components are primarily radiated forwards from the speaker, while deep bass is radiated spherically, i.e. all around the speaker. Therefore, most of the energy that travels from the speaker to the rear wall is in the low frequencies. These are reflected off the wall and thrown back at the listener, while high frequencies are not. This results in an overemphasis of the speaker's low-frequency spectrum when placed closer to the wall. Placing it in the corner further reinforces this effect.
In addition, the bass components reflected on the wall reach the listener with a delay due to the additional path. They are superimposed on the original signal, which leads to bass peaks or extinctions at some frequencies. As a result, certain tones no longer reach the listener in the way they were originally reproduced by the loudspeaker. The result is discolouration and falsifications in the sound image.
Continuous Soundfield Sampling, CSS
The calibration system takes into account all the effects described above and is able to compensate for them. Each room mode can be precisely identified thanks to the spatial measurement. This then enables a filter to be calculated which eliminates these room modes. The placement of the loudspeakers and the subwoofer in the room and the distances to the walls, floor and ceiling are also identified and then corrected. The method behind the measurement is called Continuous Soundfield Sampling (CSS). It is a continuous collection of measurement data, which continuously generates several thousand data sets during calibration and then evaluates them. The advantage of this method is that significantly more data and information about the room can be collected and taken into account than when only discrete point measurements are made. With discrete point measurements, on which most calibration systems are based, the cause of abnormalities in the measurement is completely unknown. The risk of making corrections on this basis is therefore very high.
Since the effects described only have an effect at frequencies below the Schröder frequency, the calibration system only corrects the sound below this frequency. However, data in the high-frequency range is also recorded. This data is used to match the bass response to the treble response. Otherwise, it would be difficult to identify the characteristics of the loudspeakers to be measured due to the sometimes significant room influences in the low-frequency range. The high-frequency range data is used to perfectly match the two ranges.
