Control Interface Backes Müller Active Speakers
The outstandig feature of these speakers is their linear phase. This can only be achieved with built-in non-causal FIR filters.
Thanks to this internal signal processing these speaker offer the option of programming frequency response to compensate for room acoustics. The benefit of doing this in the speakers and not in the amp is that phase linearity is not compromised.
Additionally it is possible to adjust the sweet spot by adding some delay to one channel. In practice this setting has proven to have a great effect on the sound. Even a slight missadjustment destroys the impression of sound comming exactly from the midst "and not from the speakers".
The speakers have a RJ-45 plug for remote control and programming. But it's not Ethernet, its RS-232. Each speaker has a control input and a control output for daisy-chaining. Each speaker has a number: 1 for left, 2 for right. All this and much more is coded on the little SD-card on the back-side.
| Pin | Color | Function |
|---|---|---|
| 1 | blue | none |
| 2 | orange | none |
| 3 | black | GND |
| 4 | red | data to speaker |
| 5 | green | data from speaker |
| 6 | yellow | +5 V supply |
| 7 | brown | none |
| 8 | white | none |
You have to build an adapter that connects these lines to a standard SUB-D connector.
Data level is +/- 6 V = 12 Vss. Serial parameters are 115.2 kBaud, 1 start bit, 8 data bits, no parity, 1 stop bit. The data is pure ASCII with only 7 bits. Since the MSB is always 0 in this case, the serial parameters could also be 1 start bit, 7 data bits, no parity, 2 stop bits.
Each speaker accepts a query or config line and always responds with a status line.
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Query line to speaker: <speaker no> <setting>CRLF
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Config line to speaker: <speaker no> <setting> <parameters>CRLF
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Status line from speaker: <speaker no> <setting> <parameters>CRLF
"#1" for left, "#2" for right speaker. A query or config line may also start with "#0", which addresses each speaker. This will generate two status lines as response, one starting with "#1" from the left speaker, and one starting with "#2" from the right.
| setting | parameters | meaning |
|---|---|---|
| VOLUME0 | -120.0 ... 24.0 | Gain / Volume in dB. Always 0 after power on. Different values serve as balance control. |
| EQx | 1 <F> <G> <Q> | One of the 10 parametric equalizers x = 0 ... 9. F = frequency in Hz, 20 ... 20000 G = gain in dB, -12.0 ... 12.0 Q = Q factor, 0.1 ... 20.0 Usually same parameters for both speakers. |
| EQx | 2 <F> <G> | Low frequency (Bass). F = below this frequency boost/damp ramps over 3 octaves. G = final boost/damp in dB, -12.0 ... 12.0. e.g. with F = 400 Hz and G = +12 dB level is +4 dB @ 200 Hz, +8 dB @ 100 Hz and +12 dB below 50 Hz |
| EQx | 3 <F> <G> | High frequency (Treble). F = above this frequency boost/damp ramps over 3 octaves. G = final boost/damp in dB, -12.0 ... 12.0. e.g. with F = 1 kHz and G = -3 dB level is -1 dB @ 2 kHz, -2 dB @ 4 kHz and -3 dB above 8 kHz |
| DELAY0 | 0.0 ... 30.0 | If one speaker is closer to the listener than the other, its signal can be delayed as a compensation in milliseconds. Usually this value should only be applied on one of both speakers. 30 milliseconds are quivalent to 10 meters. |
| SUB0 | 0 0.0 | For additional subwoofer(s). On/Off and level. |
Usually these parameters are programmed using a "Pegel Programmier Gerät" (PPG). The PPG provides access to the first 6 parametric EQs and to delay control. The EQs 6...9 are not accessible. EQs 0...5 always work in mode 1 as parametric equalizers. Bass and Treble control are not provided. Perhaps these functions are reserved for the special pre-amp from the same firm. Maybe it does no own signal processing and leaves it to EQs 6...9, using filter mode 2 for bass control and filter mode 3 for treble. This pre-amp also uses the speakers internal volume control. This is a great advantage compared to volume control in the pre-amp because the speaker is always driven with full scale level and attenuation is done very late in the signal chain, right before each power stage. A high precicion low distortion analog attenuator is used for this. This way the full digital processing resolution is always used and noise coupling into the cables (if analog is used instead AES digital) is reduced.
While common in professional audio, this seems to be a very unique design in the home hifi world.