Hi everyone
I recently made a digital subwoofer system in which all the filtering is done in the digital domain. It was designed as an accompaniment to my all-digital amplifier, and receives AES/SPDIF audio from the amp (post selection), and a serial control bus to control volume etc. The power amp part is presently analog but I plan to replace that too with a digital system in the future.
I really don't like the sound of these off-the-shelf subwoofer systems which tend to make their presence felt by slight boom on the lower male voice and so designed a serious 900-tap Brick Wall FIR Filter for my sub (and appropriate delay matching for the main amps). I find it really works well and have christened it - on suggestion from one of my younger colleagues - 'The Bass Bitch'. 😎
Has anyone out there done something like this before? If anyone is interested in this technology ...or anyone thinks it's really crazy ... let's discuss it here....
Cheers
John Hope
I recently made a digital subwoofer system in which all the filtering is done in the digital domain. It was designed as an accompaniment to my all-digital amplifier, and receives AES/SPDIF audio from the amp (post selection), and a serial control bus to control volume etc. The power amp part is presently analog but I plan to replace that too with a digital system in the future.
I really don't like the sound of these off-the-shelf subwoofer systems which tend to make their presence felt by slight boom on the lower male voice and so designed a serious 900-tap Brick Wall FIR Filter for my sub (and appropriate delay matching for the main amps). I find it really works well and have christened it - on suggestion from one of my younger colleagues - 'The Bass Bitch'. 😎
Has anyone out there done something like this before? If anyone is interested in this technology ...or anyone thinks it's really crazy ... let's discuss it here....
Cheers
John Hope
John,
There's several off-the-shelf solutions for a digital sub amp that accepts a digital input in SPDIF format, I'm not at my desk right now so don't have the url handy, but I'll get back to you on that.
But to satisfy my own curiosity, how did you manage to extract the LFE/sub-channel from the digital signal, I'd be interested to know that, as I plan to build a full digital subwoofer as well.
Regards,
Sander Sassen
http://www.hardwareanalysis.com
There's several off-the-shelf solutions for a digital sub amp that accepts a digital input in SPDIF format, I'm not at my desk right now so don't have the url handy, but I'll get back to you on that.
But to satisfy my own curiosity, how did you manage to extract the LFE/sub-channel from the digital signal, I'd be interested to know that, as I plan to build a full digital subwoofer as well.
Regards,
Sander Sassen
http://www.hardwareanalysis.com
Hi
There's a post-selection AES digital feed from the PowerDAC amp to the subwoofer box. In the PowerDAC, volume is controlled by a combination of digital attenuation and control of the power bus voltage. This means that volume information must be sent down to the sub box, which it is via a serial control link (RS232 at 38400 baud.
In the sub box, the AES digital audio for both channels is converted to I2S and passed through an AD1890 ASRC, where it is resampled to 24kHz. (BW now 10kHz or so). From there it is passed to a Motorola 56303 DSP, where the L and R channels are monofied by L/2 + R/2. The mono signal is passed through a 900-tap FIR in the same DSP which has fc set at 72Hz. The FIR is good here because of it's linear phase characteristics. Transition zone from fc to -90dB down is around 100Hz if I recall.
The LPF'd mono signal is then scaled for volume by means of the volume scaling control data sent down from the PowerDAC amp.
The output from the digital processing section is fed to an AD1853 DAC which produces an analog signal for driving a 100W analog Mosfet power amp module.
The delay in the subwoofer filter is fixed (because of the FIR) and can therefore be compensated for by an equal delay in the PowerDAC's main signal path. This ensures that the bass from the subwoofer and that from the main channels is properly in phase.
The whole project was implemented with odds and ends from the junkbox and all the DSP stuff done on a modified Vistek V1635AA tracking delay module which I just happened to have.
I'll post some pics soon
Cheers
John Hope
There's a post-selection AES digital feed from the PowerDAC amp to the subwoofer box. In the PowerDAC, volume is controlled by a combination of digital attenuation and control of the power bus voltage. This means that volume information must be sent down to the sub box, which it is via a serial control link (RS232 at 38400 baud.
In the sub box, the AES digital audio for both channels is converted to I2S and passed through an AD1890 ASRC, where it is resampled to 24kHz. (BW now 10kHz or so). From there it is passed to a Motorola 56303 DSP, where the L and R channels are monofied by L/2 + R/2. The mono signal is passed through a 900-tap FIR in the same DSP which has fc set at 72Hz. The FIR is good here because of it's linear phase characteristics. Transition zone from fc to -90dB down is around 100Hz if I recall.
The LPF'd mono signal is then scaled for volume by means of the volume scaling control data sent down from the PowerDAC amp.
The output from the digital processing section is fed to an AD1853 DAC which produces an analog signal for driving a 100W analog Mosfet power amp module.
The delay in the subwoofer filter is fixed (because of the FIR) and can therefore be compensated for by an equal delay in the PowerDAC's main signal path. This ensures that the bass from the subwoofer and that from the main channels is properly in phase.
The whole project was implemented with odds and ends from the junkbox and all the DSP stuff done on a modified Vistek V1635AA tracking delay module which I just happened to have.
I'll post some pics soon
Cheers
John Hope
More on Digital Subwoofer system
Hi again
Attached are some pics of my 'Heart Of Gold' subwoofer controller/amp. It also performs the function of video switching under control of the main amp input source selection. In short, anything and everything I couldn't fit into (or forgot to fit into) the main amp (PowerDAC 2), went in the subwoofer amp box.
As mentioned before, the sub tracks the volume from the main amp, but there is an offset gain (-16dB. . .+15dB) that can be set on the sub to adjust for room conditions/speaker efficiencies/personal tastes. The video switching commands from the main amp can be temporarily overridden by controls on the sub amp box, and there are two video 'record' outputs. These are normally under control of the main amp, but can be also be temporarily switched to different sources by controls on the sub amp box.
The DSP processing capacity is only 50% utilised, as I wanted to leave room for a SigmaDelta noise shaper on the (already heavily oversampled) LPF output data with a view to one day making an all-digital power amp module of the sort used in the main amplifier. This would then replace the present analog p/a module.
The speakers used are two downward-firing 1980's vintage Vifa 25cm woofers which are arranged in acoustical series, electrical parallel in an 80 litre cabinet which has two ports tuned (if I remember correctly) to about 26Hz. If anyone wants more details I can dig them out . . .
Adios
John Hope
Hi again
Attached are some pics of my 'Heart Of Gold' subwoofer controller/amp. It also performs the function of video switching under control of the main amp input source selection. In short, anything and everything I couldn't fit into (or forgot to fit into) the main amp (PowerDAC 2), went in the subwoofer amp box.
As mentioned before, the sub tracks the volume from the main amp, but there is an offset gain (-16dB. . .+15dB) that can be set on the sub to adjust for room conditions/speaker efficiencies/personal tastes. The video switching commands from the main amp can be temporarily overridden by controls on the sub amp box, and there are two video 'record' outputs. These are normally under control of the main amp, but can be also be temporarily switched to different sources by controls on the sub amp box.
The DSP processing capacity is only 50% utilised, as I wanted to leave room for a SigmaDelta noise shaper on the (already heavily oversampled) LPF output data with a view to one day making an all-digital power amp module of the sort used in the main amplifier. This would then replace the present analog p/a module.
The speakers used are two downward-firing 1980's vintage Vifa 25cm woofers which are arranged in acoustical series, electrical parallel in an 80 litre cabinet which has two ports tuned (if I remember correctly) to about 26Hz. If anyone wants more details I can dig them out . . .
Adios
John Hope
John,
Thanks for the informative reply, I'd be very interested in some schematics of that nifty box you've got, if you're willing to share them of course.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
Thanks for the informative reply, I'd be very interested in some schematics of that nifty box you've got, if you're willing to share them of course.
Best regards,
Sander Sassen
http://www.hardwareanalysis.com
Cool project. My mate has been messing around with digital pots and now digital processing for a couple of years now and not made great progress. I'm too entrenched in my analogue, so am waiting for something nice digital to turn up 😉
- Status
- Not open for further replies.