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#11
News Updates / Group-delay correction of a cr...
Last post by Tranquility Bass - January 18, 2025, 10:50:09 AMThere is a school of thought that says the only way to accomplish a linear phase crossover is to use linear phase filters which are a special class of FIR filter that sums to a unity response with a fixed time delay. So when both high pass and low pass sections are added together the response is a delayed version of what is being fed into the crossover. As I have discussed in another thread this type of filter has issues with proper impulse response cancellation due to imperfect matching between drivers which can result in severe pre-ringing leakage among other topics. The proponents of these filters will never talk about this aspect of this type of filter topology, instead pretending that it doesn't exist or can easily be minimized so that it is never a problem.
The other way of linearizing the crossover is to take an existing non-minimum phase crossover and apply a global group-delay correction filter which is just an FIR filter with a large number of taps. The advantage here is there are no FIR filters in the actual crossover to generate any pre-ringing artifacts. This is the approach taken by a few commercial designs as well as here, although the correction filter usually requires a large number of taps and has a large delay depending on the sampling rate and tap count. So this should be taken into account if you want to use these speakers with other speakers or in conjunction with a video display in say a home theater system in order to avoid lip-syncing issues.
We took a four-way 8th-order Linkwitz-Riley crossover from one of our clients who own one of our Preamps and applied a global group-delay correction filter to linearize the phase response. The LR crossover was already flat but the phase response was anything but linear so a square wave fed into it did not look like a square wave coming out of it. A linear phase response means that there is just a time delay in the signal path whilst still maintaining a flat magnitude response but there is no waveform distortion.
To measure the phase distortion of the crossover in isolation we needed to sum the outputs of the crossover before they were level adjusted and time delayed so we could focus on the crossover alone without the effects of the speaker and the room etc which we know adds its own artifacts to the response. Later on, we will look at compensating for those as well but for now we are just focussing on fixing up the crossover and making sure the group-delay correction filter is doing its job properly.
To evaluate the crossover we just looked at one channel and summed the outputs to measure the net frequency response. We also fed the input to the crossover to one of the outputs of the preamp as a reference for the analyzer to create the frequency response. The summed outputs of the crossover are fed to another channel on the preamp. We also added a few multiplexers or selector switches to enable us to switch in and out various filters quickly so we can quickly evaluate the effects of different filter configurations. Both the input to the crossover and the summed outputs are then connected to our dScope III audio analyzer. The dScope III can measure both amplitude and interchannel phase. The following Audioweaver test bench is what we used to evaluate the effectiveness of the global correction filters.
All of the DSP is done on a PC instead of the Preamp itself because the group-delay correction filters required are beyond the resources of the onboard SHARC DSP. The Ultimate Preamplifier Plus (UPP) has a unique feature in that it allows one to run DSP on an external device such as a PC as though it was running inside the Preamp. In fact, in another thread, we proved that the audio path was transparent and the performance of the Preamp was unaffected by the noisy environment of a PC ! The Ultimate Preamplifier Plus can also share DSP between its own onboard DSP as well as an external PC so for example you could build a non-minimum phase LR crossover using the onboard SHARC DSP whilst running the correction filter on an external PC and then switch the correction filter in or out from the Preamp menu. This is the approach our client is taking but for proof of concept and convenience, we ran everything on an external PC which allowed us to quickly evaluate and test everything in the one environment. The main aim was to validate the effectiveness of the global group-delay correction filters and prove that you don't need to use dedicated linear-phase filters to achieve this.
First, we measured the frequency and phase response of the crossover filter alone using a log frequency axis. We note the flatness of the filter as expected but the phase is anything but flat ! The top blue trace is the sweep of the input which is ruler flat as expected and the bottom red trace is the crossover summation of all drivers which is also ruler flat as expected.
To evaluate the phase response we reran the frequency response test using a linear frequency axis instead of the usual log axis. If the phase is linear then the phase response should look straight and not curved. Since the analyzer cannot distinguish phase greater or less than 180 and -180 degrees respectively it wraps or folds the phase so the response always is contained within a 360 degree envelope which makes it look like a saw-tooth. From the phase plot below we can see the phase is anything but linear which was expected !
And now for the money shot. What does this crossover filter do to a square wave? The square wave is important because it is not just a single tone but rather a series of odd harmonically related tones with a monotonically decreasing magnitude. If the phase is not linear, it will manifest itself as distortion in the waveform even though the magnitudes of the harmonics are still correct. So, let's look at the test results on a scope.
Firstly at 1kHz and then at 100 Hz !! This is what a non-minimum phase crossover does to a square wave !! Can we fix this ??
The other way of linearizing the crossover is to take an existing non-minimum phase crossover and apply a global group-delay correction filter which is just an FIR filter with a large number of taps. The advantage here is there are no FIR filters in the actual crossover to generate any pre-ringing artifacts. This is the approach taken by a few commercial designs as well as here, although the correction filter usually requires a large number of taps and has a large delay depending on the sampling rate and tap count. So this should be taken into account if you want to use these speakers with other speakers or in conjunction with a video display in say a home theater system in order to avoid lip-syncing issues.
We took a four-way 8th-order Linkwitz-Riley crossover from one of our clients who own one of our Preamps and applied a global group-delay correction filter to linearize the phase response. The LR crossover was already flat but the phase response was anything but linear so a square wave fed into it did not look like a square wave coming out of it. A linear phase response means that there is just a time delay in the signal path whilst still maintaining a flat magnitude response but there is no waveform distortion.
To measure the phase distortion of the crossover in isolation we needed to sum the outputs of the crossover before they were level adjusted and time delayed so we could focus on the crossover alone without the effects of the speaker and the room etc which we know adds its own artifacts to the response. Later on, we will look at compensating for those as well but for now we are just focussing on fixing up the crossover and making sure the group-delay correction filter is doing its job properly.
To evaluate the crossover we just looked at one channel and summed the outputs to measure the net frequency response. We also fed the input to the crossover to one of the outputs of the preamp as a reference for the analyzer to create the frequency response. The summed outputs of the crossover are fed to another channel on the preamp. We also added a few multiplexers or selector switches to enable us to switch in and out various filters quickly so we can quickly evaluate the effects of different filter configurations. Both the input to the crossover and the summed outputs are then connected to our dScope III audio analyzer. The dScope III can measure both amplitude and interchannel phase. The following Audioweaver test bench is what we used to evaluate the effectiveness of the global correction filters.
All of the DSP is done on a PC instead of the Preamp itself because the group-delay correction filters required are beyond the resources of the onboard SHARC DSP. The Ultimate Preamplifier Plus (UPP) has a unique feature in that it allows one to run DSP on an external device such as a PC as though it was running inside the Preamp. In fact, in another thread, we proved that the audio path was transparent and the performance of the Preamp was unaffected by the noisy environment of a PC ! The Ultimate Preamplifier Plus can also share DSP between its own onboard DSP as well as an external PC so for example you could build a non-minimum phase LR crossover using the onboard SHARC DSP whilst running the correction filter on an external PC and then switch the correction filter in or out from the Preamp menu. This is the approach our client is taking but for proof of concept and convenience, we ran everything on an external PC which allowed us to quickly evaluate and test everything in the one environment. The main aim was to validate the effectiveness of the global group-delay correction filters and prove that you don't need to use dedicated linear-phase filters to achieve this.
First, we measured the frequency and phase response of the crossover filter alone using a log frequency axis. We note the flatness of the filter as expected but the phase is anything but flat ! The top blue trace is the sweep of the input which is ruler flat as expected and the bottom red trace is the crossover summation of all drivers which is also ruler flat as expected.
To evaluate the phase response we reran the frequency response test using a linear frequency axis instead of the usual log axis. If the phase is linear then the phase response should look straight and not curved. Since the analyzer cannot distinguish phase greater or less than 180 and -180 degrees respectively it wraps or folds the phase so the response always is contained within a 360 degree envelope which makes it look like a saw-tooth. From the phase plot below we can see the phase is anything but linear which was expected !
And now for the money shot. What does this crossover filter do to a square wave? The square wave is important because it is not just a single tone but rather a series of odd harmonically related tones with a monotonically decreasing magnitude. If the phase is not linear, it will manifest itself as distortion in the waveform even though the magnitudes of the harmonics are still correct. So, let's look at the test results on a scope.
Firstly at 1kHz and then at 100 Hz !! This is what a non-minimum phase crossover does to a square wave !! Can we fix this ??
#12
News Updates / Re: On the drawing board is ou...
Last post by Tranquility Bass - January 14, 2025, 03:17:40 PMAfter version 6.17 of Audioweaver DSP concepts changed the base support package so you could no longer integrate your own code with the Audioweaver code so from thereon the Audioweaver code has exclusive control of the DSP and you talk to it through another host micro. Because of that, you have to write special routines in the Audioweaver base support package just to do simple things like change the master volume etc. As I do a large amount of housekeeping functions on the SHARC DSP this is not practical nor do I have a host micro on the board. Having said that, you can still leverage the latest version of Audioweaver by running it on a PC and use the audio-conduit functionality offered by the Ultimate-Preamp Plus and this will not impact on the sound quality at all whilst expanding the DSP functionality. If you don't want to use a PC or don't need to use Audioweaver then you could use something like a Raspberry Pi-5 which uses a Broadcom BCM2712 with 4-cores based on an ARM-Cortex A76 with plenty of Level 1/2/3 cache and really cheap too at $133 AUD a piece the last time I checked 
And I just checked there is a version of Audioweaver that runs on a Rasberry Pi. Not sure if this will interface with the UPP but it's a possibility
https://dspconcepts.com/forums/audio-weaver-general/362-does-dsp-concepts-have-runtime-raspberry-pi-3
And I just checked there is a version of Audioweaver that runs on a Rasberry Pi. Not sure if this will interface with the UPP but it's a possibility
https://dspconcepts.com/forums/audio-weaver-general/362-does-dsp-concepts-have-runtime-raspberry-pi-3
QuoteDoes DSP Concepts have a runtime for the Raspberry Pi 3 ?
Yes, we offer two different products that can run on the Raspberry Pi 3.
1. AWE_command_line - This is the Audio Weaver Server command line application, which relies on ALSA to handle audio inputs and outputs. On the Raspberry Pi 3, to handle real-time audio we usually use a USB sound card (something like the "Plugable USB Audio Adapter"). Usage of AWE_command_line can be seen in our 'Linux Integration Guide' here: https://dspconcepts.com/support
2. AWELib - Delivered as a set of libraries with example/reference integrations, AWELib provides an API to create and tune your audio processing system and is easily integrated into other applications. With AWELib, it is up to the user to implement the audio inputs and outputs.
Both AWE_command_line and AWELib running on your Pi can connect directly to Designer on your PC for testing and tuning purposes. Alternatively, user created scripts or applications can be used to interact with and tune your Audio Weaver driven systems.
If you're interested in evaluating one or both of these products, please email info@dspconcepts.com to begin an engagement.
Thanks
-Axel
#13
News Updates / Re: On the drawing board is ou...
Last post by hoschi - January 14, 2025, 06:02:51 AMWith the right firmware the amanero should be able to do 512 i think.
Not to be stuck on the 6.17 Version would be great!
I upgraded to the amanero768, better clock and better Isolation. Could even run dsd2048, but don't think that will be a needed option😃
A later sharc model would allow higher sample rates like 384 or 767 pcm. I think that could be a great improvement. But yeah, is it worth a complete redesign, I can't answer that.
Your Measurements look great for sure!
Not to be stuck on the 6.17 Version would be great!
I upgraded to the amanero768, better clock and better Isolation. Could even run dsd2048, but don't think that will be a needed option😃
A later sharc model would allow higher sample rates like 384 or 767 pcm. I think that could be a great improvement. But yeah, is it worth a complete redesign, I can't answer that.
Your Measurements look great for sure!
#14
News Updates / Hard truth: most vinyl is digi...
Last post by Tranquility Bass - January 09, 2025, 01:25:38 PMHate to break the bad news !!
#15
News Updates / Vinyl sound quality myth destr...
Last post by Tranquility Bass - January 05, 2025, 06:48:27 PMNot as simple as the more expensive turntable the better !!
#16
News Updates / Re: On the drawing board is ou...
Last post by Tranquility Bass - January 05, 2025, 12:44:45 PMQuote from: hoschi on January 05, 2025, 11:40:47 AMAny update on the Board?
Upgrade to all latest chips would be amazing.
ES9039PRO
ES9842PRO
Latest sharc dsp.
Dsd 512 or 1024 capability. That would be amazing😄
From what I can see from the datasheets the only thing that the 9039 offers over the 9038PRO is support for MQA but that is now dead in the water so I wouldn't want to change that part. The Rev:D board includes the new ADC but I am still using the Rev:C board for prototyping. I'm not sure if I would want to upgrade to a newer SHARC part because that means a complete board re-design using a BGA part and migration to a new development platform and for the additional performance it is probably not worth the effort. Also I don't think I can easily integrate the newer versions of Audioweaver as they changed their format after 6.17 which relies on the base support package running exclusively on the SHARC DSP so the necessity for another slave micro just for house-keeping. This means you have to write special custom functions for Audioweaver just to control things like DAC volume and input source selection etc and with all of the options that are on the existing Preamp this would become unwieldy very quickly
From a DSP perspective and given what I have been able to achieve on the UPP using an external PC it would probably be more prudent to offer an upgrade to existing UP2 owners for this capability rather than redesign a new board when the existing Rev:C board works so well
Not only that you get to use the latest version of Audioweaver on the PC whilst the SHARC DSP is still locked into using the earlier version of Audioweaver. The DSD limit of DSD-256 was limited by the master clock from the Amanero USB audio interface device.https://analog-precision.com/forum/performance-and-measurements/ultimate-preamp-vs-deqx/msg937/#msg937
#17
News Updates / Re: On the drawing board is ou...
Last post by hoschi - January 05, 2025, 11:40:47 AMAny update on the Board?
Upgrade to all latest chips would be amazing.
ES9039PRO
ES9842PRO
Latest sharc dsp.
Dsd 512 or 1024 capability. That would be amazing😄
Upgrade to all latest chips would be amazing.
ES9039PRO
ES9842PRO
Latest sharc dsp.
Dsd 512 or 1024 capability. That would be amazing😄
#18
News Updates / A PERSONAL PERSPECTIVE ON THE ...
Last post by Tranquility Bass - November 13, 2024, 01:29:13 PMA great read for anyone interested or skeptical of class-D amplifiers
LIFE ON THE EDGE – A PERSONAL PERSPECTIVE ON THE PAST, PRESENT, AND FUTURE OF CLASS D AUDIO AMPLIFIERS
BRUNO PUTZEYS OCTOBER 22, 2024 30TH ANNIVERSARY THE NOW
LIFE ON THE EDGE – A PERSONAL PERSPECTIVE ON THE PAST, PRESENT, AND FUTURE OF CLASS D AUDIO AMPLIFIERS
BRUNO PUTZEYS OCTOBER 22, 2024 30TH ANNIVERSARY THE NOW
#19
News Updates / Latest Firmware Update - 24080...
Last post by Tranquility Bass - August 05, 2024, 07:32:50 PMPlease click on the following link for the latest firmware updates for the UP1,UP2 and UPP. (You need to be a registered member of this forum to access this page.)
https://analog-precision.com/forum/firmware-updates/firmware-update-24/
https://analog-precision.com/forum/firmware-updates/firmware-update-24/
#20
News Updates / Latest Firmware Update - 24052...
Last post by Tranquility Bass - May 28, 2024, 09:49:56 PMPlease click on the following link for the latest firmware updates for the UP1,UP2 and UPP. (You need to be a registered member of this forum to access this page.)
https://analog-precision.com/forum/firmware-updates/current-firmware-update/
https://analog-precision.com/forum/firmware-updates/current-firmware-update/