ALLCONTROL DSP-428II DSP Audio Processor Instruction Manual

DSP-428II DSP Audio Processor

“`html

Specifications:

• Model: DSP-428II

• Functions: FIR and RTA

• Measurement Options: 1/3 Octave, 1/2 to 1 Octave, Moving
  Average, Smoothing

• Weighted Result: Available

• Measurement Saving: Yes

• Full Screen Mode: Supported

Product Usage Instructions:

Using the RTA Function:

1. Enable the RTA function.
2. To start the measurement, click on the Play button.
3. To stop measuring, click on Pause.

Measurement Options:

1. Click on the Settings button to access measurement
   options.

2. Adjust settings for Octave, Moving Average, and Smoothing based
   on your requirements.

3. Ensure microphone is unmuted before each measurement.

Saving and Combining Multiple Measurements:

1. After making a measurement, analyze the curve for
   plausibility.

2. Click the Save button to save the measurement.

3. Repeat measurements in different locations to account for room
   interference.

4. Combine multiple measurements for weighted results.

Full Screen Mode:

1. To view more details, click on the Full Screen button.

FAQ:

Q: How many measurements can be saved?

A: The product allows saving multiple measurements and provides
options to combine them for analysis.

Q: What is the recommended setting for Octave adjustment?

A: It is recommended to set the Octave to 1/3 for accurate
measurements.

Q: How can I improve measurement accuracy?

A: Increase the Moving Average value for more accurate
measurements, and consider adjusting the Smoothing option as
needed.

“`

Application Note
DSP-428II: Using FIR and RTA
Introduction
The ALLCONTROL Software provides FIR calculation functions for hardware that supports FIR filters, and an RTA (Real Time Analyzer) option that can be used in combination with the FIR filter, but is independent from the hardware. This document describes the use of both options. It is assumed that you know what a FIR filter is, what the difference between IIR and FIR filters is and that you understand the concepts of truncating, convolution, group delay and related topics, as well as transfer function, power compression, directivity etc.
Using the RTA function
The RTA enables you to visualize the spectrum of the audio signal. By using this to measure the output of the loudspeaker to replay white noise or pink noise or 1KHz sine wave, you can find the transfer curve of the loudspeaker. Although other applications exist, we will focus on this application. To measure a transfer curve, select a signal from input channel’s signal selection to play white noise or pink noise and assign those channel into your specific output channel by AllControl software and adjust the volume to a reasonable level.There is no need for high SPL measurements unless you want to test power compression. Now go into the channel you want to adjust. For a 2-way system this may be the HF or LF channel, or it may be the input channel to adjust the overall transfer function. If you plan to use the RTA measurement to control a FIR filter, choose a channel that supports FIR. STARTING A MEASUREMENT First you need to enable the RTA function. To do so, click the RTA icon in the graphical window.
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
The RTA function will be enabled. To start the measurement, click Play:
This will start the RTA with the default settings. A measured response will be shown: Click Pause to stop measuring:
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
MEASUREMENT OPTIONS Now that you have made a first measurement, let’s have a look at the measurement options. Click the Settings button:
A window will open with the measurement options:
· Smoothing: Choose the frequency smoothing. For manual measurements and settings, 1/6th
or 1/3 Octave is recommended. For automatic FIR adjustment, this may lead to unexpected results due to room interference; in this case, a setting of 1/2 to 1 Oct is recommended.
· Avg: Choose the number of measurements that are averaged. The averaging function is a
moving average over Avg samples. For fast indications, 1 to 5 is OK; for accurate measurements, you may want to increase this value to 10 or more.
· FFT size: The size (length) of the sampled data. With a size of 4k, lower frequencyes are not
accurately measured, but the measurement is fast. Larger size leads to more accuracy in the low region, but longer measurement times. To see the accuracy, try setting the smoothing to “Off”; then you will see the actual data points.
· One Shot: When selected, one reading consisting of “Avg” measurements will be taken.
When deselected, the measurement is continuous.
· Auto Mute / Unmute: When selected, the software will automatically mute the hardware at
the end of each measurement, and unmute it before the next measurement. Use this in combination with the One Shot setting to go easy on your neighbours.
· Input: Select the source fo the measurement. A proper measurement microphone is
recommended; the measurement is only as good as the microphone.
· Auto Scale: Manual scale is not supported yet. Please leave this check box selected.
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
· Show Filters: When deselected, the filter (PEQ, HPF, LPF, FIR) response curves will not be
drawn. Below the options, a list of saved measurements appears. When no measurements have been saved yet, only the default curves are listed:
· Last Measurement: This is the as-yet-not-saved measurement. Select if it is visible or not by
clicking “Visible”; select the colour by clicking “Colour”.
· Weighted Result: When saving multiple measurements, these can be combined into one
Weighted Result. Select visibility and colour here. Feel free to play around; it will not explode.
SAVING AND COMBINING MULTIPLE MEASUREMENTS After making a measurement, look at the curve and see if it is plausible. Measurements can be distorted by nearby objects (walls, floor, people, bookshelves). Try measurements in different places to see what happens. When you are satisfied with a measurement, click the Save button.
One more line is added to the measurements. You can set some options:
· Enabled: When deselected, this measurement will not be visible and will not be taken into
account for the weighted result.
· Name: You can set the name for each measurement. · Weight: The weight of each measurement determines the relative importance of each
measurement when calculating the weighted result. For instance: When you have one measurement with weight 2 and another with weight 1, the first measurement will be twice as influential in the result. Choose any number other than zero.
· From: The bottom end of the frequency range where this measurement is used. More on this
on the next page.
· To: The top end of the frequency range where this measurement is used. More on this on the
next page.
· Visible: When deselected, the measurement will not be visible, but will still be taken into
account for the weighted result.
· Colour: Select the colour of the displayed curve plot.
Normally, one would make several measurements, e.g. for different listening positions to take the room into account, or from different angles w.r.t. the loudspeaker to take the directivity of the loudspeaker into account. I will not go into the details of why and how to measure for optimal results; that is not the scope of this document, and there is plenty of information on that in the Internet.
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
After saving several measurements, the Settings window looks like this:
And the graphical window like this:
Now you can combine these measurements into one averaged result. In the image above, measurements 2 and 4 have a greater weight than the other two; these would be e.g. on-axis results, or my favourite listening position. Now let’s assume that measurement 4 (the blue one, also the last one, so shown light grey here) was done close to the woofer, and I think this data is very useful for the low end up to 300Hz, but irrelevant for the rest of the frequency spectrum; and the other measurements are relevant for 300Hz and higher. I can se the “From” and “To” fields like this:
I also set the Last Measurement to invisible so as not to be in the way, since I am done measuring for now.
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
The graphical window will look like this:
For more detail, I will go to Full Screen Mode. Click the Full Screen button:
The graphical window will fill the size of the unit panel. Click the same button again to use the whole display area for maximum detail.
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
You can play around with different frequency and weight settings to reach the desired weighted result. One more example: Settings: And the result:
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
Using FIR filters
There are 3 ways to define a FIR filter in ALLCONTROL: Load from a file, create by using PEQs and crossovers to draw a target response, and by inverting the RTA measurement. When a unit is connected, the filter that is loaded will be shown on the display as a white line. In this example, no FIR filter has been loaded yet. Let’s change that, shall we? CREATING A FILTER FROM PARAMETRIC EQUALISERS First, set some PEQs to create a target response, then click “CALC” in the FIR corner:
A pop-up window will appear with some options:
· Number of taps: You can select how many taps of the FIR
filter are used. More taps means more accuracy in the low end, but also means that the filter will cause more latency. The filters we generate are all linear phase with a symmetrical impulse response, and the latency is therefore equal to half of the filter length.
· Currently active filter: You can add to the filter that is
currently active in the hardware, or you can start from scratch by deselecting this option.
· HPF, LPF and PEQs: You can choose which filter will be
converted to FIR and which will be ignored. It is common practice to use IIR for low frequencies and FIR for higher frequencies to keep the latency down to a minimum.
· Track changes: The FIR calculation will be updated
automatically as you change PEQs.
· Invert RTA measurement: We will come to that later. · Data from File: We will come to that later.
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
Select 256 taps (the maximum number for the hardware that is used for this example), deselect “Currently Active FIR filter” and click OK. In the graphical screen you will see a dotted red line indicating the calculated FIR response. As you can see in the screenshot above, it is perfect as far as the display resolution allows us to see. The red dotted line is the calculated response from the FIR coefficients; so any deviations will be visible here. This becomes clear when we try to use this FIR filter to change the low frequency response:
As you can see, the deviation below 200Hz is quite serious. In other words: A 256 tap FIR filter is useless in this frequency region. Which is to be expected, it is caused by the “finite” in the name FIR. You can use the red dotted line to see the result of the approximation.
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
INVERTING THE RTA MEASUREMENT We can also create a filter by inverting the RTA measurement. Using the measurement we did in the previous chapter, the result looks like this:
As you can see, it only works above appr. 300Hz. Below that, the response is automatically reduced, taking the amount of taps into account. The red dotted line again shows the expected response.
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
LOADING A FILTER FROM A FILE Click CALC, then “Load File”. Select your file. If the number of taps in the file fit in the FIR filter, it will be loaded automatically. If the number of taps exceeds the available filter size, the filter will be truncated. ALLCONTROL supports 24 and 32 bit fixed point files, and floating point files. Any lines that do not contain a number (such as comments) will be ignored. You can generate FIR files with many 3rd party programs, or even online. Here is a 1k HPF, generated with ScopeFIR:
Again, the dotted red line indicates the calculated frequency response. SENDING A FILTER TO THE HARDWARE When you are satisfied with the response indicated by the red dotted line, click APPLY to send the filter to the hardware. A window will appear with two options:
· Reset converted IIR filters: When selected, the
software will automatically disable the IIR filters that were converted to FIR.
· Do not show this dialog again: Speaks for itself,
doesn’t it? The communication can take some time to complete, depending on filter size and connection speed.
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
If no other filters are active, the response will be shown as a white line:
When you add PEQs now, you will see the white line will follow the PEQs, as well as the red dotted line; the red dotted line is the newly calculated FIR filter, following the changes you are doing with PEQs, and the white line is the overall frequency response. If some PEQs are not taken into account in the FIR calculation, the white line will include them (because that is the overall frequency response) but the red dotted line will not. There will also be a solid red line, which is the response of the FIR filter that is currently loaded in the hardware:
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

DSP-428II: Using FIR and RTA
SAVING FILES
Click the disk icon next to the FIR block to save a filter as a text file. The filter that is loaded in the hardware will be saved to disk, and can be loaded into other channels or saved for later use. Here is the FIR file of this example (omitting most coefficients):
FIR filter generated by ALLDSP ALLCONTROL software. Sample rate: 48828Hz, 255taps -0.00251117721317 0.000136785209243 0.000140666030413 0.000148858875106 0.000161494128481 0.000177634880029 0.000197263434621 0.000219509005649 0.00024409592163 0.000270059332377 0.000297261402149 0.00032448116705 0.000351532362565 0.000377209857282 0.000401332974621 0.000422636978525 0.000440970994737 0.000454998575363 0.000464653596498 0.000468616374055 0.000467024743774 0.000458568334793 0.000443564728109 0.00042071472873 0.00039056222904 0.000351847149596 0.000305717811131 0.000250644981978 0.000188274309127 0.000117114745135 3.95923853105e-05 -4.697032275e-05 -0.000135075300995 -0.000237385742477 -0.000340500846664 -0.000444556586651 -0.000553787686189 -0.000663964078139 -0.000775675289694 -0.000885844231064 -0.000994163565801 -0.00109791569463 -0.00119658745881
Conclusion
We believe that these functions provide you with powerful tools to use FIR filters and RTA measurements conveniently. Development is ongoing (as always), so functionality will be added in the future. The examples in this document were made with software version 3.8.23 build 117010. Earlier software versions will have more limited functionality. Please always upgrade to the latest version as found on https://www.alldsp.com/software.html
AllDSP GmbH & Co. KG · Küferstr.18, 59067 Hamm, Germany · +49 (0) 23 81 3 73 06 29 · www.alldsp.com

References

• Audio Technology Made in Germany | ALLDSP
• Audio Technology Made in Germany | ALLDSP

Read User Manual Online (PDF format)

Read User Manual Online (PDF format)  >>

Download This Manual (PDF format)

Download this manual  >>