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Old 28th July 2013, 06:08 AM   #21
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With the user interface more or less worked out on the Arduino, I decided to build up a test circuit for my input switcher on a breadboard. Even though it is a bit of a rats nest, it works - at least I can get SPDIF to run through it at 48kHz. The multiplexer IC that I am using has an enable pin, and I can change the state of that to mute the system without having to generate an "all zeros" SPDIF signal. Very convenient.

Tomorrow I check that 192kHz will pass through unscathed. Then I need to add code to the Arduino to set the pins that control the multiplexer so that I can test its input switching functionality. I don't expect any issues there.

After that I will need to test out the volume control circuitry. There is a thread around here somewhere on that already:
NEED ADVICE: controlling Mini2x4 volume when using digital inputs

So far this project is looking promising!

-Charlie
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Old 28th July 2013, 09:23 PM   #22
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I tested out the input switcher at a few different sample rates - the highest that it will pass is 96kHz. But in another forum thread, someone pointed out to me that the stray capacitance and contact resistance of a breadboard tend to cause circuits that have signals above a few MHz not work, and this is probably the case with my little hack job:

Click the image to open in full size.

I'll have to transfer the circuit over to a prototyping PC board where everything can be soldered down, component leads trimmed, etc. and then retest it.
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Old 9th December 2013, 01:16 AM   #23
Dresden is offline Dresden  Czech Republic
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Default Interesting Project

What is the present state of the project?

I have for quite some wanted a system which can send separate signals to powered speakers.

In the last few days, I resumed my search, and not finding anything comparable, I had resolved to making a list of potential retail components (such as the SONOS product), for possible use in sending a signal to two powered loudspeakers. (I also only want a power cord for each speaker, nothing more.)

If you can design something which can match the performance of the SONOS (albeit at a more affordable price) in the foem of a module for DIY/OEM, I am interested. (Perhaps you have already accomplished such?)


Quote:
Originally Posted by Charl
3576854
I tested out the input switcher at a few different sample rates - the highest that it will pass is 96kHz. But in another forum thread, someone pointed out to me that the stray capacitance and contact resistance of a breadboard tend to cause circuits that have signals above a few MHz not work, and this is probably the case with my little hack job:

Click the image to open in full size.

I'll have to transfer the circuit over to a prototyping PC board where everything can be soldered down, component leads trimmed, etc. and then retest it.
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Old 15th July 2014, 07:36 PM   #24
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Quote:
Originally Posted by minidsp View Post
That's a neat idea Charlie. Keep us updated on your progress...
We don't have any wireless audio solution so providing your system works, we can certainly build a section of our website to advertise your final tweak.

Keep us updated.
OK, at last I can offer an update.

First, my apology that this project became a little stale. I have been wrestling over how to implement the volume control and remote on/off, etc. I needed to design and build some custom circuitry for this in addition to the digital input selector. This is where I got bogged down - I just could not convince myself of a "best" way to do it. I designed and simulated various circuits, but these always left me scratching my head about how they would perform or fall short, etc. I even went as far as to purchase a bunch of ICs and other parts, but then it hit me...

To send SPDIF using the AV wireless senders you are limited to a sampling rate of 48kHz. This is because the bandwidth capability of the RF wireless circuitry (that is normally used for analog video) is not high enough to accommodate a higher sampling rate. But often people have SPDIF sources at other (higher) rates, and to make sure that these sources can be sent using the wireless system the digital signal must first be downsampled to 48kHz. After some Q&A with the MiniDSP DevTeam, I discovered that their miniDIGI can be used as an ASRC, and its output stream is always at 48kHz. Great! As a bonus, the miniDIGI has built in input selection from its four spdif inputs (two coax, two Toslink) although you have to add a MBB switch to automate this. No prob. So I picked up another miniDIGI (once they were back in stock) so that I could use it in this capacity at the source. This is in addition to the two other miniDIGI boards that are used as spdif receivers for the 2x4 boards I will put in my loudspeakers.

The ah-ha moment that I had recently was about the 2x4 + miniDIGI stack. Thanks to the flexibility of these products, you can not only route the incoming spdif stream from the miniDIGI to the 2x4 (this is the normal usage) but you can also route a pair of processed channels from the 2x4 (e.g. the 2x4 outputs CH1 & CH2) back to the SPDIF output of the MiniDIGI. Because the 2x4 has the capability of using an external potentiometer for master volume control this gives us the possibility, with the addition of one more 2x4 board, to route the spdif stream to the 2x4 so that its master volume control can be used. This setup makes it possible to adjust the volume of the signal digitally before it is sent over the wireless system rather than at the loudspeakers using my previous control circuitry concept. This has several advantages...

It's been noted that when using the external volume pot, the volume = max position results in a gain loss of about 3.3 dB when using the onboard voltage source (3.3Vdc) on the potentiometer header for the 2x4. With my setup, I can make up this loss by programming a boost into the "crossover" section using the advanced biquad mode. But there is another advantage: I have often complained to the DevTeam about the lack of input PEQ/biquads in the 2x4 and 4x8 products. I use these to flatten the frequency response of my loudspeakers instead of the PEQ bands for individual drivers, which has certain subtle but important advantages. The "input" PEQ bands are globally applied to all channels in the crossovers, which are downstream. Since we have added another 2x4 board to control the volume, we can use its PEQ biquads for global PEQ as well, and this increases the number of available global PEQ "bands" by 8+6 = 14 bands! Awesome!!! My prayers have been answered.

I have been testing the volume control setup over the last couple of days. I had to iron out a couple of odd problems with the plug-in, but it seems to be working great now.

There is another advantage of this setup over my previous volume control scheme: you can connect as many wireless receivers to the system as you would like and they will all get the same volume controlled digital signal. This makes it possible to mix digital crossovers (e.g. the 2x4+miniDIGI stacks that I will use in my main loudspeakers) with analog crossovers by simply connecting the wireless receiver to a DAC feeding the analog crossover. For instance, you may only need a relatively simple crossover for a subwoofer and don't need all the processing power available with a 2x4. You could have multiple subwoofers located anywhere you would like throughout the listening space as long as they can be connected to mains power, without having to run any signal cabling. This would make it possible to investigate distributed subwoofer setups such as those espoused by Geddes:
Serious Audio: Two Great Articles on Multiple Subwoofers by Dr. Earl Geddes
I plan to do this kind of thing using four identical subs that I am building. These will be powered by plate amps that have their own built in analog LP crossovers, so there is no need for any DSP. This situation is likely quite common with plate amp powered subwoofers, so its likely to be a useful tip for others as well.

Finally, one significant advantage of the wireless SPDIF bridges is that they eliminate long and/or multiple cabling runs. These can be a source of noise pickup and can result in ground loops and other very undesirable effects. I have definitely encountered these in the past when running multiple 10-foot-long line level cables between a MiniDSP crossover and the amps located in my powered speakers. It was pretty amazing that when I would pick up and separate the cables I could form a very nice antenna that easily picked up AC hum present in the room! By eliminating cable runs between equipment this kind of problem should no longer be a concern.

When I finally manage to get my nascent system fully working (likely this will take a little while) I will post all the details of the setup for those who want to assemble something similar.

-Charlie
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Old 26th July 2014, 11:42 PM   #25
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I'm making slow progress... I have everything running off of one power supply so that I can have the different boards tied together and communicating with each other. I'm temporarily making all of these connections via a solderless breadboard. At this point I have only implemented the volume control. Next up are the source selection MUXes.
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Old 27th July 2014, 06:22 PM   #26
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Decided to try and add an encoder to the project. This took some time to track down some interfering code, but it is working great now and was definitely worth the effort.

Contact bounce is totally eliminated by using the approach described here:
Buxtronix: Rotary encoders, done properly

There is just something so familiar and soothing about a knob...
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Old 28th July 2014, 11:35 PM   #27
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Default problems... problems... problems.

I've encountered a problem in which I can't get enough control range from the master volume control. This is bad enough that I will have to give up on the idea that I was so enthusiastic about just a few posts ago. It's just not going to be satisfactory as a volume control.

Here's how I've implemented the master volume control of the 2x4 and tested it out:
  • The hardware includes an Arduino controller and a 12-bit DAC, running off of the same power supply and grounds tied together.
  • The user sets the volume control setting (in dB) in the Arduino
  • The Arduino sends a 12-bit level code to the DAC.
  • The DAC is acts like a potentiometer between ground and a 5Vdc voltage supplied by the Arduino board. The ground is tied to the ground common point.
  • The output of the DAC is connected to the input pin of the volume control on the 2x4 board.

I did a regression of some data I received from another DIYer (Dave) and developed some code that converts gain to the DAC setting. This allows the user to directly set the gain (attenuation) in dB from the user GUI of the Arduino controller. It is definitely working but after using it for awhile I noticed that at the low end the attenuation range was not working properly, meaning that at some point continuing to reduce the volume did not increase the amount of attenuation (see plot below).

In order to try and spot the trouble I made some measurements of gain versus DAC setting (control voltage). A plot of my data (in blue) compared to Dave's (in orange) is provided in the attachment.

I made these measurements by doing an impulse measurement using ARTA and processing it into a frequency response. I recorded level at 1kHz for a range DAC output voltages. The voltages were calculated as: Dac setting * 5V / 4092. Using a multimeter I measured the voltages for gain settings below about -30dB and these closely agree with the calculated values. You can see that , at the "loud" end of the volume control range (near 0dB) things seem about right. When we get down to about -30dB things start to go wrong and it becomes increasingly difficult to reduce gain to the full -52dB or so that Dave reported.

If I switch the 2x4's wiper contact between the DAC set to 0 (e.g. at ground) and the wiper of a 5k ohm potentiometer with the pot also set to 0 ohms the sound level does not change. This seems to indicate that the DAC and the pot are experiencing the same lack of attenuation at the minimum setting as illustrated in the plot above.

Because of these problems, and the fact that the total available gain/attenuation range is too small to be useful for the main system volume control, I am going back to some of my original ideas for remote volume control and trying those out.
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Old 31st July 2014, 05:28 AM   #28
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While I am waiting for some parts to arrive for the new(est) attempt at a volume control circuit I thought I would go back and try to get the input switching circuit working again. The MUX will be connected to the spdif selection header on the miniDSP and will select from the four spdif input channels that are available through the board. But I also want to be able to connect additional spdif inputs to the MUX and route them to the miniDIGI board as well. My original input switcher circuit had an spdif -> TTL converter and I was able to get this working again and clean up the breadboard to all the components are not just sticking off into space and picking up noise. But alas my MUX is an HCU type and only operates at 5V, so I need to get a 3.3V compatible HC type MUX before testing this out further. Since the part is only $1.50 I will probably wait until I need some other parts before ordering. Since I got the identical pinout HCU part to work before, it's just a matter of getting the new part in and plugging it into the board. Well, it's that simple on paper at least!

The next attempt at a volume control circuit will use some venerable 4-channel VCAs that use an analog DC input to set levels. These aren't as good as modern volume control ICs in terms of noise or distortion, but they will be fine in the short term at least. If I can get this concept working I will test it out and post some measurements of the performance. The VCAs can provide both gain and attenuation. Having access to global gain in the signal chain will be very handy, and will allow me to more easily interface with amplifiers that have higher input sensitivities.
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