Topic: I want to build a DAC for Raspberry Pi 2

[emanuel]

It is very easy to test soundcard's jacks under raspberry linux using:


speaker-test -Dsurround71:Device -c6 -twav


all 7.1 jacks are in use while testing (headphones)
after that I'm using:


aplay -Dsurround71:Device C.wav


C.wav = the white circle at 20000 Hz.
It also brings up noice on headphone excluding the 2 channel that are not connected to the amp.
So soundcard works in 7.1, but amp not.


As I wrote I can come neer to zero at the channels, but while playing the wave there are no volts on the pins (1,2,5,6,7) of the DB25 respected to GND.
All channel values could only be changed using the black marked pots to come to the zero. All other cannel pots are dead.


There is not even a minimal change screwing the other pots X,Y,R,G.B (blue 500 Ohm), while playing the wave.


I've spent next 4 houres for nothing...


same as yesterday :-(

[emanuel]

exact the same under Windows C.wav + spiderplayer loop-> no volts om DB25 pins  (cannels are the same as under linux in my case, not twisted), I think some part is not working on amp.

[James]

Well, you know the negative voltage regulator is getting power.


        -[    ]-*
        -[    ]-
        -[    ]-
       *-[    ]-


Use your volt meter with one lead grounded and check for voltages on the IC sockets (from the top of the board). You should see something close to +9VDC and -9VDC.

That will tell you if the DC/DC converter is working.

The -9VDC must be working. It powers the negative voltage regulator.

I find it hard to believe that none of the channels of the correction amp are working.

I tested that board thoroughly and had it set for a USB SND8 that I have.

The SND8 offset voltage that I get are about +1.3VDC.

We'll figure it out. If needed I will replace that board.

If you want to send it back to me, you could also send your sound device and I'll set it up for you.

James.

[emanuel]

I also thought at first to the converter, but this provides -9V / + 9V to TL072 so it is ok.
Offset master tuning also works up from -1.28V so LM337 it is also ok.


Channel offset triming only works at uppermost position of the 100K pots so pots are ok.
Grain 500Ohm pots are with out any function but measurement was ok.


I think it could be a problem about the input resistance of the TL072, maybe they are out of range with that soundcard. Or what do you think?


I once the soundcard channels measured in operation (white circle wave) X and Y bring 2.26V, RGB, however only 0.68V
with out playing sound all channels have got about 2.26V.


What can I do/test with out oscilloscope?

[James]

I think everything is OK.

It just seems like there is a misunderstanding.

The 100K pots are gain. I marked them black.

The 500 ohm pots are offset trim. (Blue).

The effect of the offset trim pots is certainly relative to the overall gain of each channel. Gain is a multiplying factor, so it multiplies the measure of offset.

The offset trim pots are only 500 ohm so they add between zero and 500 ohms to a 15000 ohm resister. They are just there to tweak out the tiniest differences between the channels.

Most of the offset adjustment is in the master negative voltage regulator adjustment.

Since the correction amp is an inverter, waves made in LB are inverted, so it stands to reason that the voltage should drop on the color control signals from the silent offset (measured driectly from the sound card) of some positive voltage to one that is lower and closer to zero when circle.wav is playing. That is correct.

James.

[emanuel]

LOL
Oh my God! Am I stupid, yes I am.
Something I must have mixed up while reading and translating in my brain.

Now comes power from the amp to the laser.
I made the first attempt in Windows.
The white circle works at 25000Hz. The blanking is not fit. Settings in Laserboy I still need to test. It all seems to be too fast (other test files are still to bad for images/movies) and the scanner is very noisy compared to the SD interface operation.

I have analogous colors !!!

James, many thanks for all the trouble until now.

[emanuel]

In the first test I only have the [TAB> h output wave sample rate.....] is set to 25000. What settings do I have to adjust for a 20k-25k scanner?

[James]

The points-per-second rating of scanners is very ambiguous.

It really relates to the point rate of a DAC clocking out the ILDA standard test pattern.

Proprietary laser DACs ususally allow you to change the DAC clock rate.

It doesn't exactly apply to LaserBoy time optimized waves.

That USB sound card DAC you have really only has one fixed rate; 48KHz.

All waves of any sample rate are up or down sampled to 48KHz in the sound driver.

Remember, you are using a sound device. So it is a sound device! It plays waves. You just happen to be sending the signals to a laser projector.

You should set the wave sample rate in LaserBoy to 48000. That is the default.

That means that the sound card will output at its native sample rate.

You control the scanner velocity with settings 3 & 4 in the [Tab] menu.

This is how far the scanners can move in any direction in one sample. Points are relative to signed short integer numbers (16-bits). So the whole cube of defined 3D space is 65535 points in X, Y & Z.

The default settings are probably OK for your scanners.

If you think you are scanning too fast, you can decrease the values for options 3 & 4.

If you think the scanners need more time to settle in the corners of your images, you can also control the dwell time in micro-seconds. (This setting is relative to the fact that your waves are made for some samples-per-second (micro-seconds translates to a number of samples at rate).

Think of it this way... a laser projector is entirely an analog device. It takes ANALOG signals to control it. It doesn't care about DAC clock rates. It only responds to change in voltage over time. You can divide that into samples any way you want.

Centimeters per second... kilometers per hour.... points per sample (1/48000 seconds).... It's all the same thing.

Actually, your scanner velocity is measured in radians of rotation per second. And that is relative to the gain setting of your correction amp for X and Y, giving you a maximum scan angle.

James.

[emanuel]

So I've setup all on raspberry, I still have got magenta blanking Lines. I'm at 5.0V at RGB, juppers do reduce down to 4.85V.


And Colors are very TTL like. Do I have to reduce power on the RGB Channels?

[James]

Well, considering the fact that your projector is almost definitely TTL color, you could try dropping down the gain on the color channels to a point where they just come on when they should.

I don't know!

Something to think about.... Magenta is an equal mix of red and blue... No green.

That might tell you something.

Your green laser is a DPSS, not a direct injection diode like your blue and red.

Maybe you can get inside of the projector and measure the color control signals that are coming from the internal SD card reader to get an idea of what the projector is designed to respond to.

James.

[emanuel]

about TTL: it is analog, I can reduce color intensiveness with pot on all RGB channel.


Please take a look at the video (sorry bad phone cam) there you can see the issues:
the whole art will be circled by a white or sometimes magenta copy of it self including the blanking lines.
Some kind of interference on the RGB channels.
 
https://goo.gl/photos/vBoNMaHmtd81P2GS8


If I reduce all RGB channels to 0V, it is still there as you can see:


https://goo.gl/photos/eSFFWDLJ5qvYvWzh9


Have you seen somthing like this before? What can I do?

[James]

I've seen enough video of laser display to know that it can look a lot different than the real thing. It's all about the frame rate and exposure time of each frame of the video camera / recorder.

Anyway....

WOW. I don't know.

You could try an experiment.

If you open a LaserBoy wave in Audacity you can play one or two channels in "solo" mode.

So you could plug speakers into channels front left and right and listen to each channel all by itself.

You can then compare the sound to what you hear when you plug the speakers into the color channels and use ALSA Player to play the full 6 or 8 channel wave.

BTW if you set LaserBoy to make 8 channel waves,  there should be total silence in channels 7 & 8, when you play an 8 channel wave.

If you hear anything in those channels then you know that information is getting mixed up in all the channels.

It's weird though.... The color version of the LaserBoy logo that you scanned looks SOOOOO CLOSE!

Good work so far! I know this can be very challenging. I've been through it many times!

You could try lowering the number for option 4 in the [Tab] menu. Then the laser will have more time to trace the blank lines. You can also increase the time for option 5 so the laser dwells longer in the corners and transitions from lit to blank.

What parameters are you using to make waves?

James.

[emanuel]

What parameters are you using to make waves?
>>>3,4,5, pots, ... lots, but the white RGB noice do not go away :-(

Even if I swap one of the RGB cannels wire with channel 6 of the DAC there is missing a color in the main picture, but the white RGB noice picture is shown with full colors. So the white noice is on the other non RGB channels also. I do not know what I can do any more.

[James]

Well, I hate to say this.... but you should probably try the sound card plugged into a Windows machine!

Install the drivers from the disk that came with the sound device.

Install Spider Player.

Set the sound device driver to 8 channels. Go into Windows Sound System settings and set the speaker arrangement to 8 speakers.

See if it works that way.

If it works, the problem is in the configuration of ALSA on the Pi2.

I think we could fix that with a copy of the image that I have here.

We'll figure it out.

Did you make a big red, blue and green circle wave for testing each color channel?

That can be very helpful.

James.

[emanuel]

I've tested all in Windows at the very first try + Spiderplayer + Disc driver + full 8 cannels, also on Linux pc amd64. Always the same white noise. It does not depend on which host I'm running the soundcard + amp + projector. Also I've mad the colored Gears in R/G/B, at that point I've noticed my projektor's RGB channels are twisted (DB25), but that was fixed simple by swaping DAC inputs. I can not get rid of that white noise circling around the picture and the blanking line. The Setup only edits quality of the main picture.


Also I've noticed looping a wave of 10 sec produces a double picture with a small offset, maybe this is also this white noice but on X and Y cannel. Using a one frame wave does make one circled picture.


I think I have no chance to find the source of the white noice. It could be the soundcard, projector..