Saturday, May 12, 2018

Behringer X32 and S16 Preamp Noise

I got myself a Behringer X32 Compact mixing desk and Behringer S16 Stageboxes. Because sometimes I record classical concerts, or have ambient microphones that get very little signal, I wanted to verify the preamp performance to avoid nasty surprises.

To establish the voltage scale when recording, a sine-generator generating 1kHz was fed into a passive DI box, and adjusted in amplitude, so that a 1Vrms signal was present on the DI box XLR output. This 1Vrms signal could be easily measured with a TRMS multimeter. Then both the 20dB pad on the DI-box and a 40dB pad on the function generator were engaged, so that a 1mVrms signal is output on the DI box. This sinewave was fed into all devices under test at maximum gain. (See Fig. 2)

Sucessively the XLR connection to the DI box was removed and the input was shorted. A 2nd recording was then made, with identical gain settings only capturing the noise. (See Fig. 1)

Preamps tested were:
  • EchoAudio EchoFire 4, which uses a now obsolete SSM2017 from which noise performance is known and documented
  • Preamps within the X32 Compact mixing console, using the local XLR input
  • Preamps within the two S16 stage boxes
In python (using numpy), the rms of the sinewave-recording was calculated relative to digital full-scale and the noise recording scaled with the resulting factor (e.g. -21.0 dBrms/full-scale for the EchoFire4 recording). From this data the noise density in nV/√Hz was calculated using Scipy's scipy.signal.welch results.

The spikes present are most likely caused by switching voltage regulators within, or in vincinity of, the devices. Generally the Behringer preamps have slightly higher noise than the very respectable SSM2017 within the EchoAudio firewire interface, but all in all the difference is pretty much irrelevant (approx. ~1.5dB more noise than the  SSM2017, adds approx. 3 dB to the thermal noise of the 200Ω resistor) for practical use.

Fig1: Noise Analysis (with inputs shunted by 200Ω)

Fig2: Reference Recording of 1mVrms, 1kHz Sine Wave

Useful Links

Monday, September 11, 2017

VHDL Testbench using Oscilloscope Waveforms

I got a little tired of writing a generator for synthetic data in a VHDL testbench, so I thought, maybe just use an oscilloscope trace with real-workd data for this purpose.

So, here's the VHDL code, I've only captured one channel and it's stored as unsigned 8-bit characters, so everything is very easy.

signal aes3_in : std_logic;
signal aes3_analog : natural;
type t_stim_file is file of character;
data_in: process
   -- waveform captured on a rigol oscilloscope, 1ch, 8bit, 1GS/s
   file stim_file : t_stim_file open read_mode is "aes_48khz_24bit.wfm";
   variable v : character;
   variable n : natural range 0 to 255;
   -- skip over the first 3300 bytes, header of the wfm file
   for i in 0 to 3300 loop
      read(stim_file, v);
   end loop;
   while not endfile(stim_file) loop
      read(stim_file, v);
      n := character'pos(v);
      aes3_analog <= n;
      aes3_in <= '1' when n > 90 else '0';
      wait for 1 ns; -- 1GHz sampling rate
   end loop;
   assert false report "end of test" severity failure; -- end testbench here
end process;

Monday, July 24, 2017

Mixing Station X Air Pro XR18 Default Layout

Working with a Behringer XR18 Mixer and using the brilliant Mixing Station Air software, I've made a custom layout for it. It can be downloaded from github.

Thursday, March 23, 2017

Roland TD-25 (Drum Sound Module) Pinout and Waveforms

Debugging an issue with a Roland TD-25K electronic drum set.

First, here are the outputs of the trigger-pads. Yellow is "tip" of a TRS connector, cyan is "ring". Both the bell and the dampening signals of the cymbals connect to the ring and there's a constant voltage offset (which you don't see because the 2nd (cyan) channel was AC coupled.

PDX-100 Snare Pad

CY-13R Ride Cymbal, "Cymbal" connector:

Ride Cymbal, "Bell" connector:

Also, I traced the pinout of the Roland TD-25 multipin/dsub connector.

                +-/   |
                |   O |  1 KIK-T
   T1-T      14 | O   |
                |   O |  2 KIK gnd
   T1-R      15 | O   |
                |   O |  3 SNR-T
   T1 gnd    16 | O   |
                |   O |  4 SNR-R
   T2-T      17 | O   |
                |   O |  5 SNR gnd
   T2-R      18 | O   |
                |   O |  6 HH+T2 gnd
   T3 gnd    19 | O   |
                |   O |  7 HH-T
   T3-T      20 | O   |
                |   O |  8 HH-R
   T3-R      21 | O   |
                |   O |  9 HHC gnd
  RD/RDB gnd 22 | O   |
                |   O | 10 HHC-T
  RD-R       23 | O   |
                |   O | 11 CR1 gnd
  RD-T       24 | O   |
                |   O | 12 CR1 T
  RDB-R      25 | O   |
                |   O | 13 CR1 R
                +-\   |

gnd: sleeve/ground, T: tip, R: ring
T1/T2/T3: Tom 1, 2, 3
RD: Ride
RDB: Ride Bell
HH: Hihat
HHC: Hihat pedal
KIK: Kick/Bassdrum
SNR: Snare
CR1: Crash Cymbal

Wednesday, November 09, 2016

Archlinux on the Toradex T20 (with Archlinux' kernel)

This is a followup on my earlier blogpost Archlinux on the Toradex T20 from 2012.

I happened to resurrect my old Toradex NVidia Tegra T20 / Colibri / Iris eval-board, which I had running using Toradex' kernel back in 2012 already. And it's amazingly smooth to run the stock Archlinux-Arm distribution for ARM7 CPUs on it, as I found out:

You'll need:

  • a SD-card formatted using the EFI partition table format (the stock uboot for whatever reason didn't like it when I had it partitioned as MBR)
  • two partitions, one for boot, one for the rest (I used vfat / ext4)
  • mkfs.vfat on the first partition (will be mmcblock0p1 on the Toradex), mkfs.ext4 for the 2nd.
  • mount the first partition (vfat) as "boot" in the second partition (ext4, rootfs)
  • unpack in the 2nd partition
  • in "boot" create a uImage from the zImage (the stock u-boot of Toradex can't boot zimages directly), I use the following script (on the toradex module itself, useful after a kernel upgrade!) You'll need the "mkimage" utility from uboot-tools.
[root@alarm ~]# cat /usr/local/sbin/ 

mkimage -A arm -O linux -T kernel -C none \
-a 0x00008000 -e 0x00008000 \
-n "Linux Kernel" -d /boot/zImage /boot/UImage

Interrupt the bootloader (in the stock configuration, bootdelay is zero, so you'll have to hit your keys hard to get it to interrupt the boot!) and configure the following environment variables like so:

(copy & paste every line separately, I got lost characters when I paste then as a whole!)

env default -f
setenv dtaddr 0x1000000
setenv dtbname tegra20-iris-512
setenv myload_img fatload mmc 0:1 \${loadaddr} uimage
setenv myload_fdt fatload mmc 0:1 \${dtaddr} dtbs/\${dtbname}.dtb \; fdt addr \${dtaddr}
setenv myload mmc part\;run myload_img\;run myload_fdt
setenv myargs setenv bootargs console=ttyS0,115200 root=/dev/mmcblk0p2 rootdelay=2
setenv myboot run myargs \; run myload \; bootm \${loadaddr} - \${dtaddr}
setenv bootcmd run myboot
setenv bootdelay 5

If you just want to test, leave out the last "saveenv" and type "run myboot", after a reset, your Toradex module will still be completely unchanged. If you save the environment (u-boot configuration) with "saveenv", the module should boot up archlinux automatically.

Update: I just realized, the variable "fdtaddr" I used initially is used by u-boot internally and overwritten on every boot, don't use it then :-).


U-Boot 2011.06-dirty (Dec 18 2014 - 22:27:35)

DRAM:  512 MiB
NAND:  1024 MiB
MMC:   Tegra2 SD/MMC: 0
Board: Toradex Colibri T20
Net:   Net Initialization Skipped
No ethernet found.
Hit any key to stop autoboot:  0 

Partition Map for MMC device 0  --   Partition Type: EFI

Part  Start LBA  End LBA
gpt1  0x800    0x407FF
gpt2  0x40800    0x1D47BDE
reading uimage

5623992 bytes read
reading dtbs/tegra20-iris-512.dtb

28979 bytes read
## Booting kernel from Legacy Image at 00408000 ...
   Image Name:   Linux Kernel
   Created:      2016-11-08  21:31:41 UTC
   Image Type:   ARM Linux Kernel Image (uncompressed)
   Data Size:    5623928 Bytes = 5.4 MiB
   Load Address: 00008000
   Entry Point:  00008000
   Verifying Checksum ... OK
## Flattened Device Tree blob at 01000000
   Booting using the fdt blob at 0x1000000
   Loading Kernel Image ... OK
   Loading Device Tree to 01ff5000, end 01fff132 ... OK

Starting kernel ...

[    0.000000] Booting Linux on physical CPU 0x0
[    0.000000] Linux version 4.8.6-1-ARCH (builduser@leming) (gcc version 6.2.1 20160830 (GCC) ) #1 SMP Mon Oct 31 23:22:19 MDT 2016

Sunday, April 17, 2016

Yamaha DGX-620 LCD Repair/Retrofit

UPDATE: Ulrich Borghoff did his repair slightly different and sent in photos. Thanks Ulrich!

Here's a Yamaha DGX-620 keyboard with a broken LCD. Judging from the fact that there are quite a few videos and tutorials on how to exchange a broken LCD with the proper spare part, I'd reckon these LCDs break quite easily and often.

 (Broken LCD)

(Original LCD module: The slightly kink in the flexfoil is enough to ruin the module, this slight damage is, unfortunately, not repairable.) 

I was a little surprised to get a quote of 130€ ($150) for the replacement (Yamaha Part # WG299100), and I was looking for a replacement that's cheaper. As this is a fill-in instrument, and long out of warranty, I wouldn't mind not getting the original part. Also I don't mind the tinkering.

Enter Pollin, a company selling electronic remnants, such as bags with 1kg of capacitors, sold as “perfect for tinkering”. By chance, they sell a “LCD-Modul NAN YA LTC79H202T50K, 240x320” (order #121307, €4/$5) which, on closer examination, is electrically 100% compatible to the Yamaha display. Only with a slightly different, smaller shape, and with a different pinout. Also, they sell the exact same flex-cable that Yamaha uses to connect the LCD to the CPU board: Flexprint-Kabel AXON FFC1.00A14/0200L5-5-10-10 (#562251) (€1).

(Pollin: Product Page)

(cheap replacement LCD)

Looking into the datasheet of the LCD provided by Pollin and the Yamaha DGX-620 service manual easily found online, it's clear that electrically the display is compatible.


(Pollin LCD Module)

FR (first row) is FLM (first line marker), LP (line pulse?) is CL1 (data latch), XCK (x? clock) is CL2 (shift clock). Everything else has identical designators. Sanding off the back of the replacement LCDs PCB a little to have good bonding to epoxy glue...

(replacement LCD, before gluing down flex cable)

(sanding off the back of the PCB and the white flex-cable, so that the epoxy sticks better, you can see the residue from a first attempt: The blue plastic on the flex cable didn't stick)

(jumper wires)

Ugly patching of wires from the 14-in cable to the pads on the LCD according to pinouts shown above. After soldering the wires (which is hard on the flexfoil, as the plastic melts and the copper traces of the flexfoil will move around, creating shorts) and emitting a few prayers to the deity of choice, a picture appears! :-)

(first signal on replacement LCD screen)

Then I added the display to the original plastic part from which I removed both the LCD logic and all the light spreading works. The complete LCD module almost fits in the cutout for the old glass, only a small plastic ridge had to be removed with a xacto-knive.  I had to add a small slot for the flexcable, to feed through and added a hole for the big blob of goo by which I glued down the cables so that they don't become loose.

(cutout for big lump of wires and goo)

The end result: A replaced LCD screen for €4. But I had to invest a few hours of work, which in this case I didn't mind.

(replaced screen on the Yamaha keyboard)

Update: As Ulrich pointed out, one can directly solder the cables for the LCD backlight to the cables going to the two-bin connector on the power supply on more recent models of the keyboard. Some onlder ones have high-voltage for an electroluminiscent foil, though, so beware.

Here are Ulrich's Photos: