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AMBM - The Analog Meter Bridge Module Project

( Deutsche Version: AMBM - Das Analog-Meterbridge-Modul-Projekt)

Modules as kits for DIY multichannel analog PPM or VU meter bridges

With LED bars containing 51 dual-color LEDs such a meter bridge is not only technically valuable but an exceptional eye-catcher, too

Contents

• Videos
• Features
• What You Get and What Else You Require
• Modules as Kits and Fully Assembled
• Some Views of the Assembled Modules
• The LED Scale
• Operation
• DIY Aspects
• Documentation
• Pricing and Availability

Videos

Watch (after a short introduction) the AMBM in action:

→ Medium resolution, (1280 x 720, 25p, 727 MB, 12 Mbit/s): The download is often fast enough

→ High resolution, (1920 x 1024, 50p, 1570 MB, 26 Mbit/s): The download is normally not fast enough

Features

• Two channels (two LED bars + dual channel controller) per kit
• 51 dual color LEDs per LED bar, 77.5 mm long
• Separate LED bars: Any LED bar pitch > 17.4 mm
• Single 12 V (7 to 15 V) power supply
• Two balanced analog inputs
• Input sensitivity +4 dBu or -10 dBV @ 0 dB display
• True PPM or VU measurements
• High precision: Error < ±0.1 dB (~ ±1%)
• Scale range from -50 dB to +5.5 dB
• 6 switchable display options (opened switch mentioned first):
  1. PPM or VU measurements
  2. Bar display or dot display
  3. Bar display: With or without peak hold or
     Dot display: With yellow or green-yellow-red dot
  4. Background LEDs on or off
  5. Color pattern green-yellow-red or green-red
  6. Display intensity high or low

What You Get and What Else You Require

The AMBM modules are meant to be kits, so you are not only free to install them in various ways - the drawback is that you also need to decide how to do it and to care for the necessary mechanical design and parts.

The kit contains:

• 1 Control module AMBM-Ctl with all SMDs soldered, programmed, tested and calibrated
• 2 LED modules AMBM-LED with all SMDs soldered, tested
• 2 LED bars containing 51 dual-color LEDs
• 4 Pin headers, 2 x 13 pins (LED module connections)
• 2 IDC ribbon cables, 26 wires, 45 mm long (LED module connections)
• 1 Pin header, 2 x 3 pins (option switches)
• 1 IDC ribbon connector, 2 x 3 pins (option switches)
• 3 Screw type terminals (power supply and analog inputs)

That's it. No screws, no nuts and bolts, nothing else. Thus you have to care yourself for all these screws, nuts and spacer bolts, i.e., all mechanical parts.

And you must care for a power supply unit, e.g., 12 V, at least 350 mA per AMBM module set (2 channels).

Should you want to install the LED modules further away from the controller than the 45 mm IDC ribbon cables allow, let me know. I can alternatively supply the IDC ribbon connectors and sufficient cable separately.

Modules as Kits and Fully Assembled

The kits come with the SMDs solderered and tested, but you must solder the few THDs (through-hole-devices) yourself. This allows you to assemble and wire them differently from my original intention and the description here.

		The modules as they come as kits...		... and assembled with screw type terminals, pin headers and LED bars
The Control Module AMBM-Ctl
Two LED Modules AMBM-LED

Some Views of the Assembled Modules

The LED Scale

The AMBM's scale is identical for both VU and PPM. It reaches from -50 dB to +5.5 dB. 0 dB scale indication refers to either +4 dBu or -10 dBV (others are possible), which I think is widely used in non-broadcast studios. Levels above 0 dB are displayed with red LEDs, levels between -5 dB and 0 dB either yellow or green and all other LEDs below are green. The scale values are:

Scale Lettering Template(s)

Maybe similar to this:

  or    or    or  

Anyway, yet to come...

+4 dBu, -10 dBV and other Reference Levels

Selection between +4 dBu and -10 dBV for 0 dB display is made by 4 solder joints (solder jumpers) on the solder side of the AMBM-Ctl.

• Solder joints open (as on the photo): +4 dBu
• Solder joints shorted: -10  dBV
  

Should you need another reference level for 0 dB display than +4 dBu or -10 dBV you can achieve it with 2 external resistors for each channel at its input pins, provided the reference level is higher than -10 dBV (= 316.2 mV_RMS), read more below.

Operation

The settings in detail are:

Switch 1: PPM or VU measurements

PPM and VU measurements differ a lot from each other.

• VU (Volume Unit) measurements were very early introduced when hardly anything else but electromechanical instruments were available to indicate measured values. Thus somebody took a simple rectifier and an available moving coil instrument and called it a VU meter. (Ok, that's simplified.) The dynamic characteristics of that instrument were important, of course, and, in order to make comparable VU measurements, all other VU meters ought to have the same dynamic characteristics. And so the VU meter was standardized in ANSI C 16.5 / IEC 268-17:
  
  
• The attack time, i.e., the time it takes for the display to reach 90% of the actual value, is 300 ms.
• The overshoot is within 1 to 1.5%.
• The decay time is the same as the rise time, 300 ms.

The VU meter response time is high so that short peaks are widely invisible. This is how the AMBM in VU mode behaves, too. But in contrast to original VU meters, which have a linear scale from 0 to +3 VU (+3 VU is 3 dB above 0 VU), the AMBM uses the same logarithmic scale as for the PPM scale. Anyway, the measured values are identical.


• PPMs (Peak Program Meter) were developed and standardized much later. These standards are not identical worldwide. Particularly the reference levels differ. Here I adopted the IEC 60268-10.
  
  
• The attack time, i.e., the time it takes for the display to reach 90% of the actual value, is 10 ms.
• The decay time is 20 dB per 1.5 s.

PPMs respond much faster to peaks so that any overdrive is much better visible. Usually PPMs would be used in mixing consoles, VU meters possibly more in tape recorders.

The PPM mode is selected as default, i.e., when switch 1 is off.

Switch 2: Bar display or dot display

 Bar display

 Dot display (dot color = yellow)

Switch 3, bar display: With or without peak hold

 Bar display with peak hold

(This is the default display, i.e, when all switches are off resp. when there are no switches)

Switch 3, dot display: Yellow or multi-color dot

 Dot display with dot color = background color

Switch 4: Background LEDs on or off

 Background LEDs off

Switch 5: Color pattern green-yellow-red or green-red

 Color pattern green-red

Switch 6: Display intensity high or low

 Display dimmed

DIY Aspects

Mounting

The LED modules can be mounted stacked on the control module using spacer bolts with a length of at least 30 mm, as shown on the photos here. When both LED bars are mounted this way, a pitch of either 21.59 mm (0.85") or 27.305 mm (1.075") is possible.

When the LED modules are mounted separately from the control module, a minimum pitch of 17.4 mm (0.675"), the width of the LED PCB, is possible. The LED module's PCB can be shortened by cutting or breaking it at the provided perforation.

The mounting holes are prepared for M3 (approx. #4-40 UNC) screws.

Should you want a huge distance between control and LED modules, you can also use very long ribbon cables. Up to 1 m is no problem. I provide ribbon cables with a length of 45 mm, intended for stack mounting of the module, as shown on the close-up view on the right.

The LED bars' protective foils are meant to be removed before they are finally built in. They are usually invisible but can produce reflections while the LED bars' surface itself is matt. Anyway, on all photos here you can still see the foils and, when you take a close look, reflections produced by them, too.

Wiring

Correct wiring is crucial.

• Power supply: Usually you have one central power supply for all modules.
• Use one pair of power supply wires for each control module and join them at the power supply (→ star wiring).
• Use twisted wires if possible or at least run both wires close to each other.
• Keep the power supply wires apart from all audio signals, particularly those with low signal levels (microphone inputs).
• The longer the wires are, the thicker they should be. For short distances 0.25 mm² (AWG 23) should do, but under normal circumstances I would use 0.5 mm² (AWG 20) or 0.75 mm² (~AWG 18).
• Audio input:
• For each channel run one pair of wires to the source.
• Even with unbalanced (usually -10 dBV) signals connect one input at the signal source's ground.
• Use twisted wires if possible or at least run both wires close to each other.
• The polarity of the audio signal input is irrelevant.
• Display option switch:
• The display option input is ST2 on the AMBM-Ctl module. These inputs can be paralleled with all control modules by one 6 wire IDC ribbon cable with any length and any number of taps so that one set of switches can control the display options of all modules.
• The switch and pin numbers at ST2 are identical. Pin 1 (= switch 1) is marked on the PCB.
• There is no ground pin on the 6 pin header, that is important: A ground connection between the control modules would cause strong AC-containing equalization currents between the modules which again adds strong noise on the measured signals. Thus the switches' common ground must be connected at the power supply's ground by a separate wire.
• Last but not least the meter bridge's ground must be connected to the console's ground, preferably from the power supply ground to the console's chassis.

Other Reference Levels

As mentioned above, when you need another reference level for 0 dB display than +4 dBu or -10 dBV you can achieve it with 2 external resistors for each channel at its input pins, provided the reference level is higher than -10 dBV (= 316.2 mV_RMS). Connect these resistors in series with the AMBM's analog inputs. Each input wire requires one resistor. The values for these resistors can be calculated from:

R = (U_Source - U_Input) × 42 kΩ/V

Example:

• The reference level shall be +6 dBu (U_Source = 1.545 V)
• The AMBM-Ctl is configured for +4 dBu (U_Input = 1.228 V)
• → the difference is 0.317 V
• → the resistor values are 0.317 V × 42 kΩ/V = 13.3 kΩ

Tip: 13.3 kΩ is not a common value from the E24 series (but it is at least a value from the E48 series). To make it easier to use available resistor values, you can also use two unequal resistors per input. The following applies:

• For a precise setting of the reference level, the sum of the two resistor values must be correct, i.e. in the example 2 x 13.3 kΩ = 26.6 kΩ.
• For optimum common mode rejection, both resistor values must be identical.

This means that if no great demands on the common mode rejection in the system are required, it may be easier to achieve the reference level with two different resistors. In the example, for example, 12 kΩ + 15 kΩ = 27 kΩ, which is close to the desired 26.6 kΩ. Even with only one 27 kΩ resistor, only the common mode rejection would be further reduced.

Contrast Filter Windows

I recommend to use a gray filter glass in front of the LEDs. That makes the display darker, but it enhances the contrast in illuminated environments significantly. With the photo on the right I tried to illustrate that, but it does not really show the effect in practice.

You can buy well suitable acrylic display windows from TTV GmbH, even in customized sizes. A gray color with a transparency of 30% is a good compromise. A scratch resistant and non glare surface is favorable. Such a material is ANTIFLEX-AR, which is usually in stock as ANTIFLEX-AR 1220 with thicknesses of 1.5, 2 and 3 mm.

Power Supply

The AMBM can be supplied with 7 to 15 V. The typical maximum average and peak supply currents are:

(Note: On the workbench, 5 V is possible, too, but with very little or no headroom for cable losses and tolerances.)

Due to the multiplexed LEDs there is a strong ripple on the supply current with peaks up to 25% more then the maximum average current listed above, even when not all LEDs are on. This is also why the AMBMs' grounds must not be connected to each other anywhere else but at their power supply inputs, preferably at the PSU.

Documentation

• Circuit diagram AMBM-Ctl
• Circuit diagram AMBM-LED
• Dimensional Drawing AMBM-Ctl
• Dimensional Drawing AMBM-LED (Note: The LED bars are exactly centered between the screws at their ends.)
• Data sheet LED bar

Pricing and Availability

For this project, which was started some time ago, I now have the module sets in stock.

AMBM kit, consisting of

• 1 Control module AMBM-Ctl with all SMDs soldered, programmed, tested and calibrated
• 2 LED modules AMBM-LED with all SMDs soldered, tested
• 2 LED bars containing 51 dual-color LEDs
• 4 Pin headers, 2 x 13 pins (LED module connections)
• 2 IDC ribbon cables, 26 wires, 45 mm long (LED module connections)
• 1 Pin header, 2 x 3 pins (option switches)
• 1 IDC ribbon connector, 2 x 3 pins (option switches)
• 3 Screw type terminals (power supply and analog inputs)

  * A kit is regarded to be additional even when you order it somewhen later.
** Refer to How to order for details and excluded countries.

Let me know if you want single parts instead of 45 mm pre-fabricated IDC ribbon cables. And feel free to mail me for other questions.