Friday, March 9, 2018

Review: SSL 1900 O'Tool

Always wanted an oscilloscope integrated into your modular?  Euro modular users have the Doug Jones Design O'Tool, a handy scope with a little color LCD display mounted in a module.  Fortunately for us 5U guys, Doug Slocum took it upon himself to reformat some into MU format, and the result is the Synthetic Sounds Labs Model 1900 O'Tool.  Since the days of Keith Emerson, modular users have wanted to find a way to mount an oscilloscope and be able to conveniently route signals to it.  Problems with this have always included the size and weight of traditional scopes, their incompatibility with the panel formats and mounting methods that modulars use, and signal compatibility issues.  (Who has room for a Tektronix 464 in their case?  Or the $$$ for a Tek MDO3000?  Me neither.)

The O'Tool solves these problem neatly, and provides many more capabilities than your average Ebay-special analog Tektronix.  The O'Tool consists of a digital signal processing system coupled to a color LCD screen, packaged in a modular synth panel format.  It is powered from conventional +/- 15V power, and easily accepts the usual modular synth signal levels and types.  No heavy CRT, no high voltages, and no four-figure price tag.  Available functions consist of scope screens, voltage measurement, frequency measurement, signal level metering, and spectrum analysis.

This version of the O'Tool is physically packaged as a 1U wide Dotcom/MU format module.  When I received it, I was a bit concerned at first because the screen is pretty small, and my eyesight is not what it used to be.  However, the contrast and resolution are excellent, and I've had no trouble reading the screen.  The screen does take up as much of the width as could fit without structurally compromising the panel (which would make it difficult to package this in an MOTM-format module).  There are six input jacks, a pair for each of the input channels, and a pair for an external trigger signal for the scope modes.  Each pair is simply wired together; this allows them to be used to "patch through" a signal that needs to go somewhere else, so you can conveniently insert the O'Tool into a patch without needing a mult.

Underneath the screen is a row of four small pushbuttons.  The leftmost one selects the operating mode and screen to be displayed.  Pressing the mode button repeatedly cycles through the screens.  The other three buttons are "soft keys" whose functions vary depending on the selected screen.  Each screen has a small legend at the bottom showing what the soft buttons do in that screen.  The available screens are:
  1. Single channel voltage/time scope, displays channel 1 only
  2. Dual-channel voltage/time scope, with the two channel signals overlaid.  Channel 1 is displayed in red, and channel 2 in green.
  3. Dual-channel voltage/time scope, split screen.  Channel 1 is displayed in the top half, and channel 2 in the bottom half.
  4. Bar-graph averaging voltage display, described further below.
  5. VU/peak level meter
  6. Spectrum analyzer
  7. X-Y oscilloscope
  8. Frequency counter
  9. Digital voltmeter

Scope Mode Screens

In the first three screens, the three buttons allow the user to select the time per horizontal division, the displayed voltage range, and the trigger source and mode.  Pressing each button cycles through the available values (which can be a bit tedious in the case of the time/div setting since there are many possible values).  The screen is underlaid with a division grid, shown in dark blue, which appears behind the signal traces.  The available time per division settings range from 100 microseconds per division to 5 seconds per division.  There are six horizontal divisions across the screen, so at the maximum setting, the time for one screen sweep is 30 seconds.  The voltage range differs from oscilloscope convention, and from the time range, in that it applies to the entire vertical span instead of per grid division.  Available ranges are

  • Plus/minus 5V DC
  • Plus/minus 10V DC
  • Plus/minus 10V AC (DC signals/offsets are filtered out)
  • 0-5V DC
  • 0-10V AC

Here are some screen shots of the three scope mode screens.  (All of the photos from here through the end of this post were photographed from the actual screen.  I cropped the shots, and added some contrast enhancement in order to get rid of room light reflecting off of the screen; otherwise, the photos are unretouched.  The somewhat fuzzy look is caused by magnification of the photos, and the fact that I had to use the camera's digital zoom because I don't have a proper macro lens.  Bear in mind that these photos are larger than the actual screen.  All of the waveforms are from a Q106 VCO.)

This is the single-channel mode, showing a sine wave:

The dual-channel stacked mode, showing two waveforms from the same VCO.  Channel 1 is shown in red and channel 2 is in green.  Here, channel 1 (the sine wave) is chosen as the trigger signal.

The dual-channel layered mode, with the same two waveforms.

The same screen, but with channel 2 (the sawtooth wave) chosen as the the trigger channel  

The trigger can be set to trigger on either of the two input channels, or on the signal connected to the external trigger input jacks.  It can also be set to no-trigger mode, in which the scope free runs.

Triggers and Triggering Modes

The concept of triggering, for a scope in general, can be a bit difficult to understand at first.  The reason that scopes have triggered modes is to make the waveform "stand still" on the display.  Considering what would happen if the scope was free running; that is, if it scanned continuously.  Unless the waveform you are trying to display happens to be divisible by the scan rate, the wave won't be stationary on the screen; it will begin in a different place in its cycle on each scan, resulting in a display that jumps around.

To solve this problem, a scope has some sort of detection of a certain part or feature and then generates a trigger signal, not unlike the trigger signals that we use in our synths.  The trigger causes one horizontal scan to happen; after that scan is completed, the scope waits until it sees the trigger again, and then it does another scan and updates the display, etc.  By doing this, the scan always starts at a chosen point in the signal cycle, so that the displayed waveform remains stationary and you can actually look at it. 

The O'Tool can use either input channel as the source to the trigger detector, or it can use the signal at the "Trigger" input  There are five trigger modes:

  • Trigger 1.  Uses channel 1 as the trigger source.  If the ±5V or one of the ±10V ranges are selected, the trigger is generated when the signal crosses the horizontal axis in the positive-going direction.  If the 0-5V or 0-10V range is selected, the trigger is generated when the signal crosses 1.25V in the positive-going direction.
  • Trigger 2.  Same as trigger 1 except that it uses channel 2 as the trigger source.
  • Ext 0V uses the external trigger input as the trigger source.  The trigger is generated when the signal crosses the horizontal axis in the positive-going direction.
  • Ext 1V is the same except that the tigger is generated   when the signal crosses 1.25V in the positive-going direction.
  • No Trigger is a free-running mode; the scan runs all of the time, unsynchronized to the input signals.  

The trigger selection allows you to select either channel to be fed to the trigger generator.  You can even do this in the single-channel mode; you can display channel 1 and trigger off of channel 2.  The display may look different depending on which channel you trigger from.  Consider the screen shots of the two stacked-mode screens above.  In the top one, the trigger is on channel 1, so it triggers when the sine wave crosses the X axis going up.  The nature of the Q106 VCO (as with most sawtooth-core VCOs) is that the positive peak of the sine wave is where the positive peak of the sawtooth wave is.  So the top half starts with the sine wave heading up from zero towards its peak, while the sawtooth display starts with the last 90 degrees of its cycle.  In the second photo, we switch the trigger to channel 2. Now we are triggering on the positive-going zero crossing of the sawtooth, which is pretty much instantaneous.  So we see the display start with both of the waveforms descending from their peaks.  

Level Displays

The bar graph display is interesting but kind of hard to describe.  Basically, what it does is show how much time -- what percentage of the cycle -- a signal spends at a given voltage level.  The more the signal is at that a given level, the brighter the bar will be at that level.  In the shot below, channel 1 is a square wave and channel 2 is a sawtooth.  The square wave, of course, alternates sharply between the positive and negative peaks; hence the two discrete bars.  The sawtooth falls linearly and so all of the voltage steps get the same saturation, resulting in a spread of evenly lit bars.  (Not sure why the top one is a bit dimmer; may have to investigate how the sawtooth waveform is looking coming out of that VCO.)  The display range can be adjusted, and "fast" or "slow" averaging can be selected.

The VU and peak level meters do what you expect them to: display the average and peak voltage level of an alternating signal.  The display shows VU and peak levels for both of the input channels; the VU displays are grouped on the left, and the peak displays on the right. The VU indicators appear to be a true RMS measurement, as they display identically to the peak levels when a sine wave is input.  I cannot say, however, that the ballistics of a true VU meter are emulated properly; I don't have any means to measure it.  There are three selectable scale modes, which effect what reference level is used for the meters, and how the scale on the peak meters is displayed.

Like many such meters, the display uses color bars to display different regions of the measurement levels.  The blue horizontal line indicates the reference signal level (the level that is considered a "100%" signal) for whatever scale mode is in use.  Levels below and at the line are displayed using green bars.  Above the line, on the peak side, the first three steps are displayed using yellow bars, and levels above that are displayed with red bars.  On the VU side, all levels above the line are displayed using red bars.

This screen has three modes, which effect where the "100%" line is, and how the peak display is scaled.  The modes are:
  • +4dBu: In this mode, the VU scale conforms to the standard recording industry definition, in which zero VU = +4 decibel volts RMS, or dBu.  This in turn is defined as 1.228 volts RMS.  (It's defined at 1000 Hz, but that is not supposed to matter across most of the audio range.  I'll have more to say about this further down.)  The peak scale displays dBu and the blue line will pass through +4 on that scale.  (It always passes through zero on the VU scale.)  Red bars on the peak scale start at +8 dVu.
  • +2.5V: In this mode, zero on the VU scale corresponds to 2.5V RMS.  The peak scale will be re-scaled to show voltages up to 10 volts, and the blue line will pass through the 2.5V mark.  Red bars on the peak scale start between the 3.5V and 5V marks.
  • +5V: In this mode, zero on the VU scale corresponds to 5V RMS.  The peak scale will be re-scaled to show voltages up to 10 volts, and the blue line will pass through the 5V mark.  Red bars on the peak scale start between the 7V mark and the 10V mark.
An issue with the VU/peak display is that it does some preliminary high-pass filtering before it processes the signals for display.  This is common for VU meters; it prevents a DC offset in the signal from creating a false high reading.  However, it prevents the meters from working properly with low-frequency signals.  If you want to look at levels from an LFO, use the bar graph display, or one of the scope screens.

X-Y Display

The X-Y display emulates a feature of many of the old analog scopes, in which the X-axis, which is normally controlled by the scope's time base, can instead be driven by an external signal, producing two-dimensional patterns on the scope screen.  In this implementation, channel 1 drives the X axis and channel 2 drives the Y axis.

The old analog scopes depended on the persistence of the display phosphor for the user to be able to perceive the drawn figures.  The O'Tool attempts to emulate that with a setting that defines the "persistence" of each dot drawn; the dot is removed from the display after the equivalent of what would be that amount of time has passed, which determines how long each part of the figure remains on the display (which is, of course, also a function of the frequencies of the two waveforms driving the display).  To my eye, it doesn't work all that well; the continuously redrawn form is hard to perceive at faster settings, and it quickly fills the entire screen at slower settings.  The photo below was taken at a 1/15 second exposure and captures more of the drawn figure (which is made from a triangle wave driving the X axis and a sine wave on the Y axis) than was visible to the eye in real time.

Spectrum Analyzer

The spectrum analyzer surprised me with how well it works.  The update rate is pretty fast, and it seems to not have much of a problem with quantizing noise.  It has two display modes, "linear" and "log".  In the linear mode, there are four frequency ranges available, with a choice for the upper end of 20, 10, 5, or 2.5 KHz.  Vertical scaling is relative, but you can choose from 1x up to 4x.  If, in one of the higher vertical magnifications, one of the peaks exceeds the vertical range, the peak displays a red top, as you can see in the shots below.

This one is with a square wave on channel 1 (top) and a sawtooth on channel 2 (bottom.)  On the square wave, you can see the odd-harmonics pattern typical of square waves. 

This one is with a 25% pulse wave on channel 1, and a triangle on channel 2.  Notice how little harmonic content the triangle has. 

I have found the log mode to not be as useful in general, because it groups all of the frequencies into octaves and displays one bar per octave.  Depending on the range setting, it displays between 5 and 7 octaves.  Here's an example; unfortunately, I forgot to write down what waveforms I was using for this shot.

Frequency Counter

The frequency counter is straightforward and works well.  You can select channel 1 only, channel 2 only, or both.  There are three elements on the display.  At the top is the frequency, in Hz, for each channel.  Not having a calibrated frequency source, I can't really speak to whether these are actually precise to two decimal places.  In the middle, it displays the closest equal-tempered note for the frequency of each channel, and how far away in cents it is from the ideal equal-tempered value.  The bottom portion shows this deviation graphically.

For the note display, you can select concert A to be either be 440 Hz (the usual standard) or 432 Hz.  There is a lot of nonsense surrounding 432 Hz tuning; there's nothing special or magical about it.  Prior to the 20th century, orchestras were all over the map as to what standard they tuned to; Bach, Beethoven and Haydn are thought to have used an A of about 422 Hz.  Nonetheless, if you want to try something different, and you're using the O'Tool to tune instruments, you can give 432 Hz a try.  Note that some polysynths may not be capable of being tuned this far off of 440 Hz.


\The last display is a simple voltmeter, displaying the voltage present on each channel.  Only DC voltages can be displayed.  Keep in mind that the O'Tool is (in this case) being powered from a 15V supply; it most likely cannot display voltages exceeding the supply rails, and trying to do so could potentially damage it.  (I haven't tried.)  So don't use it to check the supply voltages on your Roland System 700.


The O'Tool is a useful thing to have in your setup.   And it looks cool.

No comments: