This article explains what a scope (oscilloscope, waveform monitor) is, and whether or not you need one for video.
This article builds on Driving Miss Digital, where we looked at analog and digital systems, waves and signals.
What is an oscilloscope?
A scope is a testing instrument that looks at stuff in a signal and displays it as a graph. The name comes from two sources:
- Oscillating wave
- Scope – something to look through
Visualize a person looking into a microscope at a video signal.
The most important part of a signal is the voltage amplitude, and this is a slippery customer. On most signals, it changes frequently over time. Therefore, the x-axis of any scope is usually time.
Interestingly, the first CRT oscilloscope was made by Karl Ferdinand Braun in 1897, while the first CRT TV was demonstrated only in 1925 by John Logie Baird (Source: Wikipedia).
The scope we are most interested in is the DSO, or digital storage oscilloscope.
What is a DSO?
From the words ‘Digital Storage’ you might have gathered that:
- The signal ‘chain’ is supposed to be digital
- The signal is ‘stored’ for some time
There are also analog scopes, which are usually CRT based, but these technologies have almost evaporated from professional video use (though they are still important scientifically), so there’s no point talking about them. For the rest of this article, scope means a digital scope.
Why would you want to store voltages? Well, if you have a signal with rapidly changing properties, the scope without storage ability will tend to show the most current signal. That’s not very helpful if you wanted to look at the signal from two seconds ago, or whatever.
Digital technology makes it easy to save the signals on a temporary drive or media so you can look at it at your own sweet time. Just like cameras, you can’t run out of media space!
The heavyweight champion of the scope world is Tektronix, the first manufacturer of a calibrated oscilloscope.
What can a scope do?
A scope can do some or all of these (don’t worry if they don’t make sense):
- Read BNC (SDI, HD-SDI, 3G-SDI, etc.) signals.
- Read HDMI signals.
- Display signals ‘as-is’ on the LCD monitor.
- Allows you to manipulate the way you want to view the signals, by varying scale (think of it as zooming in or out of a wave).
- Be triggered automatically or at a set event.
- Compare signals. E.g., the incoming signal to a test or predefined signal.
- Allow the use of probes for low level signals – probes are special tools that can do extra stuff, like measure current, magnetic flux, etc.
- Has graticules for easy viewing and ball-park measurement.
- Amplifies (if necessary) and samples the signal according to the Nyquist criterion.
- Change the sweep speed (time-base for video).
- Give you minute control over every aspect of measurement.
- Years of calibrated, dependable, trouble-free use.
Here’s some of the stuff it might not do:
- It can’t make your signal better or worse, it just shows you the signal as-is.
- It won’t show you ‘unscientific’ properties, or help you understand in ‘layman’s speak’. You really have to know your signal processing.
- Cheap scopes don’t offer much control, and won’t be consistent in their use. They also won’t be able to read signals slightly out of their ‘bounds’. The expensive ones will, though.
- Make things easy for you.
Do you really need a scope?
The answer is simple. Ready for it? If you don’t know the difference between an Ultrascope and a Tektronix or Leader, you don’t need a scope.
If you feel you need a scope for whatever reason, make it your priority to learn signal processing. Little knowledge is dangerous. Good scopes are scientific tools that serve specific functions, and they’ve been around for more than a hundred years.
For specific scope and waveform monitor advice, check out the Chapter on External Monitoring in the Comprehensive Rigging Guide.
Don’t take oscilloscopes lightly, and never rely on ‘scope’ advice from a non-engineer or a manufacturer.