This article explains what a signal generator is and why it is used. If you’re new to video, you’ll also want to know when it is necessary to use a signal generator for video production.
What is a Test Signal?
A test signal is one that is designed to test a system. Because there are myriad systems, there’s no one single test signal that will satisfy all requirements (it’s not as simple as running water through a pipe). Each system needs a specifically designed signal, and there are many kinds of tests that can be performed.
If you had a chain of video tools, like cameras, monitors, recorders, audio, switchers, etc., what would you want to test? Here are a few options:
- Sync, Frequency
- Signal Strength
- Signal integrity (no bit errors)
- Color and Luma information
- Audio properties
For each property, you have one or more test signals that are forced through your data pipeline, and measured at the other end (or ends). If it ‘matches’ (at least within acceptable tolerances), the pipeline is okay.
You might want to know, why not test with an actual video feed itself? If it looks and sounds okay at the end, the pipeline should be okay, right?
That’s like building an oil pipeline and testing it with expensive oil. No, you test it with water (unless you’re living in a country where water is more expensive than oil).
More seriously, you use monitoring systems like a waveform monitor, vectorscope, etc. to test your video feed. However, to test these monitoring systems, you need a standard test signal. Otherwise, how would you know if the errors you see are a result of the monitoring system or your video itself?
A standard test signal is well-known thing, with clearly defined scientific properties. It is the foundation on which all video systems build their fortresses. Without test signals, there is no video.
What is a Signal Generator?
A signal generator is a device that produces (creates from scratch) a test signal every time you need it. When it comes to video work, they’re called video-signal generators.
Cheap generators are limited by the kind of systems they support, the number of devices they support, or the kinds of test signals they can produce. The expensive ones tend to offer more features, and can handle more systems. It would take a thorough knowledge of digital signal processing to find the right signal generator for each workflow. That’s why you don’t see them in supermarkets.
Some important signal generator features are:
- Colorburst – to check color, and other color and luma testing mechanisms like the SMPTE Color bar, etc.
- Blackburst (for SD mainly)
- Tri-level genlock
- Sync Pulse Generator – to check synchronization, frequency, phase and other characteristics
- SDI Checkfield signal, standardized as SMPTE RP198 (1080p) which includes a PLL (Phase-locked loop) test signal to test signal lock and an equalizer test signal to check for bit losses due to long cable lengths
- Jitter test
- Bit errors or data corruption
- Vertical interval tests
- Different patterns, like the saw-tooth wave, sinusoidal wave, square-wave, etc.
- HDMI signal tester, for home networks
- Audio test generators
Remember, HDMI is a consumer format which allows for a bit more leeway when it comes to signals. SDI, HD-SDI and 3G-SDI are clearly defined signal standards, and the room for error is far lesser.
When you hear about manufacturers sending 4K or whatever over ‘SDI or HD-SDI’, they are using the cables and connectors, but not the standard. Big difference – it’s like using your camera to hammer a nail, but that doesn’t make your camera a hammer.
Do you need a video-signal generator?
Only if you’re setting up a network of video devices that need to perform in perfect sync, and under the rigors of broadcast.
For basic monitoring, you need only trust a monitoring solution like a scope. However, a chain of devices cannot be served reliably only with scopes. There are too many links in the chain, and each link should be tested systematically for weaknesses with good testing equipment. This means a solidly reliable video-signal generator.
If you don’t know whether you need a video-signal generator or not, you don’t need it. These things are usually only found with video engineers or instrumentation engineers.