# What does the term “railed” mean in signal processing?

I'm having trouble finding a definition of "railed" that relates to signal processing.

Am I correct in my guess that this term is in fact from this field?

My signal data comes from an EEG device. The lightly documented open source software I'm using doesn't define it, but it shows that term when there is no signal data being displayed.

Is that all it means (no data)? Or does it mean something like the signal being read is too great to be displayed or correctly measured?

A railed signal, or a railing signal, seems to indicate a flatline. On BIOPAC, Railing signal (flatline) says:

When the amplified signal for any given channel exceeds the range -10 to +10 volts, the signal will rail. You will see a straight line at -10 or + 10 volts (more likely the reading will be close 9.99 volts). The MP system is designed to work only in the range -10 to +10 volts. The signal could rail for several reasons (which are not exclusive)...

With respect to analog signals, a “rail” is a boundary that a signal has to work within.

• Good, You'll find some reasons at the end of the link I gave. I'll upvote the other answers – Laurent Duval Jun 3 '19 at 17:46
• To me, 'flatline' means no signal, but in range (say, 0 V DC input). Railing, on the other hand, indicates that your signal is out of range - too big, too much offset, etc. – alex.forencich Jun 5 '19 at 6:38
• To me as well, even with background noise. However, note that in the graph you have no $y$ axis, so one cannot tell – Laurent Duval Jun 5 '19 at 11:06

Just a guess.

From analog electronics, amplifiers typically have a DC voltage that supplies power to the circuit. The amplified output is typically limited to that voltage. When an output is clipped, it has been said that the output is at the rails.

Not really a dsp term but is a way to say that a waveform is clipped via the dynamic range of the system.

• Thank you! This sounds accurate and matches the other info that was also just posted by Laurent then myself. :) – Hack-R Jun 3 '19 at 17:33

The answer given by @LaurentDuval is correct. I just wanted to also post the explanation I found in an ancient forum post right before reading his answer:

[Signal processing software] shows "Railed" when microvolt magnitudes for the channels are off the top end of the scale. In other words, generally it means something wrong with the differential voltage measured between the channel and the reference. Either one could have a poor connection leading to "Railed".

One way to check consistency is to connect all three leads together, SRB2, Bias, and the channel(s) you are trying to measure. You can do this with a glob of Ten20 paste or alligator clips. Once you have all leads connected, that should produce 0 microvolts, or close to it.

The term comes from the power "rails" or buses that provide the positive and negative voltage supply for an operational amplifier. (Op amps are typically used for preamplification of the raw signal before analog-to-digital conversion). If the input signal is so low or high that the amplifier's output goes as far as it can to the full positive or negative supply voltage, then it is said to be "at the rails" or "railed".