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Variable Time Delay and Variable Transport Delay - what is their main difference?

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Dear fellow Matlab Simulink users. I've really taken time to gather information about this Variable Time Delay and Variable Transport Delay function (for simulink). I'm just trying to find out what is the main difference between the features of each of these blocks. I realise that they're the same function block, but with a user settable feature that allows us to set its property - of either Variable Time Delay, or Variable Transport Delay. I understand the concept of buffering etc. But I don't understand what the difference is between these two blocks. Both appear to have a settable time delay (for input). Both appear to do the same thing. I've read Matlab details about this block, and all I understand right now is that both provide a time delay. Is there an example where one mode should be (or shouldn't be) used over the other?
The Matlab help for these block says "Apply a delay to the first input signal. If the delay type is variable time delay, the second input specifies the delay time To. The block implements the function y=u(t-To(t)). If the delay type is variable transport delay, the second input specifies the instantaneous delay time Ti at the input."
So, my understanding (from reading that help description) is that the Time Delay block provides a TIME DELAY, while the Transport Delay block provides an INSTANTANEOUS TIME DELAY. Both are time delays, so what's their point?
I have attached a very small simulink model that uses both the Variable Time Delay and Variable Transport Delay blocks. Each block delays a sinewave by half a second. I'm really trying to understand what conditions should I not use a particular block (ie. Variable Time Delay or Variable Transport Delay). Thanks for your help in advance!! Thanks very much!

Accepted Answer

Hari Desanur
Hari Desanur on 14 Nov 2016
The basic idea is that both blocks implement a delay but in slightly different ways:
- For the Variable Time Delay, the delay is directly used (input To).
- For the Variable Transport Delay, the delay is computed by solving an equation involving the Ti input. This helps to smooth the output and avoid instability.
Refer to the two demo examples 'sldemo_VariableTransportDelay' and 'sldemo_VariableTransportDelay_pipe', which show how you can use the Variable Transport Delay block. By switching the parameter of the block from Variable Time to Variable Transport, you can see that the results are different, the results of the Variable Transport being smoother.
  1 Comment
Kenny on 15 Nov 2016
Hello Hari! Thanks for your time and for helping me understand the difference between the features of these delay components. Very much appreciated. I've marked your answer as accepted answer. Thanks again for your help. Best regards.

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