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F612 2014

Posts: 19
Just sent this email to Sandra Marsh about the most recent Signal Processors paper. If anybody (!) has any thoughts.
Thanks for a very enjoyable Electronics F612 Signal Processors exam (well, at least from my point of view). It was satisfyingly analytical, which is quite rare for AS, but that may not be to the student’s taste.
Either way, I’m really sorry about this (I became a pain to Mark Judge as well). However, could you put the following two queries to the marks standardisation meeting.
5 (a) (ii) There seems to be a flaw in this question.
I assume that the setter expected the first stage of the answer from students to be X = nC.nB.nA + nC.B.nA + nC.B.A, which can be reduced to X = nC.B + nC.nA, as required by the question. However, as the truth table is not complete a number of other solutions to X are possible which will not reduce to X = nC.B + nC.nA (for example X = B + nC.nA).
Are you accepting these other possibilities for X for the first mark? And how, if it arises, do you intend to deal with this mismatch for the second mark? Should this mark be discounted?
5 (a) (iii) As with part (ii) will you accept alternative logic solutions for this which are not equivalent to X = nC.B + nC.nA ?
7 (c) and (d) There is also appears to be quite a bad flaw in this question.
This flaw results from the time delay between operations inherent in the PICAXE microcontroller. The PICAXE is used to teach this section and is the only microcontroller which can be programmed to use flowcharts like this.
In (c) the problem arises from the moment that any key is recognised to be pressed. This passes control to ‘C’, and through two operation boxes to ‘D’ and then through another input box (let S3 = input).
This means that the user has the time taken for the program to go through about three operation boxes before the important input code (VWX pressed) is checked by the program. If each operation box takes 4ms (as used to be quoted by Michael Brimicombe) then the user has about 3x4m=12ms to get their fingers in place. This would be a struggle for even the most agile user. If, on the other hand, zero time is assumed for each operation box then unlocking the door would be impossible as it would be impossible to simultaneously press three keys (in fact this is a common problem with student microcontroller projects).
(I don’t know how you intend to allocate the five marks in this section but would be interested to know as I can only see three obvious marks.)
In (d) the problem is the opposite to part (c). A frequency of 250 Hz for the sound means that microcontroller output O3 needs to produce a square wave with a period of 1/250 = 4ms. This means that O3 needs to be low for 2ms then high for 2ms repeatedly.
If the student assumes zero time period operation then two 'Pause 2' boxes could be added, which I guess is what the examiners expect. However, if a student assumes that the microcontroller takes more than 1ms to complete each operation (as the real ones seem to) then generating a frequency of 250 Hz is impossible using a flowchart.
This means that (c) depends on there being a reasonably long time for each operation and (d) depends on there being almost no time for each operation. The two questions therefore conflict.
A time delay should either be stated to be acknowledged or discounted in exam questions (perhaps this information should go on the front of the exam) and the questions written accordingly.
However, until such time how are you intending to deal with the (admittedly rare) answers that are going to come from students who realise the problem and have struggled to come up with solutions to this?
Once again, sorry about this. I do it every year.