How to reduce delay of QueueOutputData() during session

I am trying to queue some output data during a session. I am using s.startBackround() to collect data in real time and an event listener to queue some output data if a certain condition is met. The delay between when the queueOutputData() is called and the actual time when it outputs the data is about 40ms. I am trying to reduce this delay as much as possible. How can I do this? Is there a way to queue the output data before the function handle, and if it is in the function handle, immediately output this data?

Below is my code:

% initialize session
s = daq.createSession('ni');

% analog inputs
addAnalogInputChannel(s,'Dev1', 'ai1', 'Voltage');
addAnalogInputChannel(s,'Dev1', 'ai2', 'Voltage');
addAnalogInputChannel(s,'Dev1', 'ai3', 'Voltage');

% analog outputs
addAnalogOutputChannel(s,'Dev1', 'ao0', 'Voltage');
s.Rate = 80000;

% not sure how to not start background session without this
data0 = zeros(1,40001)';
data0(end) = [];
queueOutputData(s,repmat(data0, 5, 1));
s.IsContinuous = true;

% event listener where output data can be queued
lh = addlistener(s,'DataAvailable', @kick);

s.startBackground();

s.wait(1000);
stop(s);

handle function:

function kick(src, event)

          % collects all data in single file
          fp = fopen('DATA.bin', 'a');
          X = event.Data;
          T = event.TimeStamps;
          fwrite(fp, [T, X]', 'double');

      if (X(end,1) > 0 && X(end,1)<2.5)
              % row of zeros to determine when queueOutputData() was called
              fwrite(fp,[0 0 0 0]','double');

              % create and queue output data
              outputSingleValue1 = 9;
              outputSignal = [outputSingleValue1*[0:0.01:1 1:-0.01:0] zeros(1,38000)];
              outputSignal(end) = [];
              queueOutputData(src,outputSignal');
      end

          fclose(fp);

   end

Answer 1

[Walter Roberson]

To improve performance, do not fopen in a loop: fopen before you designate the callback, and pass the fid to the callback. Also, since your output signal is constant, pre-compute it.

outputSingleValue1 = 9;
outputSignal = [outputSingleValue1*[0:0.01:1 1:-0.01:0] zeros(1,38000)] .';
outputSignal(end) = [];

fp = fopen('DATA.bin', 'a');
lh = addlistener(s,'DataAvailable', @(src,event) kick(src, event, fp, outputSignal));

function kick(src, event, fp, outputSignal)
  % collects all data in single file

    X = event.Data;
    T = event.TimeStamps;
    fwrite(fp, [T, X]', 'double');

    if (X(end,1) > 0 && X(end,1)<2.5)

      % row of zeros to determine when queueOutputData() was called
      fwrite(fp, [0 0 0 0]', 'double');

      % queue the pre-built output data
      queueOutputData(src,outputSignal);
    end
  end