How to plot real time EMG signals without delay?

[Igor Muniz]

Hi guys,

I'm trying to plot a real time signal with python and PyQtGraph has improved the speed a lot, but still with some delay. The data comes from a DLL and it's store in a NumPy Vector.
I wan't to plot this buffer every 0.1s with 100 points(the signal has 1kHz sampling rate). At the beginning this seems to work but after few seconds starts to get behind the plot.

This is part of my code:

def coleta():
    global cnt
    global t

    nsample = c_ulong(int(0.1 * sampleRate.value))

    status = emgSampleNSeconds(handle.value, 50)

    if status == 0:
        print("Inicia a amostragem")
    else:
        print("Não foi possível iniciar a amostragem")
    emgIsSampling(handle.value, byref(isSampling))


    canal = c_ulong(0)
    emgbuffer = (c_double * nsample.value)()
    



    emgDataPlot = np.zeros(10000, dtype=float)

    curve1 = p1.plot(emgDataPlot, pen='r')
    sleepVar = 0.1
    pltStart = pg.ptime.time()
    while isSampling:
        time.sleep(sleepVar)
        emgGetRealUnitDataBlock(handle.value, canal.value, emgbuffer, byref(nsample))
        now = time.time()
        for value in emgbuffer:
            emgDataPlot = np.append(emgDataPlot, value)
            if len(emgDataPlot) > sampleRate.value * 10:
               emgDataPlot = np.delete(emgDataPlot, 0)

        if t == 0:
            p1.enableAutoRange('xy', False)  ## stop auto-scaling after the first data set is plotted

        t += 1

        curve1.setData(emgDataPlot)
        app.processEvents()

        #print(t)
        emgIsSampling(handle.value, byref(isSampling))
        
    #p1.enableAutoRange(True, True)
    print(pltStart - pg.ptime.time())

What I need to do?
Thanks.

[vas...@gmail.com]

Hi.

I think that you need to put all pyqtgraph and other graphic stuff (including window initialization, processEvents etc.) in a separate thread and to plot a shared data you have every 0.1s. Look at FuncThread class at bottom of page http://softwareramblings.com/2008/06/running-functions-as-threads-in-python.html, could save you a time. So, one thread for graphic stuff, one or calculation.

Good luck,

Vasilije

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[vas...@gmail.com]

.replace('one or calculation', 'other one for a calculation')

[vas...@gmail.com]

You could start with something like this, just use collectData function to calculate your data and put it to self.collected_data variable:

import pyqtgraph
import numpy
import threading
import time

class FuncThread(threading.Thread):
 def __init__(self,target,*args):
  self._target=target
  self._args=args
  threading.Thread.__init__(self)
 def run(self):
  self._target(*self._args)

class MainApp(object):
 def __init__(self):
  plot_loop_time = 0.1
  self.sleep_time = 0.001
  self.number_of_points = 100
  self.collected_data = numpy.zeros([self.number_of_points])
  app = pyqtgraph.mkQApp()
  self.graphics_layout_widget = pyqtgraph.GraphicsLayoutWidget()
  self.graphics_layout_widget.show() 
  plot = self.graphics_layout_widget.addPlot()
  plot.setRange(yRange=(-200,200),disableAutoRange=True)
  cd = FuncThread(self.collectData)
  cd.start()
  current_time = time.time()
  while self.graphics_layout_widget.isVisible():
   time.sleep(self.sleep_time)
   plot.plot(self.collected_data)
   app.processEvents()
   while time.time() - current_time < plot_loop_time:
    time.sleep(self.sleep_time)
   plot.clear()
   current_time = time.time()
  
 def collectData(self):
  while self.graphics_layout_widget.isVisible():
   self.collected_data += numpy.random.uniform(-1, 1, size=self.number_of_points)
   time.sleep(self.sleep_time)

MainApp()