recording with pyaudio giving junk values even if MIC is muted or in pin drop silence - python-2.7

Question

I am working on recording using pyaudio on windows. I am plotting recorded sound using matplotlib.
Recording length is 60 sec.
buffer size is 1024
What i am getting is, while recording, for first few iterations i am getting junk values. After that it starts recording actual sound.
I also found that, even if the MIC is muted, its giving junk values wjen i plot it.
These junk values are affecting result of my computations.
Any idea, why this junk values/sound is getting recorded?? Any quick solution??
Code:
class record(Thread):
def __init__(self):
#Thread.__init__(self)
super(record, self).__init__()
self.lock=threading.Lock()
self.project=projectutils.getActiveProject()
self.outfile=self.project['name']+'.wav'
self.tw=tool.getToolWindow()
self.duration = 60 #record for 1 second. Pretty long duration don't you think
self.buffer=1024
self.pin = pyaudio.PyAudio()
self.channels=2
ccare.rate=self.rate=8820
self.format=pyaudio.paInt16
self.inStream = self.pin.open(format=self.format, channels=self.channels, rate=self.rate,input=True, frames_per_buffer=self.buffer)
self.flag=1
self.out = []
self.upper_lim = self.rate / self.buffer * self.duration
def run(self):
ccare.val=[]
x=[]
if not self.inStream:
return
self.lock.acquire()
data = self.inStream.read(self.buffer)
self.lock.release()
x=list(struct.unpack("%dh"%(len(data)/2),data))
self.lock.acquire()
ccare.val=ccare.val+list(x)
self.lock.release()
self.out.append(data)
for i in xrange(1, self.upper_lim):
x=[]
if not self.inStream:
break
data = self.inStream.read(self.buffer)
x=list(struct.unpack("%dh"%(len(data)/2),data))
self.lock.acquire()
ccare.val=ccare.val+list(x)
self.lock.release()
self.out.append(data)
if self.inStream:
self.inStream.stop_stream()
self.inStream.close()
self.pin.terminate()
self.save_file()
Simple Code:
import pyaudio
import wave
import struct
val = []
def record(out_file):
duration = 60 #record for 1 second. Pretty long duration don't you think
buffer=1024
pin = pyaudio.PyAudio()
channels=2
rate=8820
format=pyaudio.paInt16
inStream = pin.open(format=format, channels=channels, rate=rate,input=True, frames_per_buffer=buffer)
out = []
upper_lim = rate / buffer * duration
val=[]
x=[]
if not inStream:
return
data = inStream.read(buffer)
x=list(struct.unpack("%dh"%(len(data)/2),data))
val=val+list(x)
out.append(data)
for i in xrange(1, upper_lim):
x=[]
if not inStream:
break
data = inStream.read(buffer)
x=list(struct.unpack("%dh"%(len(data)/2),data))
val=val+list(x)
out.append(data)
if inStream:
inStream.stop_stream()
inStream.close()
pin.terminate()
The values stored in 'val' variable will be plotted in different thread using matplotlib.