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IPTV can suffer from
a wide range of
problems affecting
quality. The
demonstrations below
illustrate several
types of impairment,
for additional
examples visit the
IPTV Troubleshooter
web site.
IPTV is encoded
using MPEG-2 or
H.264 video codecs.
These codecs send
intra-frame encoded
"I" frames at
regular intervals
(typically every
500ms); I frames can
be completely
decoded without
reference to other
frames.
Between the I
frames, motion
encoded P and B
frames are sent.
I frames are
generally large, and
may take several
hundred IP packets
to transmit, whereas
P and B frames are
usually smaller and
take much fewer
packets.
If some of the
packets that carry
an I frame are lost,
this will degrade
the quality of that
I frame and also the
quality of all the
following P and B
frames that are
based on this frame.
When the next I
frame is received
this error extension
will stop.
This means that a
packet loss that
impacts an I frame
could cause errors
that persist for
500ms or more and
are hence quite
visible.
Losses occurring in
B frames will not
generally lead to
error extension and
hence will last only
for one frame (1/30
or 1/60 seconds).
Losses in P frames
may lead to some
error extension
however this will
not last for such an
extended period as I
frame loss.
There are two
example video clips
below - both of
which have the
same packet loss
rate. The
quality of each clip
is quite different,
which illustrates
why the packet loss
rate is not a
good measure of
video quality.
Telchemy's VQmon/SA-VM
and VQmon/HD video
analysis technology
does analyze
the impact of packet
loss on specific I/
B/ P frames and
would report Example
1 as having a lower
MOS score than
Example 2.
These example video
files can be played
using VLC -
obtainable here
Example 1: Packet
loss in I Frames
Packet loss may
occur in I frames
due to network
congestion. I
frames are much
larger than P or B
frames and hence
bandwidth spikes can
occur during the
transmission of an I
frame.
This example shows a
video sequence with
1% packet loss
occurring in
I frames.
Example 2: Packet
loss in B Frames.
Packet loss may
occur in B frames
due to deliberate
discard within the
network. As B
frame losses are
less visible, the
packets carrying B
frames are sometimes
marked as "discardable".
This example shows a
video sequence with
1% loss occurring in B frames.
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