User`s manual

User’s Manual v.13 QVidium
®
H.264 HD Video Codecs™
Copyright 2011-2013 QVidium
®
Technologies, Inc. Page 36 of 67
5.5 Configuring Packet Transport & Error Correction
This section explains how to configure the video transport and error correction capabilities of the
QVidium H.264 HD Video Codec™ and how to configure its decoder for proper operation of these
error correction mechanisms. Section 5.5.1 describes the configuration of the decoder’s network
transport and Section 5.5.2 describes the ARQ error correction mechanisms of the QVidium H.264
HD Video Codec™.
5.5.1 Configuring the Decoder’s Network Transport
Within the Network Parameters section of the decoder profile, select among ARQ, RTP, or
UDP packet transport. Also, specify the UDP port number. The Multicast Address specifies
the IP multicast address, if any, to listen to for the Video/IP stream. If expecting a unicast video/IP
stream, leave this field blank. The decoder interprets the video and audio signals as UDP/IP
packets in all cases, regardless of the type of packet transport you select. Specifying UDP treats
the packet payload as raw MPEG-2 Transport Stream data. All the other transport types will
assume that the incoming packets have an RTP header at the start of the payload, as per
IETF/RFC 2733 and related standards.
The network interface to the decoder includes QVidium’s patented synchronized network de-jitter
buffering mechanism. In the decode interface, you may specify a delay for this de-jitter buffer in
milliseconds. When using ARQ or RTP network transport mode, the de-jitter buffer holds the
specified number of milliseconds of packets to give the decoder ample time to account for random
delays and to allow the decoder to correct packet ordering. The decoder uses RTP sequence
numbers of incoming packets to insert incoming packets into their proper place in the de-jitter
buffer before releasing those packets into the MPEG decoder. The de-jitter buffer also uses the
RTP timestamp to synchronize the decoder’s network clock with the video source to adjust the de-
jitter buffer hold time.
In addition to the de-jitter packet buffer, you can specify the size (in milliseconds) of the decoder’s
low-level video data buffer. In contrast to the de-jitter buffer, this video input buffer always exists,
even when error correction is disabled. It buffers out variations due to both network-induced jitter
and operating-system jitter, to insure that the decoder never runs out of video or audio data,
despite variations in CPU processing and packet arrival times.
5.5.2 ARQ: Automatic Retransmission Request
Automatic Retransmission Request (ARQ) tries to recover any packets lost during transport to the
decoder by adding a small amount of delay at the decoder during which time the decoder would
have time to detect and request any missing packets. The size of this delay should also include
adequate time for the missing packet to be received and inserted into the play out queue so that
the video stream can continue to flow smoothly and unimpeded to the MPEG decoder.
To enable ARQ, you must first select ARQ transport from the Profile dialog. Also, ARQ
transport must be enabled at the encoder. With ARQ selected and the decoder started, the
decoder will watch for gaps in the RTP sequence numbers from the output of the jitter buffer.
When it detects missing packets, it sends retransmission requests to the encoder so that the
encoder can quickly resend any missing packets. Incoming retransmitted ARQ packets bypass the
network de-jitter queue and find their correct place in the ARQ buffer, so that by the time packet
are output by the ARQ buffer, the output of the ARQ buffer should no longer contain any missing
packets.
ARQ has three parameters that can be configured in manual mode: Round-Trip Time,
Number Retries, and Burst-Drop time. In automatic mode, the system automatically