EN5990 Encoder for MPEG-4 Part 10
18
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Latency associated with the transmission medium and decoder
- this
cannot be varied by any of the other parameters associated with the
Encoder.
5.5
How do I Transmit Transport Stream over an IP Network?
The product has an optionally fitted card that can output the internal Transport
Stream (TS) over an IP/Ethernet network. This option can be purchased as part of
the unit with the marketing code EN5900/HWO/IPTS.
The card implements RFC 2250, which specifies the carriage of MPEG-2 TS over
IP using unicasting or multicasting. DVB-IPI has enhanced the reliability of the
UDP transmission by defining a forward error correction (FEC) scheme to handle
the loss of IP packets without loss of the underlying TS. More detail on the
operation of this card is given in the
Reference Guide
.
Alternatively an external IP streamer or ASI adapter can be used. A range of
suitable products is available from TANDBERG Television.
5.6
Why are There no IDR Pictures in the Video Stream?
An IDR (instantaneous decoding refresh) picture is a type of I picture which states
that no picture after the IDR may reference a picture prior to the IDR. An IDR can
always be used for random access into a stream, however it is possible and
desirable to have random access points which are not IDRs. The reason for this is
that if every random access point is an IDR, it would restrict the way in which B
frames could be used. Consider a random access I picture which has an
associated B frame that follows it, in encode order, but precedes it in display order.
This I picture cannot be an IDR as the B picture following it references pictures
prior to it. However this I picture is still a valid random access point as no picture
following it in display order ever references a picture preceding it.
All I pictures from TANDBERG TV encoders can be used for random access, this
is signaled by the Random Access Indicator flag in the Adaptation Field of the
video transport stream packet. When this flag is set, there will be a PES header
starting at that packet and the decoder can start to decode the video stream. The
presentation time stamps of the following frames need to be monitor to ensure that
the following B frames are not displayed.
If this is still not sufficient for the decoder, there is a menu switch in the video
encoder menu which forces the video encoder to switch off B frames at the end of
each GOP so that the I frame can be identified as an IDR. This is at the expense
of video encoding quality.
