Live Encoding - A Practical Guide to Video and Audio Compression - page 271

Setting up a triangulated feed with an alternate leg will be important if your service is a
high-profile and popular broadcast.
Your delivery to the end user may suffer glitches in the connection from their client
to the service. Those service interruptions would only affect that user. Service interrup-
tions on your incoming master feed will affect all users. The amount of time, effort, and
cost you put into these links will vary accordingly.

15.8 Encoding Latency

Processing groups of pictures introduces a delay because the GOP should be processed
from end to end before any of it is delivered for output. This allows the maximum encod-
ing benefits to be gained.
Because of the way that video compression works, there is a built-in latency, which
cannot be any less than the length of a GOP. Arguably, you might allow that latency to be
reduced to the distance between an I-Frame and a P-Frame (see Chapter 10 for details of
GOPs and frame types).
The H.264 (AVC, MPEG-4 part 10) codec has an even larger potential window for
complex macroblock dependencies, and the motion-vector computations could be based
on a macroblock that is a large number of frames in the past or future relative to the cur-
rent frame being coded. This increases the latency.

15.8.1 Rate-Control Buffering

The rate-control process introduces a delay because it needs a buffer to measure in order
to generate the throttling values for the quantization scale factor.

15.8.2 Consequences of B-Frames

Once you introduce B-frames, the bi-directional nature increases the latency because the
look-ahead buffer must see a larger number of frames into the future in order to compute
the bi-directional difference information and to reorganize the frames into the correct
transmission order before loading them into the output bit-stream buffer.
Eliminating B-frames altogether allows the GOP to be assembled in transmission
order and the latency to be reduced to a minimum.
Latency is the delay between the input video arriving and the output bit stream
being transmitted and then received at the client playback device. Zero latency is achieved
by eliminating B-frames altogether at the expense of coding efficiency.

15.8.3 Latency in Broadcasting Systems

The complexity of your content pipeline will also introduce some latency in addition to
that caused by the coding process. Distance is not just a physical aspect of video systems.
Buffers, repeaters, and communications links will all contribute small delays. These may