This article - Albis Technologies

from the industry
from the industry
Building TV Services
With OTT Technology
By Achim Bosse, Product Line Manager, Albis Technologies
This article explores the issues that need to be considered when building TV
services for the “big screen,” and discusses whether it may make sense to copy
legacy TV service features to a new and flexible, but not entirely TV-centric, OTT
technology such as HLS.
Over-The-Top (OTT) delivery platforms offer a unique way of
Achim Bosse,
Product Line Manager,
Albis Technologies
delivering content to various kinds of consumer devices, including
Achim has more than 15
Every device has different capabilities, screen sizes and connectivity
years of experience in the
telecommunications industry
and
over
five
years
of
consulting and training for
major service providers in
Germany.
He joined Siemens Switzerland in 2001, where his
responsibilities were within Solution Sales. Currently
at Albis Technologies, he is a key member of the
Multimedia team as product line manager for set-top
box solutions.
He has a broad knowledge of various aspects of
telecommunication and broadcast technologies.
also holds a Master’s Degree in Physics.
He
smartphones, tablets and set-top boxes.
to the network, and therefore different bandwidth availability.
While Adaptive Bitrate Streaming (ABR) seamlessly adjusts to the
available bandwidth on diverse networks, users also have distinctive
expectations for functionality on various devices.
therefore have also defined ways to reduce bandwidth
all DVB- and MPEG-TS-related information can be used
by offering complementary information such as subtitles
as in DVB-C/-T or -S services. However, managed service
and additional audio languages in separate HLS streams,
operators are also considering using OTT technologies as
when required. This saves bandwidth in situations where
a common delivery platform to lower operational costs. The
the information is not needed and allows for the delivery of
question becomes “what does the difference in technology
the same content assets to devices using different access
mean for the scope of service features?”.
technologies with limited bandwidth, like mobile networks.
This article explores the issues that need to be considered
DVB- and MPEG-TS-based services
when building TV services for the “big screen,” and discusses
Legacy services mainly consist of three basic features,
whether it may make sense to copy legacy TV service
which are well-defined for DVB and IPTV systems. Table 1
features to a new and flexible, but not entirely TV-centric, OTT
below shows that OTT standards such as HLS need to be
technology such as HLS.
considered.
HLS technology
Teletext
HTTP Live Streaming (HLS) is an HTTP-based media streaming
Teletext is still a commonly used service for TV and may even
protocol defined by Apple® in the Internet draft standard
be obligatory in some regions. Most broadcast signals contain
“HTTP Live Streaming -- draft-pantos-http-live-streaming”.
teletext that can potentially be used; however, it is mostly only
interesting for TV users. Teletext information could remain
Using HTTP as a protocol, HLS provides unicast-based
inside the MPEG-TS, and the comparably small bandwidth of
communication that can traverse almost any CPE, allowing
around 100Kbit/s to 150Kbit/s may still be acceptable. For
for the delivery of multimedia content to almost every device.
lower bitrate streams, teletext could even be removed.
HLS uses MPEG-2 TS as a multiplexing technology for
audio and video signals, and it defines H.264 and AAC as
However, adhering to teletext just for the TV screen implies
preferred video and audio encoding, respectively. Since these
a differentiation between streams for TV and other devices,
technologies are well known in the broadcast industry, HLS
which is not ideal for a unique and generic delivery platform.
creates a straightforward migration path for OTT delivery of
As OTT platforms were originally developed for mobile devices,
existing content.
many players do not support teletext.
The biggest value of HLS, however, is the ability to define
It’s questionable whether teletext should still be a required
different profiles for the same content. This allows the same
service or if it can be skipped for all devices including TV. This
media streams to be made available in different qualities to
might be acceptable today since teletext doesn’t have the
accommodate bandwidth fluctuation. Intelligent algorithms
importance it did a couple of years ago.
measure the available bandwidth and select the most adequate
profile, guaranteeing a smooth streaming experience.
Audio support
In many countries, multi-language audio support is vital.
HLS and other OTT technologies have been specifically
In today’s broadcast world, multiple audio streams are
developed for use in unmanaged network scenarios, and
encapsulated in the MPEG-TS with different audio PIDs. Re-
Consumers are accustomed to the quality and reliability of Digital Video
Broadcasting (DVB) systems, especially on the TV screen. Traditional
TV solutions do not experience the degraded picture quality that
Teletext
Subtitles
Audio
results from bandwidth fluctuation. While teletext is still commonly
DVB Solution
MPEG-2 TS
MPEG-2 TS
MPEG-2 TS
used on TV screens, it is not generally available on a smartphone
IPTV Solution
MPEG-2 TS
MPEG-2 TS
MPEG-2 TS
MPEG-2 TS
MPEG-2 TS
MPEG-2 TS
--
WebVTT
Alternative HLS streams
or tablet. Following the paradigm of a single delivery platform for all
devices, new services may have to be adapted in certain areas.
When it comes to OTT-based TV services, it is also important to
understand that multicast-based IPTV services for managed
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networks fulfil all the requirements for a TV service, since
Vol. 36 No. 2 - May 2014 Issue
OTT (HLS) Solution
Table 1: A comparison of DVB, IPTV and HLS standards
Vol. 36 No. 2 - May 2014 Issue
25
from the industry
from the industry
Network-based recording services are more flexible and
scalable for OTT delivery in general.
It’s questionable whether teletext should still be a required
service or if it can be skipped for all devices including TV.
use of the existing broadcast streams may therefore be the
receivers at home do not support AAC directly. Again, not all
obvious solution. However, this requires high bandwidths of
devices will be able to make use of surround-sound streams
n x 100Kbit/s, depending upon the audio coding used, which
and, ideally, they are just offered for devices such as STBs as
might not be ideal for mobile access networks.
dedicated, selectable audio streams.
As an alternative, the HLS standard describes the usage of
Subtitles
separate audio streams, as depicted in Figure 1 below. Under
Subtitles are essential in many countries due to the lack of
this method, only the relevant audio track has to be streamed.
language support on the audio side. As for multiple audio
Again, re-using the well-known broadcast option may be
languages and teletext, the straightforward approach is to
the easiest way forward — simply re-using existing MPEG
maintain the DVB subtitles included in the MPEG-TS (see
multiplexes — but is not ideal for serving every kind of device.
Figure 1), although this is again not ideal for every device.
In addition, it has to be considered whether all devices are
able to support multiple audio streams, which is not the case
The HLS standard proposes WebVTT as a common format for
today. In the long run, the use of separate audio streams as
subtitles. WebVTT is already supported in HTML 5 browsers
defined by HLS is the more efficient, common solution and
and other components. Ideally, broadcast streams should
should be targeted.
also be available in WebVTT format. However, since DVB
subtitles are based on pixel bitmaps, this requires Optical
Another aspect involved with delivering high quality live and
Character Recognition (OCR) to convert this format to a text-
VoD content to TV screens is support for high quality multi-
based format.
channel audio. Dolby Digital is commonly used in broadcast
systems today and may continue to be used in the future.
Alternatively, DVB subtitles may be converted into “timed
Multi-channel audio with AAC provides an alternative, but
metadata” as described in the HLS specification. However, this
requires transcoding capabilities in the STB since most A/V
does not define the format of the subtitles itself. Eventually, head-
end vendors may use different picture formats such as PNG,
flexible and configurable, allowing the selection of a certain
which is not ideal as it introduces a proprietary component.
target profile or bitrate. In this way PiP could easily be
introduced, since OTT platforms may already provide lower
For VoD assets, text-based subtitles like WebVTT are a
quality streams for other devices anyway.
feasible solution for broadcast linear TV. However, it may be
Network-based Start-Over TV
more difficult to achieve a common standard.
The playback of OTT-based services can vary based upon
TV Service Extensions
playlists. Usually OTT streamers generate 5-10 second chunks
Local PVR
of media and the related playlists. The playlists, however,
For legal (content rights) reasons, or just to satisfy the end
can be much larger — i.e. covering 30 minutes of content
user’s need to own a recording, local PVR services are
— allowing for searching within the time window, such as
attractive for many operators. The challenge with local PVR
in a “pause live TV” scenario. But the loading and handling
in OTT environments is the fact that, in addition to a watched
of long playlists for several profiles influences zapping times
channel, a second stream has to be received which competes
negatively. Therefore, an effective solution is to maintain good
with the first stream for bandwidth. Since the local STB can
zapping times using short playlists for live TV while offering
only record what is received, changes in quality will also be
long playlists in parallel to provide start-over TV features.
recorded. These changes might not meet users’ expectations.
Since the live event is not finished, the video player has to be
able to handle playlists without an end tag.
An alternative solution is to limit stream variants for TV to
higher quality streams. Service providers then have to think
The service may be activated by the user pressing a dedicated
about a way to guarantee service quality for the big screens
button on the remote control, a proactive action resulting in a
on the network side, in the same way it is done today for IPTV
somewhat slower start of playback than with normal zapping,
services. The latter, of course, is only possible in a managed
which is more acceptable. In addition, live events that have
network. In an unmanaged real OTT scenario, local PVR
already been finished can be available as “recorded” items.
services may not be applicable at all.
This way, OTT technology allows for the introduction of new
service variants using the flexibility of the “playlist” approach.
Network-based recording services are more flexible and
scalable for OTT delivery in general. However, if OTT-based
General Aspects
services are available across borders, content rights’ issues
The QoS question
may arise and limit possibilities.
While mobile and WLAN users may be used to degraded
connectivity and varying service quality, TV users expect the
PiP
quality they receive from DVB or multicast-based IPTV services.
As for local PVR, Picture-in-Picture (PiP) is a second stream
Although OTT platforms provide an attractive platform from an
competing with other traffic on the network side. However,
operational point of view, they were not originally built to copy
PiP in the sense of having just a small picture — such as in
a broadcast system.
the corner of the main full-screen picture — could easily be
achieved by selecting a low-bitrate profile and limiting the
Particularly in terms of service quality, a stable picture is vital
player to this profile. The video player therefore has to be
and expected. Therefore, as in multicast-based IPTV systems,
Figure 1: Transformation of an original DVB signal for OTT/HLS broadcasting
The HLS standard proposes WebVTT as a common format
for subtitles.
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Vol. 36 No. 2 - May 2014 Issue
OTT technology allows for the introduction of new service
variants using the flexibility of the “playlist” approach.
Vol. 36 No. 2 - May 2014 Issue
27
from the industry
from the industry
the bandwidth availability has to be guaranteed. Limiting the
Intense testing with realistic loads is therefore important to
profiles to be used for TV/PVR services to those with high
unveil the potential negative impact on networking elements
quality may be useful in meeting the quality standards of
and service availability.
broadcast systems. As for PiP and local PVR-based services,
a flexible and configurable player allowing the selection and
Conclusion
limitation of used profiles is required.
Several aspects, from service scope to general networking,
have to be carefully considered when building TV services
Networking aspects
using OTT technologies. Just copying legacy services is
As with the introduction of Voice over IP (VoIP), OTT platforms
not the best way. Service providers have to consider the
allow the building of unique services for different kinds
best solution to fulfill user expectations, while leveraging the
of devices, but network elements such as firewalls, load
potential of new technologies to improve operational costs.
balancers and routers also have to support the new traffic
patterns resulting from OTT services. For example, many end
Albis Technologies can support these activities, creating
devices do open a new TCP connection for every download of
solutions using this exciting but challenging new technology.
a chunk, since this is seen as a simple file download.
With its flexible Unified Media Architecture and a configurable
HLS player, the SceneGate set-top series is designed to
However, multiple devices behaving this way results in a huge
deliver compelling OTT services.
amount of TCP connections to be handled by intermediate
network elements or servers. A solution is the support of
persistent TCP connection, which remains installed at least for
the playlist and transport stream segments. In return, this is only
useful if the server side does not terminate the connection.
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