Annex R (informative):

Seamless receiver switching between DRM, DAB, AM and FM broadcasts

R.1 Overview

The AFS feature enables a receiver to detect alternative broadcasts of the same audio service (i.e. the same audio content) that is currently presented to the listener. These alternative broadcasts may be available as part of differently configured DRM multiplexes or on other broadcast systems like DAB, AM or FM.

To allow a multi-standard radio receiver to perform seamless switching between all alternative sources of a particular audio programme, both broadcasters and receiver manufacturers need to follow the rules defined in this annex when configuring their transmission networks and designing their receiver devices, respectively.

A typical use case for the seamless receiver switching feature is a broadcaster targeting a mobile audience with a multi-standard broadcast network comprising for example DRM robustness mode E, DAB and FM transmissions. On the receiver side, this feature could be particularly attractive for in-car receivers, which may experience a constantly varying coverage through different broadcast systems.

Both broadcasters and receiver manufacturers are free to support the optional seamless receiver switching feature. However, if enabled or used by either party then the timing requirements laid out in this annex need to be followed to serve as the reliable common reference point for both broadcasters and receiver implementers.

R.2 General network timing considerations

On the receiver side, every broadcast system requires a well-defined minimum processing and decoding time. This decoding delay is mainly defined by mandatory demodulation and decoding steps specific to each digital broadcast system, like de-interleaving, audio super frame management, etc. In contrast, there is basically no system inherent minimum processing delay for analogue broadcast systems like AM and FM. Processing steps in the receiver that are independent from the tuned broadcast system or those that are receiver model specific (like audio post-processing) will not be taken into account for this feature; the receiver is responsible for internally re-aligning those extra delays for all supported broadcast systems.

If all signals-on-air were transmitted simultaneously for all broadcast systems (with respect to the contained audio content), receivers would need to internally buffer and delay the uncompressed audio content received via AM and FM, to be able to seamlessly switch to and from the same audio content available after the time consuming decoding process of digital broadcast systems. This would lead to expensive buffer memory requirements for receivers and therefore needs to be avoided - if any signal buffering is required in the receiver, it should be in the decoding path of the digital broadcast systems, thereby limiting the buffer requirements (if any) to highly compressed audio content.

Therefore the signal-on-air will be delayed on the transmitter side by the broadcaster individually for each broadcast system, so that this signal-on-air has a well-defined delay transmission time difference between each broadcast system (with regard to the contained audio content). This transmission offset between broadcast systems on the broadcast side will be dimensioned such that all mandatory processing steps in the receiver are covered, and that in case any buffering will be performed in the receiver, it lies within the decoding path of the digital broadcast systems instead of the analogue broadcast systems. The benefit of this approach is that receivers are only required to buffer the compressed digital signal (e.g. the relevant compressed audio stream), if any buffering is required at all.

Figure R.1 visualizes this concept.

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transmission / reception chain

Figure R.1: Broadcast system specific transmission paths to enable seamless receiver switching

NOTE: The seamless receiver switching feature specifies well-defined content delays for the different broadcast systems with regard to the signal-on-air. It is up to the broadcaster to fulfil these timing conditions in cooperation with the involved broadcast equipment manufacturers and network operators. On the receiver side, these timing conditions are to be interpreted and implemented as well-defined relative on-air delay values between the different broadcast systems to result in a co-timed presentation of the same audio content independent from the broadcast system in use. However, it is not the intention of the present document to define or limit the absolute decoding delay introduced by a particular receiver model, as long as any additional decoding delay required by a particular receiver model is applied with equal duration to the decoding paths of all supported broadcast systems.

R.3 Network synchronization rules

This clause defines the exact delay values that need to be introduced to the audio content with regard to the signal-on- air on each broadcast system. The absolute delay values depend on the types of broadcast systems that are part of the broadcaster's network - even if some of those broadcast systems (like DRM robustness modes A, B, C and D with long interleaving) are only included temporarily each day.

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t0: earliest possible broadcast time for a particular audio signal

Figure R.2: Transmission delays per broadcast system relative to a common time reference t0

Figure R.2 describes the delays that need to be introduced for each broadcast system in relation to a common time reference t0, i.e. the earliest possible broadcast of a particular audio content in the signal-on-air. For example, the DRM

transmission frame that carries a particular piece of audio will be broadcast as soon as possible via DRM robustness modes A, B, C and D with long interleaving, while the same audio content over FM/AM will be transmitted over the air with a delay of d6 relative to the described DRM signal.

The exact delay values d1 to d6 will be as defined in table R.1:

Table R.1: Transmission delays for signals-on-air (all broadcast systems utilized)

NOTE: These transmission delay values for audio content of the signal-on-air were derived from the following assumed minimum demodulation/decoding delays in a receiver:

FM/AM: 0 ms; DAB with Layer-II audio: 408 ms; DAB with HE AAC v2 audio (DAB+): 504 ms; DAB with 96 kbps DMB-radio audio: 611 ms; DAB with 64 kbps DMB-radio audio: 704 ms; DAB with

48 kbps DMB-radio audio: 798 ms; DRM robustness mode E: 912 ms; DRM robustness modes A, B, C and D with short interleaver: 1 360 ms; DRM robustness modes A, B, C and D with long interleaver: 2 960 ms.

Note that the decoding delay for DAB broadcasts with DMB-radio audio coding slightly varies according to the assigned audio bit rate; to allow for dynamic reconfigurations of the DMB-radio subchannel without the need for adjusting the transmission delay of the signal-on-air, the highest receiver processing delay from the stated typical configurations was used to derive the transmission delay value; as for all reception paths, any varying decoding delay is left to the receiver's internal alignment management.

The derived transmission delay values for the digital broadcast systems were slightly rounded down to allow for some extra processing in the digital reception path without the need to buffer the analogue AM/FM signal in the receiver.

These transmission delay values are valid if all defined broadcast systems are used as part of a broadcaster's transmission network. However, if a broadcaster uses only some of these broadcast systems for this service, the absolute overall content delay can be reduced. In this case, the broadcast system with the smallest value, dx, sets the common

reference for the delays of all other broadcast systems being part of the transmission network.

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