PBS NETWORK

To recap, PBS distributes a complete 19.39 Mbps ATSC transport stream (see A/53b) to their DTV affiliates. This enables an affiliate station to simply add a modulator, RF amplifier, and antenna and be on the air in HD with 5.1 channels of audio. As in the case of FOX network, if a station wishes to insert local programming, it gets a little more complicated.

Adding local content can be done in several different ways. The two most common are decoding the transport stream into baseband audio and video and splicing a local transport stream together with the network transport stream. Each has advantages and drawbacks and both are presented below.

BASEBAND APPROACH

In the case of a baseband decode, the audio and video can simply appear as another source in master control, selectable in much the same way as current network feeds. The main problem with this approach is a potential loss of audio and video quality. With modern compression equipment, the results can be surprisingly good but you never know exactly how close to the edge of artifacts you might get.

KEY POINTS

• Standard IRD can be used to decode audio and video
• Baseband audio and video signals can be treated as another HD source
• A separate Dolby Digital (AC-3) decoder is strongly recommended
• Potential negative effects on audio quality due to decode/re-encode
• Keep all codecs at maximum permissible bitrate (448 kbps)

TRANSPORT STREAM SPLICING APPROACH

Relatively new to terrestrial broadcast, transport stream splicing has evolved rapidly in the cable and satellite worlds to allow insertion of local or targeted programming and commercials. Today's products have not only dropped substantially in price, but also allow some operations to occur to the compressed bitstreams without requiring a decode/re-encode.

KEY POINTS

• Highest quality as signals are encoded only one time
• Allows insertion of local video bugs in compressed domain
• Audio must still be decoded then re-encoded for local operations

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