DTV Audio: External Encoder Configuration

External Encoder Configuration

Some applications require use of an encoder that is external to the MPEG encoding system, either an encoder at the local station or the bitstream from an encoder located at the network.

Multiplexer - Source
Configure the MPEG encoder or multiplexer to "external AC-3".

Multiplexer - AC-3 Data Rate
Set the data rate to the maximum that will be encountered.

Multiplexer - Coding Latency Compensation
If using a network AC-3 bitstream, proper coding compensation setting would seem to be -10 ms, since there is no coding latency and no 10 ms decoding latency. If using a local external encoder, set the multiplexer coding compensation to 200 ms to match the latency of the local encoder (below). If a switch is provided between network and local encoder, you will need to also provide a 210 ms delay for network and server AC-3 signals to match the latency of the local encoder. These values can be adjusted to compensate for other video or audio delays within the system.

LOCAL EXTERNAL ENCODER
Currently, there is a single choice of external encoder, the Dolby DP569. It is also the only AC-3 encoder for 5.1 surround.

Encoder - Coding Delay
Following Dolby's recommendation, set the coding delay to 200 ms.

Encoder - Clock Source
The encoder clock source must be a 48 KHZ AES signal locked to the sync on the input to the video encoder. This can be video-to-AES-clock generator feeding the Ref input, input audio from a de-embedder if embedded audio is used, or house AES reference if video is always locked to house.

Encoder - Meta-Data Automation Option
The DP-569 provides four GPI inputs that can be used for automation control of meta-data presets.

Encoder - Dolby E Integration Option
The Dolby DP-569 encoder includes a meta-data input that can be connected to a Dolby E decoder metadata output. This metadata will reflect meta-data sent by the source Dolby E encoder. Currently, there is little use for this feature at PBS stations, since there is no way to feed network metadata to the Dolby E encoder input. (There is no Dolby Digital decoder available with a metadata output.)

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