SELENIO™ SEL‐1DEC1/SEL‐1DEC2
Installation and Operation Manual
Audio Decompression
© 2014 Imagine Communications Corp. Proprietary and Confidential.
Version 7.0 | Page 28
Audio Decompression
Audio Decompression provides a set of pages for each of the eight audio streams that may be decoded.
General
Mode
The Mode control sets the number of audio channels that will be allocated to the decoded audio stream.
Since stream content may vary over time, the channels are allocated to the stream even if it does not
currently contain content for them. The choices are 2.0 (stereo), 4.0, 5.1 (surround) and 8.0. The 4.0 and
8.0 choices are specifically for Dolby E and Dolby Digital Plus content, and are treated as 2.0 and 5.1,
respectively when decoding other content. (The resources for 4.0 and 8.0 channels, respectively, are still
allocated to the audio stream).
For output routing purposes, channels are grouped into pairs and labeled
stream number - pair number.
Thus, 1 ‐ 1 represents the first pair of channels for Audio 1; 1 ‐ 2 represents the 2nd pair, 1 ‐ 3
represents the 3rd pair, and 1 ‐ 4 represents the 4th pair of channels for Audio 1.
For mode 2.0, the first pair is either Left/Right for stereo content, or Center/Mute for monaural content.
For Mode 5.1 (surround) content, the first pair is Left/Right, the second pair is Center/LFE and the third
pair is Surround Left/Surround Right. (Channels that are missing in the stream are muted). When you
select mode 2.0 and the stream contains Dolby Digital or Dolby Digital Plus surround content, the
content is downmixed to stereo according to the Dolby Digital Control Audio x (on page 30) settings for
this audio stream (see ). Other surround content is downmixed using a simple formula that avoids
overflow.
When receiving SMPTE 302M content, care must be taken to set the Mode at least as large as the
number of audio channels carried in the stream. Most SMPTE 302M streams carry two channels, and
thus, the 2.0 mode will be sufficient. Since the PMT of the transport stream does not announce the
number of channels in a SMPTE 302M stream, it is possible for internal data paths to be oversubscribed
if the number of channels exceeds the mode setting. This situation can cause frequent decode errors on
all audio streams.
When receiving Dolby E content, a mode setting of 2.0 results in the encoded content being output
(with no decoding performed) according to the Passthrough setting. Mode settings 4.0, 5.1, and 8.0
result in decoding of the Dolby E content, with the contents of the two, three, or four channel pairs
defined by the Dolby E program configuration as shown in Table .
Mode settings 4.0 and 5.1 may result in discarded pairs. Some program configurations require the 8.0
mode to obtain all live output channels, even though they don't actually contain all eight channels.
