PCM Stereo and Bitstream Multichannel Modes
The SmartTUBE app for STB allows you to control the digital audio output mode. To change the mode, use the Main menu > Settings > Digital audio format option:
- PCM stereo — STB will decode audio independently and output it in the PCM format to HDMI.
- Bitstream multichannel — STB will output Dolby Digital audio track to HDMI as is. In this case, the Dolby decoding expected to be performed by an HDMI sink device (device with HDMI input, ex.: sound bar, A/V receiver, TV etc.). Changing the volume on the STB side in not possible. The user will need to change the volume on the external device.
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Decoding Dolby is only available on STBs with Dolby decoding support — the device must have a special chipset version that supports Dolby decoding to Stereo PCM. This refers to simultaneous support for the Dolby Digital (AC3) and Dolby Digital Plus (EAC3) codecs, since these codecs are not supported separately in modern devices. Similar approach has DTS decoding. Although DTS decoding on STB is rarely supported today. |
The dependence of the audio output format on the presence of the hardware decoder on the STB is shown in the table below:
|
Stream codec |
Is the STB has HW support for Dolby decoding? |
Format on HDMI output |
Format on S/PDIF output |
Format on analog audio output |
Comments |
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|
PCM stereo |
Bitstream multichannel |
PCM stereo |
Bitstream multichannel |
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|
MPEG Audio 1, 2 |
PCM |
PCM |
PCM |
PCM |
PCM |
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|
MPEG Audio 1, 2 Layer 3 (MP3) |
PCM |
PCM |
PCM |
PCM |
PCM |
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|
AAC, HE-AAC |
PCM |
PCM |
PCM |
PCM |
PCM |
Although AAC supports multichannel audio, pass-through is not supported, because it usually not supported by HDMI sink devices. Therefore, it is always decoded in stereo PCM. |
|
|
Dolby Digital (DD, AC3) |
No |
DD |
DD |
DD |
DD |
|
|
|
Yes |
PCM |
DD |
PCM |
DD |
PCM |
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|
Dolby Digital Plus (DD+, E-AC3) |
No |
DD+ |
DD+ |
No |
No |
No |
S/PDIF bandwidth is not enough to pass-through DD+, it may require transcoding to DD or decoding to PCM stereo. |
|
Yes |
PCM |
DD+ |
PCM |
DD |
PCM |
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|
Dolby AC-4 |
No |
AC-4 |
AC-4 |
No |
No |
No |
S/PDIF bandwidth is not enough for AC-4 |
|
Yes |
PCM |
AC-4 |
PCM |
|
PCM |
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|
DTS (including DTS-HD) |
No |
DTS |
DTS |
DTS |
DTS |
|
|
|
Yes |
PCM |
DTS |
PCM |
DTS |
PCM |
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|
FLAC |
PCM |
PCM |
PCM |
PCM |
PCM |
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|
Vorbis |
PCM |
PCM |
PCM |
PCM |
PCM |
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