Audio Modes
Page F-4
Reference Guide: evolution 5000 E5780 and E5782 Encoder
ST.RE.E10135.4
The encoder includes a psychoacoustic model to improve the quality. The signal is divided
into 32 multiple subbands, which correspond to the critical bands of the human ear. The
number of bits is fixed for each subband but there are additional bits that can be allocated to
any subband where encoding quality has suffered. Dolby recommends stereo signals may be
coded at
192 kbit/s, and 5.1 at 448 kbit/s, but other rates can be used if required.
The encoders have the ability to encode stereo and equivalent modes, and will also
pass-through pre-compresses Dolby Digital (both stereo and multichannel). This can be
selected in menus Audio A and B, which includes the choice of pre-encoded or uncompressed
inputs.
Advanced Menus 4A-4D include the option for pass-thru mode with optional glitch
suppression mode. When in this mode, the coding module monitors the encoded bitstream
and if the framing structure is incorrect, a valid silence frame or the last good frame is inserted
in its place. If this state occurs for more than a second, the encoder signals that the Dolby
Digital bitstream is corrupted.
F.2
Audio Coding Modes
F.2.1 Mono
This mode has a single audio channel that is encoded independently. It is seldom used in
broadcast as most viewing devices now have stereo speakers or headphones.
F.2.2 Stereo
This mode treats the incoming audio signal as a left and right channel that the viewer will
listen to simultaneously. Practically, these stereo signals can be uncorrelated where they are
coded separately or related where they combined into a sum and difference channel and each
is coded separately. Another stereo coding tool called Intensity Stereo uses the fact that the
human ear locates high frequency sounds by amplitude rather than phase. So this tool
removes phase differences between the channels at high frequency.
F.2.3 Dual
Mono
This was introduced to allow two mono channels to be carried in the same bandwidth as
stereo signal. The main use for this mode is for multilingual transmission where decoder
selects which language to decode on left or right.
F.2.4 Multichannel
Sound/5.1
A stereo signal produces a very focused audio field so unless the viewer is sitting in the
correct position, the audio reproduction suffers. More audio channels are required to generate
a larger audio field in which the viewer can listen. The current standard for the multichannel
configuration is 5.1 where:
•
1
st
audio pair: Left front and right front, coded as a stereo pair and can be used when
there is only stereo speakers.
•
2
nd
audio pair: Centre channel for speech and low-frequency enhancement (LFE)
channel to be feed to a bass speaker for good low frequency reproduction, these are
coded as mono channels with restricted frequency on the LFE channel.
•
AES 2: Left surround, Right Surround.
