User manual

55 000 026 – Ed. B SCOOP E-Z - User Manual 11
This document is the property of AETA and can not be duplicated without authorisation November 2001
5.1.4. Notes about TDAC
As an option, the codec can also include the TDAC algorithm. TDAC is for Time
Domain Aliasing Cancellation ; this is a transform coding based on an MDCT
(Modified Discrete Cosine Transform), encoding a 15 kHz bandwidth mono signal at a
64 kbit/s bit rate. When the option is installed, three modes are available :
? TDAC mono full-duplex, running at 64 kbit/s, with a 15 kHz bandwidth ;
? G722/TDAC : G722 encoding, TDAC decoding, running both in mono at
64 kbit/s ;
? TDAC/G722 : TDAC encoding, G722 decoding (with SRT), running both in
mono at 64 kbit/s ; this mode is symmetric to the previous one.
5.1.5. Symmetric or asymmetric codec modes
The codec allows two communication modes:
Symmetric communication: in this mode, the encoder and decoder both use the same
coding algorithm with the same configuration (channel mode, etc.). In this case, the
communication is strictly symmetric full-duplex, with exactly the same coding
configuration used in both directions (local to remote and remote to local). This is
usually required when using proprietary algorithms.
Asymmetric communication: this mode is used for applications requiring different
coding configurations in the two directions. The J52 protocol allows such mode. To
give some examples, it is possible to transmit MPEG in one direction and G722 in the
other one.
With the TDAC option, asymmetric modes are also available wherein one direction is
G722 coded while the other one is TDAC coded. Such mode is useful e.g. in order to
get a low delay return path encoded in G722 while the send path is encoded with higher
quality but a higher delay.