Dolby Decoder Downmix(Lo/Ro and Lt/Rt)

    From the Dolby Metadata Guide:
     In each of these devices, the analog stereo output is one of two different stereo downmixes.
     One type is a stereo-compatible Dolby Surround downmix, of the multichannel source program that is suitable for Dolby Surround Pro Logic   decoding.
     This kind of downmix is also called Pro Logic or Left total/Right total (Lt/Rt).
     The other type is a simple stereo representation (called Left only/Right only, or Lo/Ro) suitable for playback on a stereo hi-fi or on headphones,
     and from which a mono signal is derived for use on an RF/Antenna output.
     The difference between the downmixes is how the Surround channels are handled.
     The Lt/Rt downmix sums the Surround channels and adds them, in-phase to the Left channel and out-of-phase to the Right channel.
     This allows a Dolby Surround Pro Logic decoder to reconstruct the L/C/R/S channels for a Pro Logic home theater.
     The Lo/Ro downmix adds the Left and Right Surround channels discretely to the Left and Right speaker channels, respectively.
     This preserves the stereo separation for stereo-only monitoring and produces a mono-compatible signal.
     In all downmixes, the LFE channel is not included.

 

AC3 decoder Downmix

      Downmixing: Combining(or mixing down) the content of n original channels to produce m channels, where m<n

     If the number of channels required at the decoder output is smaller than the number of channels which are encoded in the bit stream,
     then downmixing is required.

     Since the inverse transform is a linear operation, so Downmixing can be implemented in the time domain(sample source code), also possible to downmix in the frequency domain prior to transformation.

         Let L, C, R, Ls, Rs refer to the 5 discrete channels which are to be mixed down to 2 channels. In the case of a single surround channel(n/1 modes),
     S refers to the single surround channel. Two types of downmix should be provided: Downmix to an LtRt matrix surround encoded stereo pair; and
     downmix to a conventional stereo signal, LoRo.

     The downmixed stereo signal(LoRo, or LtRt) may be further mixed to mono, M, by a simple summation of the 2 channels.

     If the LtRt downmix is combined to mono, the surround information will be lost.

     The LoRo downmix is preferred whhn a mono signal is desired.

     Downmix coefficients shall have relative accuracy of at least +25dB or -25dB