Technical details of the encoder
There are two major goals of the Logic 7 encoder. First, it should be able to encode a 5.1
channel tape in a way that allows the encoded version to be decoded by a Logic 7
decoder with minimal loss. Second, the encoded output should be stereo compatible - that
is, it should sound as close as possible to a manual two channel mix of the same material.
One factor in this stereo compatibility should be that the output of the encoder, when
played on a standard stereo system, should give identical perceived loudness for each
sound source in an original 5 channel mix. The apparent position of the sound source in
stereo should also be as close as possible to the apparent position in the 5 channel
original.
In discussions with the IRT in Munich it became apparent that the goal of stereo
compatibility of the stereo signal as described above cannot be met by a single
adjustment of the encoder. A five channel recording where all channels have equal
foreground importance must be encoded as described above. This encoding requires that
surround channels be mixed into the output of the encoder in such a way as the energy is
preserved. That is, the total energy the output of the encoder should be the same,
regardless of which input is being driven. This will include most film sources and 5
channel music sources where instruments have been assigned to all 5 loudspeakers.
Although such music sources are not common at the present time, it is the author’s
opinion that they will become common in the future. Music recordings where the
foreground instruments are placed in the front three channels, with primarily
reverberation in the rear channels, require a different encoding.
After a series of tests (at the IRT and elsewhere) it was determined that music recordings
of this type were successfully encoded in a stereo compatible form when the surround
channels were mixed with 3dB less power than the other channels. This -3dB level has
been adopted as a standard for surround encoding in Europe, but the standard specifies
that other surround levels can be used for special purposes. As we will see later, the new
encoder contains active circuits which detect strong signals in the surround channels.
When such signals are occasionally present, the encoder uses full surround level. If the
surround inputs are consistently –6dB or less compared to the front channels, the
surround gain is gradually lowered 3dB, to correspond to the European standard.
During tests with the Institute for Broadcast Technique (IRT) in Munich I found that a
particular tape encoded incorrectly with the encoder described in the AES paper. A new
architecture was developed to solve the problem with this tape. Although the particular
tape was only marginally improved, the new encoder is clearly superior in its
performance on a wide variety of difficult material. The original encoder was developed
first as a passive encoder. It performed reasonably well with a variety of input signals.
The new encoder will also work in a passive mode, but is primarily intended to work as
an active encoder. The active circuitry corrects several small errors inherent in the design.
However even without the active correction the performance is better than the previous
encoder.
