3Com owner manual CP-1 Decoder

Theory

and

Design

CP-1 Digital Audio Environment Processor

is even more important in a home decoder than in a professional model, because the small size of the playback room makes decoding errors more audible than they are in a theater. The level detection must be very fast, and the matrix must adapt very quickly or there will be a time lag between the audibility of a sound and its correct steering. Since phase relationships determine how the sound is steered, Pro Logic decoding puts unusual demands on the accuracy of the phase and balance of the input channels. Other Pro-Logic decoders have a front panel control for adjusting input balance and for best results a user should carefully adjust this for each program. But what if the channel balance varies during playback?

The manual balancing procedure does nothing to correct azimuth errors. During the preparation of the master for a video tape or disc, misalignment of the playback heads or skewing of the film produce small time differences between the two channels. Azimuth is poorly controlled in both profes- sional video recorders and optical film chains. In the final product, which has been through many generations, it can easily be wrong by 50 microsec- onds or more, and may vary as the tape or disk is played. At middle and high frequencies it doesn’t take much misalignment to generate large inter- channel differences in phase, which are just what the decoder uses to do its steering.

Other Dolby Pro Logic decoders try to deal with this problem by reducing the treble in the surround, so the out-of-phase sibilants in the film do not splatter annoyingly from the rear. This does not, however, reduce the sibilants in the side speakers. Some non-Pro Logic decoders reduce these side-channel sibilants by narrowing the spread of the front channels in the presence of dialog; this compromise is unnecessary in the CP-1.

Pro Logic decoders remove dialog from the left and right channels, while maintaining stereo as much as possible.

The CP-1 Decoder

The CP-1 decoder is unusual in a number of ways. First of all, it is entirely digital. (Most surround decoders advertise that they are digital because there is a digital delay line for the surround channel but the matrix and the logic decoding are done in analog.)

Because the CP-1 is all digital, we can use some of the digital memory to delay all the output channels by 20 milliseconds — about the same as the acoustic delay you get in the front row of a theater. (The surround channel is delayed by an additional 16 to 32 milliseconds.) This delay allows plenty of time for the CP-1 to determine the direction of sounds and adjust the matrix before the sounds are sent to the amplifiers. This substantially improves dialog and effects cancellation, as is immediately apparent from the spread of ambient material or music, even in the presence of dialog.

The CP-1 can also sense and continuously correct both balance and azimuth errors in the incoming material. All the time the film is playing, the CP-1 is checking balance and azimuth, keeping the dialog perfectly centered. The

Pro Logic requires phase accuracy. Common azimuth errors cause ghost dialog in all channels unless the azi- muth error is corrected.

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