Notes
Stereo Synthesis
The patented Orban stereo synthesis technique creates a perceived stereo result from a mono signal by dividing the audio spectrum into several frequency bands, then directing these bands alternatively to the left and right channels. It does this by passing a mono signal through a chain of phase shifters to generate an artificial L-R signal, which is then added to and subtracted from the mono to obtain the synthesized channels. The net effect is a "complementary comb filter."
The sum of the two synthesized channels always remains equal to the original mono, ensuring complete mono compatibility.
Because the audio spectrum is divided logarithmically, the undesirable harmonic reinforcement and cancellation that can result from the arithmetic band-splitting found in digital delay-based processors is avoided.
Two types of remote-selectable stereo synthesis effects are available: wide for a dramatic sense of stereo space on music and effects, or narrow to center dialogue more accurately. The recessed SEPARATION control adjusts the amount of inter-channel difference (L-R), to establish the relative width of the stereo image.
To maximize loudness while making efficient use of the modestly-priced amplifiers in most consumer TV sets, energy below approximately 200Hz remains mono. The ear cannot detect separation in this region.
Noise Reduction
Older mono material often suffers from hiss and other forms of noise. The 275A can apply single-ended noise reduction to mono audio prior to stereo synthesis processing. The noise reduction combines program-controlled high-frequency filtering with broadband expansion. 10dB of noise reduction is typically achieved-without unnaturally reducing ambience and dialogue intelligibility when program levels are low.
Because the noise reduction system is single-ended, no encoding (or later decoding) of the program material is necessary. This makes the process ideally suited to noisy optical sound tracks and satellite feeds. Operation is exceptionally smooth and subtle, and "pumping" and "breathing" are entirely absent.
Polarity Correction
In stereo material, it is essential that the two channels be in phase with each other. If they are not, the mono sum signal will be seriously degraded as the two channels cancel each other. And that means that the viewer with a mono set (a majority of your audience) will hear disastrously inferior audio, and in some cases, no sound at all!
To ensure the mono compatibility of your stereo broadcasts, the 275A can act as a "watchdog" over your program line polarity, correcting errors when detected. The detection technique is very reliable and highly resistant to "falsing"- even when subjected to substantial high-frequency phase errors (due to misaligned heads or other mechanical problems) or when monitoring sound tracks containing out-of-phase "surround" energy.
An LED on the front (and remote control) panel lights when a polarity reversal is being corrected. The detection/correction circuit can be activated or defeated at any time.