The Dolby Model 365 Noise Reduction System was created to address the wide range of noise problems found in audio recording and transmission. It reduces rumble, hum, crosstalk, clicks, pops, buzzing, hiss and discrete frequency interference such as television synchronising pulse crosstalk. All of these noises are reduced without altering the overall frequency response or dynamics of the audio.
Within magnetic tape recording, the Dolby Model 365 effectively lowers tape hiss and also reduces print through and high frequency modulation noise. Its ability to minimise print through made it valuable in the long term storage of high quality master tapes with minimal degradation.
The system was designed for any situation where the audio signal is available for processing at both the recording and playback ends of the chain. Once correctly adjusted, system parameters remain stable and precise, and the Dolby Model 365 tolerates reasonable gain errors without audible impact. Playback level errors of up to ±2 dB do not cause any perceptible change in the restored audio.
When recording or transmission noise is reduced, other noise sources that were previously masked become more noticeable, which could require improved standards across the rest of the signal path. These include microphone amplifiers, mixers, monitor amplifiers and ambient acoustic noise from lights, furniture or movement within the studio environment.
The Dolby Model 365 is a two channel processor designed for recording, playback, editing and disc cutting, as well as applications requiring simultaneous record and playback monitoring. One channel may be placed in record while the other is in playback. All controls are located on the front panel, with pushbutton switches selecting operating mode.
The circuitry uses highly stable solid state components and requires no routine adjustments other than input and output level settings during installation. Should a module fail, the unit allows simple plug in replacement without further alignment. Each channel includes an internal Dolby Tone oscillator for establishing correct operating levels, and this tone also identifies Dolby processed material. In multi channel setups, the oscillators can be controlled by a single switch.
Processing circuitry is housed in a pair of easily accessible front panel modules. Dual meters and built in reference generators simplify level setup. Remote operation is supported for both noise reduction in or out and Dolby Level setup. Each channel also includes a Play Uncal control that provides temporary recalibration to match material recorded at a different reference level. The Dolby Model 365 could be ordered either as a Dolby Model 365 A with two Cat No 22 Dolby A type noise reduction modules, or as a Dolby Model 365 SR with two Cat No 280 Dolby SR spectral recording modules.
The Dolby A type process used in the Dolby Model 365 provides 10 dB of noise reduction from 30 Hz to 5 kHz, rising to 15 dB by 15 kHz. With noise reduction switched off, the system acts as a unity gain amplifier. Total harmonic distortion is very low, less than 0.1 percent at 1 kHz and less than 0.2 percent from 40 Hz to 20 kHz. Overall frequency response is maintained within ±1 dB from 30 Hz to 20 kHz when encoding and decoding. Matching between units is within ±1 dB at all frequencies. Crosstalk performance exceeds 70 dB from 20 Hz to 20 kHz. Signal delay is constant with frequency at 24 microseconds per channel.
The Dolby SR process available in the Cat No 280 modules represents Dolby’s later development in spectral recording. It offers negligible high order distortion and very wide dynamic range. Depending on weighting, typical dynamic range performance ranges between 90 and 105 dB, with clipping levels defined for several reference standards. Matching between SR units is within 1 dB across the full audio spectrum, with approximate 14 microsecond overall encode decode delay.
A Dolby Noise generator is included for calibration and frequency response checking, and the SR system also introduced the Auto Compare feature. This allows automatic audible comparison between the recorded Dolby Noise signal and the internally generated reference, providing quick assessment of system accuracy by ear. Status indicators show Dolby Noise mode and Auto Compare activity.
The Dolby Model 365 was suitable for a wide range of applications including mono and stereo tape recording, multitrack production, disc cutting, tape duplication, archival transfer, sprocketed magnetic film, videotape recording, landline transmission, microwave broadcast links and other fixed gain transmission paths. In each case, the Dolby Model 365 provided significant improvements in signal to noise ratio and reduction of modulation noise and print through, while preserving the original audio characteristics.
In film production, the system improved location recordings, transfer and dubbing work, and traditional optical sound reproduction which suffered from impulse noise and limited bandwidth. In electronic music and delay systems such as tape or electronic delay lines, the Dolby A type process greatly reduced accumulated noise from multiple layers of recording or long signal chains.
Digital applications also benefited, as incorporating Dolby A noise reduction before analogue to digital conversion allowed a reduction in required bit rate for a given audio quality. This preserved storage and bandwidth while maintaining low noise performance. Across all uses, the Dolby Model 365 delivered stable, repeatable results and supported the international Dolby Level standard for maintaining consistent levels between recording and playback systems.
Features
- Two channel professional noise reduction processor
- Reduces tape hiss, hum, crosstalk and modulation noise
- Supports Dolby A or Dolby SR plug in modules
- Front panel pushbutton control of all functions
- Internal Dolby Tone oscillators for calibration
- Stable circuitry requiring no routine adjustment
- Remote control capability for noise reduction and level setup
- Suitable for recording, playback, editing and disc cutting
- Allows simultaneous record and playback monitoring