Technics RS-B 905

 Dolby HX Pro-Head Room Extension  System

To record good quality sound, it is necesary to give bias current to the head.  The bias current has characteristics as follows:

Increasing the bias current reduces the distortion in low-frequency range but lowers the recording level in high-frequency renge.

Decreasing the bias current improves the recording level in high-frequency range but increases the distortion in low-frequency range.

In the case of a common deck, a specific level of bias current is determined according to the characteristics of recording head. Therefore, bias current cannot be applied to the head according to the frequency levels of music signal.

In order to solve such problems, this unit employs "Dolby HX-Pro". It picks up the high frequency of music source changing at all times, and controls the bias current level according to the changing frequency. It serves to make the bias current level then most suited for the music source.

This system always operates in recording mode irrespective of the noise reduction system, but the dynamic range in record/playback can be further increased by using this system in combination with a noise reduction system. Particularly, combination with the dbx NR system mounted in this unit will double the MOL (Maximum Output Level) in high frequency range, whi the record/playback of digital source as in CD.

 Record Calibration/Bias Control Function

There are sometimes differences in playback output level even in case of same recording level or in sound quality even in case of same recording source. This is because the characteristics of the cassette tape used are different with the makers and brands.

To solve this problem, RS-B905 is furnished with a record calibration knob and bias control knob.  These knobs can be used to adjust the sound volume and quality while comparing the music signal (original sounds of record and tuner) and recorded signal (sound recorded on tape).

Record Calibration Knob

Used to calibrate the difference in recording level of music signal and recorded signal.

Bias Control Knob

Used to correct the high-frequency sound quality of music signal and recorded signal.

In this way, the record characterisctcs will not be distorted by tape, and the characteristics can be kept nearly uniform.

The comparison by hearing can be done by only one monitor switch because this unit is of 3-head type with record, playback and erase heads.

 MPX Filter

If FM stereo broadcast is recorded by Dolby NR, then the pilot signal included in the broadcast is also processed by Dolby NR as music signal, causing deterioration of sound quality and worsening of noise reduction effect.

Pilot Signal

This signal is used to separate FM broadcast into L and R channels, which is emitted at 19 kHz very close to the music frequency band.

To solve this problem, RS-B905 is equipped with MPX filter switch to cut off the pilot signal.  It serves to cut off the 19 kHz single frequency of the pilot signal, which is different from a high cut filter installed in an amplifier, etc.

Also, MPX filter hardly causes change of sound quality in a sense of hearing.

 Class AA Circuit Recording Equalizer Amp

Recording equalizer amp is an amplifier to supply recording current to the head. Usually, loads such as recording head and bias trap circuit are added to the amplifier. Therefore, the current phase and voltage phase are fluctuated causing the recording signal to be distorted. This unit employs "class AA" amp in which two types of amplifier circuits (voltage control amp and current supply amp) different in ampllifying system. This recording equalizer amplifier is not influenced by the fluctuation of current phase or voltage phase as mentioned above, and is excellent in waveform response.

 Operation Principles of Noise reduction System

Dolby NR, B type, C type

The level of hiss noise generated during playback is constant.  So, it is more offensive to the ear when the music signal level is lower. Accordingly, raising the signal level during recording and lowering the level during playback will result in reduction of noise generated by the tape.  Dolby NR B type does it in high frequency range, and C type, in high and medium frequency range.

Dbx NR

Cassette tape is low in saturation level (limit of recording level). Therefore, distortion will be generated if music signal with large dynamic range (ratio of sound volumes of the weasound and the strongest sound) is recorded as it is.

dbx NR  halves the music signal during recording and doubles it during playback.  In this way, dynamic range as large as 110dB and noise reduction effect as high as 30 dB can be obtained, and good quality sound can be recorded even in case of music source with dynamic range as in CD and live performance.

 Monitor Switch

In order to avoid faulty recording such as low sound level or distortion, it is very important to monitor the state of recording. In the case of a common deck (2-head type), the sound that can be monitored during recording is always the sound before recording. So, when checking the state of actual recording, you have to rewind the tape and play it  back.

Technics RS-B905 is of 3-head type, and the record head is independent of the playback head. Also, the sound before recording can be compared with the recorded sound by use of the monitor switch, therefore the state of recording can be easily checked.

Specifications

Deck System:  Stereo cassette deck

Track Sysstem:  4-track, 2-channel

Heads

Rec/Play : Combination head

Erasing : Double-gap ferrite head

Motors

Capstan : Electronically controlled DC motor

Rell Table Drive : Electronically controlled Dc motor

Recording System:  AC bias

Bias Frequency:  85 kHz

Erasing System:  AC erase

Tape Speed: 4,8 cm/sec. (1-7/8 ips)

Frequency Response

Metal : 20 - 22,000 Hz (±15 dB);  30 Hz  - 21 kHz (DIN)

CrO₂ : 20 - 21,000 Hz (±15 dB);  30 Hz - 20 kHz (DIN)

Normal : 20 - 20,000 Hz (±15 dB); 30 Hz - 19 kHz (DIN)

Dynamic Range (with dbx on):  110 dB (1 kHz)

Max. Input level Improvement (with dbx on):  10 dB

Signal-to-Noise Ratio (signal level = max recording level, CrO₂ type tape)

dbx on : 92 dB (A weighted)

Dolby C NR on : 75 dB (CCIR)

Dolby B NR on : 67 dB (CCIR)

NR off : 57 dB (A weighted)

Wow and Flutter:  0,04% (WRMS) ±0,12% (DIN)

Fast Forward and Rewind Time:  Approx. 95 sec.

Input Sensiblity/Impedance

Line : 60 mV/47 kΩ

Output Voltage / Impedance

Line : 400 mV/2,2 kΩ

Headphones ; 125 mV/8Ω

General

Power Consumption:  25 W

Power Supply:  110 / 120/ 220/240 V AC 50/60 Hz

Dimensions (W x H x D):  430 x 25 x 110 mm (16-15/16" x 11-7/32" x 4-5/16")

Weight:  5 kg (11 lbs)