Advertisement
These errors arise from tape saturation at high frequencies, from biasing that didn't match the tape
when the recording was made, and from differences in equalisation and head azimuth among recorders.
Such errors are magnified by Dolby NR decoding because they cause the Dolby NR circuits to mistrack,
often resulting in dull sound.
Play Trim is a special high-frequency playback equalisation circuit, located ahead of the Dolby NR circuit
so that it can be used to restore accurate playback response before Dolby NR decoding takes place. As a
result the playback of all cassettes can now approach the high-fidelity ideal: accurate reproduction of
every original sound, with all of its brilliance and clarity preserved
These several causes of variable high-frequency playback response are significant by themselves, but
when decoded by the noise-reduction circuits, these errors are greatly magnified. For example, mild tape
saturation or skewing that rolls off the highs above 8kHz will cause a slight loss of crispness and "air" in
the sound. But if that rolloff alters the tracking of the Dolby noise-reduction circuit, it will depress
response at lower frequencies as well, so that the sound becomes dull in the mid-range too. Since the
playback error occurred ahead of the noise-reduction decoding, this problem cannot be solved simply by
adjusting amplifier tone controls.
The solution: Play Trim. The new Play Trim control not only corrects these common high-frequency
aberrations but also prevents the consequent mistracking of the noise-reduction circuits.
The Play Trim control is easy to use: you simply adjust it in playback to obtain musically correct tonal
balance, removing any false brightening or dulling of the sound. The Play Trim circuit is a narrow-band
equaliser that operates in the top two octaves of the audio spectrum, providing an adjustable boost or
cut of up to ±3 dB at 10 kHz and 6 dB at 20 kHz. The key to its success is its location in the signal path,
ahead of the Dolby NR decoder. Since any high-frequency errors are corrected before Dolby NR
decoding, playback fidelity can be as excellent in everyday use as it is in idealised lab tests.
2. DOLBY HX PRO HEADROOM EXTENSION
At high recording levels, overbiasing rolls off high frequencies and causes the tape to saturate at a
reduced level. This yields a dull sound which is commonly assumed to be an inevitable characteristic of
cassette recordings. The 6155 includes the Dolby HX Pro headroom extension circuit, which constantly
monitors the level and frequency content of the input signal during recording, and it automatically varies
the amount of ultrasonic bias so that the total effective bias (including the effect of the high audio
frequencies) is optimum at all times. Consequently, when you are recording a strong low-frequency
signal, the bias level increases to an unusually high value. providing remarkably low distortion at high
recording levels. When the input signal has strong highs, the bias decreases to provide a dramatic
improvement in high-frequency saturation level, preventing any dulling of the highs.
As a result, the useful dynamic range of the 6155 at both low and high frequencies is greater with ferric
and chrome tapes than the range many other decks achieve with costly metal tape. Best of all, the
operation of the Dolby HX Pro circuit is totally automatic. And since it works only during recording, the
superior quality of the resultant recording will be evident wherever the tape is played - in the car, in a
personal portable player with headphones, or in the 6155 itself.
3. AMORPHOUS HEAD
The NAD 6155 contains a costly amorphous record/play head that is remarkably free of flux saturation
even at the highest recording levels. In many cassette recorders, at low and middle frequencies the head
saturates before the tape does, and that is what limits the useful dynamic range of the recorder. With
the amorphous head you can record musical peaks at levels substantially higher than 0 dB (Dolby NR
reference level), confident that the sound will remain distortion-free.
The ability of the amorphous head to handle extra-high signal levels without saturating is especially
important with high-bias (chrome) and metal-particle tape formulations. These tapes excel in high-
frequency dynamic range, allowing you to capture the full brilliance and power of cymbals, bells, brass,
and synthesizers in clear, airy sound. Play Trim, Dolby HX Pro, and the amorphous head add up to a level
of performance, in both recording and playback, that normally is found only in tape decks that cost far
more than the 6155.
While the 6155 may be a mid-priced machine, it was designed to work at its best with premium-quality
tapes, producing first-class recordings. In addition, its wide-range response, low flutter, and very quiet
electronics provide a superb play-back environment for the many high-quality pre-recorded music
cassettes that are available today.
Advertisement
