Serge Auckland wrote:

"bcoombes" bcoombes@orangedotnet wrote in message


ISTR that Panasonic were established to be one of the manufacturers
that took a while to get the *new* Dolby technology right but the
defect was quite common for a while and was often commented on in
reviews of cassette decks of the time.

On the above subject here's some technical stuff on Dolby NR
implementation problems. No doubt many of the early and 'cheap'
cassette decks *incorporated* them but it seems it may also be a
problem with the recordings. Any comments from some of the people here
who obviously know about this stuff gratefully received since I'm more
interested in learning than 'being right'.

1. Pumping:
Incorrect selection of the control path bandwidth external components
can result in an audible increase in noise as the input level changes.
This is most likely to be heard on solo instruments or on speech.
Sometimes the S/N rate is too poor and masking will not be completely
effective - i.e., when the bandwidth is wide enough to pass the
program material, the increase in noise is audible. Cutting down on
the pumping will also affect the program material to some extent and
judgement as to which is preferable is required. Sometimes a shorter
decay time constant in the detector circuit will help, especially for
a source which always shows these characteristics, but for better
program material a return to the recommended detector characteristics
is imperative.
2. High Frequency Loss:
This can be caused by an improper control path gain setting-perhaps
deliberate because of the source S/N ratio as described above-or
incorrect values for the
audio path filter capacitors. Capacitors larger than the recommended
values will scale the operating bandwidth lower, causing lower -3 dB
corner frequencies for a given control path signal. Return to the
correct capacitor values and the appropriate control path gain setting
will always ensure that the h.f. content of the signal source is
preserved.


In the mid 80s, I owned a HiFi shop,

Where was it and what was it's name..being nosy.

and sold a lot of Nakamichi
machines. As part of the sale, I offered to line up the machine to the
users' preferred tapes. Few users understood that tape formulations
differ, even between batches of the nominally same tape. Nakamichis were
truly excellent machines, and were capable of exceptional performance,
but, they had to be immaculately lined up. Cassette generally is at the
limit of what is possible, and Nakamichis were at the limit of the limit.

Nevertheless, a machine could be lined up to give a 30Hz-20kHz -2dB
response, with a 60dB S/N ratio, well under 0.1% W&F and less than 3%
distortion at 1kHz and 0dB where 0dB was 200mM/mm all simultaneously
using Dolby C on metal tape, and with Dolby tracking to within 1dB. The
big however, was that changing tapes to another metal tape, say TDK from
Maxell, or That's (my favourite) to TDK would destroy the performance.
Frequency response, Dolby tracking, distortion, noise, would all get
significantly worse. Then the tape manufacturers wouldn't help by
constantly "upgrading" their tapes which meant that the machine's record
side had to be lined up all over again. I used to encourage customers
to buy a couple of boxes of tapes with their machine, to give them a
fighting chance of having a stock of tapes the machine was aligned for.

With technology comes incompatability problems it seems.
Doesn't sound like much of an improvement over vinyl in 'faffing about' terms.

I often used to hear that Dolby was awful, that people preferred to
record without Dolby and play back with, or the other way round, that B
was much better than C etc etc. C was a LOT better than B, but was
correspondingly more critical of line-up (Dolby C was effectively two
Dolby B processors in series..the first implementation of Dolby C was
actually that, then a dedicated B/C chip became available which
unfortunately wasn't totally compatible with the first implementation.

Very interesting, I had a cruise around the Dolby site (and others) and didn't
see any mention of that...unsurprisingly.

All the horror stories of Dolby essentially came down to lack of
alignment...you can't expect the Public to line up their machine every
time they used it, but that's pretty much what it took if you wanted A-B
transparency, which is what a freshly lined-up Nakamichi could do. Few
other machines gave you access to record EQ, record and bias current,
replay EQ, separate record and playback heads, head positioning, dual
capstans that took away the cassette's pressure pads etc, and so their
line-up was always compromised, if it ever happened.

ISTR that even Nakamichi car decks outperformed other manufacturers 'hi-fi
separate' decks, they used to cost an arm and a leg AIR.

With my own Nakamichi, I would do a full line-up every time I wanted to
do a "proper" recording, i.e. one to keep, but it became a real fag, and
wasn't sad to give up tape for PC recording.

Yeah with pc's everyone's a producer (for good or bad) as they say.
Fascinating post..thanks.


--
Bill Coombes