"bcoombes" bcoombes@orangedotnet wrote in message
o.uk...
bcoombes wrote:
David Looser wrote:
"bcoombes" bcoombes@orangedotnet wrote in message
...
Bill Taylor wrote:
On Sun, 10 Jan 2010 10:22:12 +0000, bcoombes bcoombes@orangedotnet
wrote:

The cassette deck might have sliced everything above 8k off, but Dolby
B didn't.
But on these decks it did because with the Dolby off there was response
up to at least 12k whereas with it on it didn't just remove the hiss it
removed everything..as per my not properly implemented comment.


What tolerance was there on the response up to12k?, and at what rate did
the response fall off above 8k with Dolby on? What level, relative to
Dolby level, did you make your measurements at?


Hardly necessary since what was missing was/is clearly audible. As it
happens I still have a Panasonic RQ-X20D cassette player which I've just
dug out of storage and had a listen to to confirm the effect and it's
totally obvious, with the Dolby on some of the 'music' is just plain
MISSING. My original 8k figure was a [mis]remembered estimate, it
actuallity it must be lower than that.
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, 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.

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. 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.

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.

S.