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Re: [ARSCLIST] Dolby S/N Stretcher
Bob,
Based on some of my expereinces, I would say that not only are there
quite a number of misaligned systems, but there are also a number of
processors which were just plain defective. Unless you have the Dolby
tester for checking the cards, there is no way to know if there is an
internal electrical problem or not.
So, many users could have indeed set up the cal levels properly, and
aligned their recorders properly, but would still end up with a bad
recording due to an internal card problem.While we have generally found
the cards to be stable, we have run across problem cards in some of the
used NR units we have bought over the years (especially ones from large
studios, with many multi-channel frames to maintain).
As you state, when moving a DBX tape from one machine to another, you
need to pay careful attention to both the frequency response, and to
some degree, the level of the recording. For example, if a recording is
done on one machine at a level of 250 nW/m, and that same tape is played
back on another machine calibrated at 185 nW/m, and the DBX processor is
located in the machine (ahead of the reproduce level control), you could
run the risk that the procesor would be driven into the unity gain
portion of the expander slope, thereby causing the louder portions of
the recording to sound compressed. I have seen situations exactly like
this occur when levels were not properly matched between machines.
Also, quite a few recorders do not allow you to adjust the LF response
properly, which, when coupled with head bump issues, could cause easily
result in a response error in excess of 2 DB. This then would be
magnified into a 4 db error by a DBX Type I processor, which would
indeed be quite audible!
However, as long as the respose is flat, and both the recording and
reproducer processor are kept within their linear region, I haven't had
too many issues. (Again, this also assumes that there isn't a problem
with the linearity of the processer itself, which I have run into,
especially on older units).
Unfortunately, there is scant information available as to how to make
any calibration adjustments on the boards. We have blueline drawings for
some of the processors, but they don't have much calibration info or
component criteria on them. Guess that DBX just wanted the units to be
returned to the factory. (To be fair, I think that there are some hand
selected components in the compander circuit which probably require some
special test equipment, and known selection parameters, which would be
outside of the abilities of most studio techs. This was also the case
with some of the UREI equipment we've had over the years.).
--Scott
Bob Olhsson wrote:
Scott D. Smith writes:
As long as the system isn't driven so hard that you exceed the range of
the level detector, then it *should* track properly at any level.
Where level calibration got challenging was in applications where you needed to move tapes between different machines. It wasn't nearly as bad as Dolby in that respect however it was far more critical than the manufacturer would have people believe. For that matter Dolby grossly understated how critical level alignment was. I've encountered a huge percentage of sloppily aligned Dolby A and SR systems.