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Richard,

This is an issue we have struggled with quite a bit over the years. There are
all kinds of factors that contribute to the problems you mention. Probably the
biggest factor is ignorance in setting up the encoding in the first place. I
have encountered a number of recording over the years that clearly did not
maintain the same levels from the setup tone to the program. It shouldn't be
the case, given the normal setup of the processor to the recorder, but it
certainly appeared to be the case.

Much of the material we deal with is on magnetic film. In some dubbing
operations, we found that the machine room operator would record the 
Dolby tone
and setup tones on a different reel of stock from that used for the actual
recording. Also, I have seen the bias drift on some mag recorders, usually
creeping upward with the length of time that they were in the "record" mode.
Obviously this will have an effect on the overall level, as well as the HF
response.

Beyond that, I have encountered more than a couple of systems that were out of
spec. Unless you have the test setup used by Dolby to confirm the record/repro
parameters, there is simply no way of knowing if the compander is operating
properly (especially with SR).

We send in all of our cards (over 60) last year to have them all 
checked. We had
this done 3 years ago as well. Five were found to have drifted out of spec.

When I was doing some work years ago with Dolby B processors, I found 
many units
I had experimented with to be out of spec. With Dolby C, it gets even worse.

In some situations, we have applied some corrective EQ to get the 
recording back
to what we *think* is the correct response. A highly subjective process, to be
sure.

We just did a whole batch of reels that were recorded in Dolby A on 35MM film
around 1975 or so, by a studio in New York. The HF response on the pink noise
was down so far that we couldn't even get it back into spec with the Jensen
preamps we use. My guess is that is was probably never flat to start with.

In situations where no pink noise or multi-frequency tones are provided, it's
really a crap shoot.

Oh, and then there are the reels labeled with Dolby stickers which 
really aren't
Dolby. Lovely......

Scott D. Smith

Chicago Audio Works, Inc.




Quoting "Richard L. Hess" <[log in to unmask]>:

> I was just checking out my Dolby 422s that I finally installed. One 
> complete unit for four channels of B/C/S repro and one that was 
> missing the "S" board, so it only has B/C repro. Got half my money 
> back from seller. Feel good overall.
>
> Anyway, I played a tape that had a tone on it and aligned the tone 
> properly on the 422 and it sounds a bit muffled, I increased the 
> level going into the processor about 2 dB and it sounds much more 
> natural, the way I remembered it. It was a tape from 1975 that I had 
> done of the Met Opera Ring broadcasts - done by remote control since 
> I was there in the audience during the broadcast.
>
> So, any thoughts/tricks/methods anyone would care to share for 
> adjusting processors for better sound?
>
> This is a topic I don't think has been covered recently. I know the 
> way one tape sounded the best was when I turned off the Dolby A 
> DESPITE a bright yellow Dolby A sticker on the reel--along with the 
> proper name for the album!
>
> Also it took three dbx I processors before I got one I liked. The 
> older 411s that I now have (rack mount cards) sound as good as the 
> best of the compact units (the 150, not the 150X that I had).
>
> Don't bother saying, "I don't like NR" as that doesn't matter now. It 
> did when we used it, but the fact is we have tapes encoded for this 
> and it's like a marriage vow: for better or for worse.
>
> As a refresher, we have:
>
> Dbx I and II
> Dolby A, B, C, S, and SR (the first and last being professional)
> Telcom C4(d) and its consumer derivative Nakamichi HighCom II
>
> I have processors for all of the above. There was at least one, 
> possibly two, Burwen companders. The one I'm aware of had a 3:1 
> linear decibel compression/expansion.
>
> I've also come to the conclusion that for rarely used formats, you 
> can ingest into the computer, then go back out and in to the computer 
> through the processor, doing as many channels as you can in each 
> pass. Not fast, but better than maintaining 16 channels of rarely 
> used NR.
>
> I did this successfully when supplied with a WAV file of encoded 
> Telcom C4 and sent back a WAV file after decoding.
>
> Cheers,
>
> Richard
>
> P.S. for those who care, here are the quantities of channels I 
> currently have available:
>
> Dbx I (16 channels in 411 cards plus 11 spares)
> Dbx II (4 channels installed, 4 backup)
> Dolby A (16 channels installed plus about 40 spare Cat22 cards)
> Dolby B (8 channels in Dolby 422s, 4 channels in Teac AN180s, built 
> into Dragons)
> Dolby C (8 channels in Dolby 422s, built into Dragons)
> Dolby S (4 channels in Dolby 422s)
> Dolby SR (I think I have about 8 cards)
> Telcom C4(d) (4 cards)
> Nakamichi HighCom II (2 channels)
>
>
> Tape Restoration Seminar:    MAY 9-12, 2006; details at Web site.
> Richard L. Hess                   email: [log in to unmask]
> Aurora, Ontario, Canada       (905) 713 6733     1-877-TAPE-FIX
> Detailed contact information: http://www.richardhess.com/tape/contact.htm