rcd@opus.UUCP (Dick Dunn) (07/09/84)
I have to copy a number of cassettes which were recorded with Dolby(tm) NR.
It seems to me that it would make most sense to copy with the Dolby
circuitry OFF on both the playback and record decks - in that way I'm not
going through the Dolby circuitry at all. This should be functionally the
same as going through it once in each direction, but not doing the extra
processing ought to help.
Is this reasonable, or is there a gotcha that I'm not seeing? Responses by
email, please; I'll summarize to the net if I get responses and there seems
to be interest.
--
Dick Dunn {hao,ucbvax,allegra}!nbires!rcd (303)444-5710 x3086
...Nothing left to do but smile, smile, smile.newton2@ucbtopaz.CC.Berkeley.ARPA (07/19/84)
There are a few things to keep in mind with consumer Dolby applications: It's desirable to optimize the flatness of the channel between a pair of encode-decode processors; the sliding-band compandor is sensitive to frequency-response errors in the spectral regions of dominant program energy. In practice this means that ragged band-edge response can cause problems when there is substantial hf energy in the program. A deck with a signle pair of processors (not a three head or "double-Dolby" job) is more likely to have truly complementary record-play response than one using two separate processors, one pair for rew3cord and another}i for replay. The components are likely to be poorly (i.e., cost-effectively) matched in the latter case, while in the former case, identically the same components (in conjugate configuration, as it were) are used for record and replay. It's very important to make a "staight dub", i.e., a direct copy of the Dolbized master to another cassettte without intermediate coding-decoding, so that the copy is made with the same absolute fluxivity as the original- that is to say "Dolby level" mustr be accurately observed so that particular signals are recorded at the same flux level on the copy as on the master. This is necessary to afford a fighting chance at accurate decoding using Dolby circuits adjusted for standard conditions. I think Dolby level on cassette is 200 nWb/M. It's essential that no superaudible tones or beats be accidentally recorded together with the intended signals, as these will cause serious decoder mistracking if they are reproduced. All acceptible Dolby-equipped products have effective multiplex pilot-tone traps and bias filters *which are incorporated in the noise reduction circuitry*. If they are omitted from the signal path when NR is switched off, a straight dub my include 19 kHz leakage from a receiver or other sources of high-frequency garbage. The presence of unintended high-frequency energy applied to the decoder will slide the side-chain filter up in frequency and markedly reduce the noise-reduction effect (more hiss, abnormally augnmented high-frequencies [seldom a complaint about Dolby!]). And so on. Why not do this: Make a noise-reduced and re-encoded dub and see if it sounds acceptible; if YES, you are DONE.