prophet@umcp-cs.UUCP (03/22/84)
I have a three-head tape deck with a bias-fine-tune control. Before recording
a tape, I tune my receiver to a spot on the FM dial where there are no
stations (just a lot of hiss) and then I use the monitor switch on the tape
deck to switch between 'TAPE' and 'SOURCE' while recording the hiss. This al-
lows me to compare the hiss a split-second before it is recorded with the
sound after it has been recorded. I turn the fine-bias-adjustment control
until the sound is the same when switching between 'TAPE' and 'SOURCE'. My
question is, should this procedure be done with Dolby off, or on? (I use Dolby
C) It seems to make quite a bit of difference. Does anyone know which way is
correct? Can anyone suggest a more precise method of adjusting the bias with-
out using lots of expensive equipment? Having a technician do this for one
brand of tape won't work since I use many different brands of tape.
Thanks,
Dennis
--
Call-Me: Dennis Gibbs, Univ. of Md. Comp. Sci. Center.
UUCP: {seismo,allegra,brl-bmd}!umcp-cs!prophet
CSNet: prophet@umcp-cs
ARPA: prophet.umcp-cs@CSNet-Relayburris@ihopa.UUCP (David Burris) (03/22/84)
The method you are trying to use to set your bias will not produce consistent results! Dolby encoders are calibrated at a level of -20 dbm. The frequency response of the tapes is referenced to this recording level also. Higher levels can cause frequency response variations due to tape saturation or mistracking of the Dolby circuit. -- Dave Burris ..!ihnp4!ihopa!burris AT&T Bell Labs, Naperville, Il.
dmmartindale@watcgl.UUCP (Dave Martindale) (03/22/84)
When adjusting bias by ear, I would say do it with Dolby off. There are three adjustments (at least) that must be made to adjust a deck properly for a particular tape formulation: bias, record EQ, and record level. With a user-adjustable bias control, you can get only one of those, so results won't be ideal. If the record level is misadjusted, you will inevitably get Dolby mistracking, as the Dolby circuits will read the same signal as being at different levels on record and playback, and apply differing amounts of high-frequency boost/compression. With Dolby switched off, at least you will be dealing with only bias-related frequency response changes. Note that setting bias by perceived frequency response won't get you the best possible bias either, unless the record EQ just happens to be correct for that tape. The normal procedure for setting up a deck in the shop seems to be to set the bias to produce some specified level of distortion (that the manufacturer has found to produce the best tradeoff of various problems), then set the record EQ so the frequency response is flat, and then set the record level so that a given signal level into the tape will produce the same level coming off, as seen by the level meters and Dolby circuitry. Without setting all three, you just won't get the best the deck is capable of. Is recording on many different varieties of tape that important?
emrath@uiuccsb.UUCP (03/24/84)
#R:umcp-cs:-606200:uiuccsb:5700030:000:1516
uiuccsb!emrath Mar 23 14:52:00 1984
You should make the bias adjustment with Dolby off.
If everything else is just perfect, then it wouldn't make any difference.
If the response changes significantly when you switch Dolby in,
then it (usually) means the Dolby en/decoders are mistracking.
The absolute sound level is important for the decoder to properly decode
the signal. This is normally achieved by adjusting the gain of the recording
amplifier (rather than the playback preamp), and should be done when changing
from one brand of tape to another. Note that this adjustment is made *after*
encoding, while the overall recording level is set *before* encoding.
It makes a difference. My deck has the appropriate test tones and record
amp adjustments, so setting record level and bias is fast, easy and accurate.
An error of 2-3dB in record level makes a noticable difference in the response
of low-level FM hiss (-15dB) when switching Dolby-B in/out (I just tried it).
Dolby C just makes the problem worse. Since tapes can vary this much, even
within the same brand, this may be your problem.
Note: FM hiss is a very good way to adjust the record bias, for it is a very
demanding signal. But just because you hear a change when you switch in the
dolby circuits, that doesn't mean the sound of typical signals (i.e. music)
will be seriously defected. If you can't live with it, you'll have to check
the Dolby record level and if this is your problem, come up with a way to
adjust it.
Perry Emrath, UofIL
...{decvax|inuxc}!pur-ee!uiucdcs!emrathron@brl-vgr.ARPA (Ron Natalie <ron>) (03/26/84)
Warning: Does your DEC to playback and record Dolby simultaneously? This may make a difference, you might be listening to unencoded sound on the monitors. -Ron