coltoff@burdvax.UUCP (Joel Coltoff) (03/06/84)
THIS LINE MIGHT BE EATEN THIS LINE MIGHT BE EATEN
How do the 70usec and 120usec EQ numbers relate to the frequency at which
the boost is done. I already know what the numbers are but can seem to
determine what the equation is. Does anyone have this information.
Thanks
--
Joel Coltoff {presby,bpa,psuvax}!burdvax!coltoff
(215)648-7258sjc@mordor.UUCP (03/07/84)
The question was what the terms "70us" and "120us" mean with regard to tape recorder equalization. They are the time constants for two different first-order (e.g. RC) filters. The equation f = 1 / (2 * pi * t) converts them to frequencies of about 2.27 kHz and 1.33 kHz respectively. Each frequency (often called a "turnover" frequency) is the 3dB point for the filter response, above which frequency the response drops (assuming you're talking about playback deemphasis) by 6dB per octave.--Steve (S-1 Project, Lawrence Livermore National Laboratory) MILNET: sjc@s1-c UUCP: ...!decvax!decwrl!mordor!sjc
fish@ihu1g.UUCP (Bob Fishell) (03/07/84)
(oo)
AC theory has been a while, but here goes:
The 120 uS and 70 uS EQ figures you refer to are the time
constants of simple RC highpass/lowpass filters used for
preemphasis and deemphasis of FM broadcasts, and in tape
decks. A highpass filter looks like this:
O-----|(---*---O
C |
>
R >
>
|
O----------*---O
And a lowpass looks like this:
O--VVVVV---*---O
R |
_|_
C '|`
|
O----------*---O
I don't remember the equations, but they're pretty simple to look
up. The important thing to remember is that the greater the
time constant (R*C), the greater the degree of preemphasis/deemphasis
present in the system.
For FM systems, the corner frequency of the preemphasis curve is
about 5Khz. I don't know offhand what it is for Dolby. Standard
FM uses 120 uS preemphasis; Dolby uses 70uS.
--
Bob Fishell
ihnp4!ihu1g!fishrfg@hound.UUCP (R.GRANTGES) (03/08/84)
Strange. I thought that 70 and 120 u sec were the EQ time constants for CRO2 and Standard Cassettes. Also I'm pretty sure that the EQ constant for FM is....well different from those two. Dick Grantges hound!rfg
fish@ihu1g.UUCP (Bob Fishell) (03/08/84)
(oo)
Sorry, folks, muffed it again. FM preemphasis is standardized at 75uS
in this country. I was wrong about the corner frequency, too; it's
2120 Hz. @+3dB. Deemphasis is exactly opposite. 120 uS is for ferric
tape;70 uS for chrome and metal, Dolby notwithstanding.
Just goes to show you what happens to the brains of a former hardware
honcho when they chain you to a terminal and take your soldering iron
away. Fortunately, I still have a few of my textbooks around; the
corrected info above is from one of them. But didn't you like the
pictures?
--
Bob Fishell
ihnp4!ihu1g!fish