brent@itm.UUCP (10/03/83)
VU (Volume Unit) meters used here in the U.S. Have ballistics
as follows: (as best I remember) The meter will rise to 99% of
its 0 VU level in 300 milliseconds given a 1000 Hz tone at 0 VU.
Most modern studios set 0 VU to +4 dBm. The Europeans use a
"peak meter" whose ballistics are a lot faster, but whose specification
I do not know. My Nak uses such peak meters and I find them satisfactory.
The lasest generation of cassette decks (such as the new Sonys) use
a linear array of LEDs as level indicators. They light up in a
continuous line to simulate VU. Dancing above this, one LED performs
"sample and hold" about every half second on the peaks. I really
like this arrangement, as it combines the best of both worlds.
By the way, "LED meters": is this like plastic glasses, and
dialing a push-button phone? :-)
Brent Laminack (akgua!itm!brent)rzdz@fluke.UUCP (Richard Chinn) (10/10/83)
re: meter balistics
ok...ok....time to set the record straight
Yes, the ASA spec for a vu meter, as I recall, is 99% of 0 VU in 300ms @
1000 hz. There is also something about overshoot, but I don't recall it.
What is important to note here is that a meter with a scale that says
"VU" does not a VU meter make. Most consumer grade equipment (maybe this
should say all) that has a "VU meter" does not. What it does have is a
microammeter movement, a bridge rectifier, and sometimes a smoothing
capacitor to make the meter less nervous. Occasionally, the meter
movement is specified to have "VU like" ballistics.
Considering the ++$30 price asked by Simpson or Triplett for a *legal VU
meter* and the parts * 5 or 6 relationship between cost of materials and
finished goods, there isn't room in the price of most consumer tape
machines for legit vu meters.
In most recording studios, the *console* VU meters are calibrated so
that 0 VU = +4 dBm or 1.23V across 600 ohms. What is really important
here is not that 0 VU = +4 dBm, but what fluxivity on the recording tape
results from a meter reading of 0.
There are several standards currently in use, all of them are referenced
to *Ampex* level or 185 nanowebers/meter. Today, many studios calibrate
their machines to 3 dB above this level (+3 or elevated level) and some
calibrate to +6 above Ampex level. This is typically done on machines
with legit VU meters as their signal level monitoring device.
In European studios, or at least on European manufactured recording
consoles (like Neve), the PPM (peak programme meter) is commonly found.
Again, I don't recall the exact ballistic specs, but the risetime is
almost instantaneous, with an integration time that is sufficiently long
to accomodate the human ear's ignorance of short term (< 10 ms)
clipping. The fall time is quite long, several seconds at least, so you
can see trends in level even after an event has occured. PPMs have a
distinct scale, very unlike a VU meter. Most of the time, the PPM is
linked (via calibration) to tape saturation, so that the reading is
truly meaningful.
I prefer the ballistics chosen by Nagra for the modulometer (that's what
they call it) used on the Nagra IV machines. It acts like a VU meter,
except that peaks are held long enough for you to see them, then the
fall time is a bit longer than a VU meter. O dB on the modulometer *is*
tape saturation.
The Sony PCM-F1 has LED meters, with a user option of displaying the
peaks, held on the display for some time, over the short term peaks,
much as on the Nakamichi machines. This is a highly usable display and
tells you what is really going onto tape.
Rick Chinn @ Fluke
tpvax:rzdz
206 356 5232