Hi David, hi John,
Thank you very much for your help! I am not trying to build a
professional VU meter; rather, what I want is to create a
microcontroller-based LM3916 alternative. These are now obsolete LED bar
driver ICs with a logarithmic output that are quite handy for building a
simple (not professional) VU meter.
Lots of things to read for me!
Best wishes
Jens
On 2019-03-28 8:10 a.m., johnk wrote:
Hey Jens...
1). Don't call it VU unless you match the actual ballistics of the ORIGINAL VU
meter
2). Europe tends to use a PPM [quasi-peak] system. Peak Program Meter.
3). Consumer and pro levels are very different. They are often misunderstood
and hence mounds of garbage appear on the web.
4). dB is a ratio and a reference must be stated. Thus you must choose what
reference your 0VU is.
5). Your headphone out MIGHT be the Line Out. There are [quite a few] standards
for Line Level.
6]. A useful fact relates to the maximum level that can be digitally encoded. A
CD can produce a certain maximum digital value. That gets converted to some
analogue value. You can work empirically by using [say] standard test tones
from the web OR an audio program like Audacity to produce known digital levels.
0 dBFS is the max level... BUT there can be output results higher than this
[long topic], so work on using either -0.3dB or even -3dB below (or do I mean
over? Semantics ) this as maximum.
7). In "recent" years the "loudness wars" and misuse of Digital Audio Workstations has resulted in
CDs and digital audio having bugger-all dynamic range. Magnetic tape was automatically a limiter [curved]. People using
digital should be working [say]20dB down from max.... but the signal level looks so tiny to them in
"oscilloscope" mode. They think that they are getting better results by working "hot" and getting
the best sig/noise ratios. Their loss of dynamic range is not a good outcome. I mention this because it DOES relate to
the choice of 0 dB[ref].
BTW.. some digital audio actually clips! The master has too much dynamic range for the
"cutting" [vinyl, cd , web etc] engineer. He wants it to sound HOTTER. Sigh.
8). Talking the voltage levels. The term dBU refers to a bridging voltage measurement.
These days you definitely do NOT want to try expecting 600 ohm impedances. The
"usual" outputs are these days exceptionally low... eg way under 20 ohms. This
way you can feed a great number of inputs from one output. Big topic. The problems start
here for your question. Many consumer outputs place a series resistor for protection
purposes. This doesn't affect Bridging (high Z meter) readings much, BUT the input
impedance of the loads does. And the number of such loads connected.
There are many manufacturers who conform to certain industry standards, and
many more who don't.
9). Read Bob Katz book Mastering Audio. Downloadable, although I didn't check which
editions. I use 2nd edit for recording because I do not do web stuff. His later
edition(s) cover the "modern" approach. [read Amateur Fiddling for modern
:-)) ]
10). I suspect that the most useful meter for your purposes is one which
relates well to the maximum signal level and has fast response. BUT VU is so
averaged and hence slow. There are many so-called VU that are fast reacting and
hence display peaks. And be careful of the definitions here when you see rms
stated. As David said, work with peak voltages that you can see on a scope.
This is where you need to start looking at the Orban meters. I haven't checked
but they were downloadable. Apart from the damping factor for Peak, his special
meters that relate to perception are interesting. He covers the standards for
TV advertising versus program content. And that leads to discussions of
loudness [not volume].
Reminder... VU can ONLY be used when the meter has the defined ballistics. See
the BTSJ papers from the 1940s iirc. And the original meter HAD/HAS to have the
attenuator incorporated. The use of the meter without recognising the lack of
sensitivity led to the actual zero level of +4dBm. The original switched
attenuator had zero attenuation in the 4 position. It takes +4dBm to drive the
meter to the zero indication. Big topic too. Watch out for web misinformation.
You can check my version by referring to the papers. I can not [easily] access
all my digital archives at present so I am not providing full details.
11). As David implied... I think you will need a sensitivity adjustment. Users
can calibrate the zero to represent whatever they want. The meter
characteristic/ballistic HAS to be known by a serious user. There are examples
on the web.
Whatever you do has to provide for the whole audio chain.. there has to be
headroom everywhere so that there is no clipping.
12). Here is a typical online statement - this one form Audio Technica :-
"Line level refers to the typical level (strength or amplitude) of the audio signal
from mixers, signal-processing equipment and other consumer and professional audio gear.
There are two line level types: consumer and professional. Consumer line level is
generally thought of as a signal whose level is at -10 dBV (0.316). CD players and DVD
players are examples of consumer line level equipment. Professional line level is
generally thought of as a signal whose level is at +4 dBu (1.23 volts or significantly
higher). Signal-processing equipment and professional mixing consoles are examples of
professional line level equipment."
Done quickly, apologies for errors and omissions,
John Kaesehagen
Australia
-----Original Message-----
From: neonixie-l@googlegroups.com [mailto:neonixie-l@googlegroups.com] On
Behalf Of jb-electronics
Sent: Thursday, 28 March 2019 11:42
To: neonixie-l@googlegroups.com
Subject: [neonixie-l] OT: audio levels
Hello Nixie friends,
I am thinking of building a simple all-in-one VU meter using a microcontroller
to visualize the audio signal on a headphone line (say, I feed the headphone
output of my PC to my circuit, and then I install a pass-thru to connect to my
headphones).
What is the typical voltage amplitude I should expect? I do realize this
question is quite general, but perhaps you can point me in the right direction?
Many thanks!
Best wishes
Jens
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